ienumerable - Page 8 - Developer IT

Why can't I create a templated sublcass of System::Collections::Generic::IEnumerable<T>?

- by fiirhok

I want to create a generic IEnumerable implementation, to make it easier to wrap some native C++ classes. When I try to create the implementation using a template parameter as the parameter to IEnumerable, I get an error. Here's a simple version of what I came up with that demonstrates my problem: ref class A {}; template<class B> ref class Test : public System::Collections::Generic::IEnumerable<B^> // error C3225... {}; void test() { Test<A> ^a = gcnew Test<A>(); } On the indicated line, I get this error: error C3225: generic type argument for 'T' cannot be 'B ^', it must be a value type or a handle to a reference type If I use a different parent class, I don't see the problem: template<class P> ref class Parent {}; ref class A {}; template<class B> ref class Test : public Parent<B^> // no problem here {}; void test() { Test<A> ^a = gcnew Test<A>(); } I can work around it by adding another template parameter to the implementation type: ref class A {}; template<class B, class Enumerable> ref class Test : public Enumerable {}; void test() { using namespace System::Collections::Generic; Test<A, IEnumerable<A^>> ^a = gcnew Test<A, IEnumerable<A^>>(); } But this seems messy to me. Also, I'd just like to understand what's going on here - why doesn't the first way work?

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Extension method using Reflection to Sort

- by Xavier

I implemented an extension "MyExtensionSortMethod" to sort collections (IEnumerate). This allows me to replace code such as 'entities.OrderBy( ... ).ThenByDescending( ...)' by 'entities.MyExtensionSortMethod()' (no parameter as well). Here is a sample of implementation: //test function function Test(IEnumerable<ClassA> entitiesA,IEnumerable<ClassB> entitiesB ) { //Sort entitiesA , based on ClassA MySort method var aSorted = entitiesA.MyExtensionSortMethod(); //Sort entitiesB , based on ClassB MySort method var bSorted = entitiesB.MyExtensionSortMethod(); } //Class A definition public classA: IMySort<classA> { .... public IEnumerable<classA> MySort(IEnumerable<classA> entities) { return entities.OrderBy( ... ).ThenBy( ...); } } public classB: IMySort<classB> { .... public IEnumerable<classB> MySort(IEnumerable<classB> entities) { return entities.OrderByDescending( ... ).ThenBy( ...).ThenBy( ... ); } } //extension method public static IEnumerable<T> MyExtensionSortMethod<T>(this IEnumerable<T> e) where T : IMySort<T>, new() { //the extension should call MySort of T Type t = typeof(T); var methodInfo = t.GetMethod("MySort"); //invoke MySort var result = methodInfo.Invoke(new T(), new object[] {e}); //Return return (IEnumerable < T >)result; } public interface IMySort<TEntity> where TEntity : class { IEnumerable<TEntity> MySort(IEnumerable<TEntity> entities); } However, it seems a bit complicated compared to what it does so I was wondering if they were another way of doing it?

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MVC's Html.DropDownList and "There is no ViewData item of type 'IEnumerable<SelectListItem>' that has the key '...'

- by pjohnson

ASP.NET MVC's HtmlHelper extension methods take out a lot of the HTML-by-hand drudgery to which MVC re-introduced us former WebForms programmers. Another thing to which MVC re-introduced us is poor documentation, after the excellent documentation for most of the rest of ASP.NET and the .NET Framework which I now realize I'd taken for granted. I'd come to regard using HtmlHelper methods instead of writing HTML by hand as a best practice. When I upgraded a project from MVC 3 to MVC 4, several hidden fields with boolean values broke, because MVC 3 called ToString() on those values implicitly, and MVC 4 threw an exception until you called ToString() explicitly. Fields that used HtmlHelper weren't affected. I then went through dozens of views and manually replaced hidden inputs that had been coded by hand with Html.Hidden calls. So for a dropdown list I was rendering on the initial page as empty, then populating via JavaScript after an AJAX call, I tried to use a HtmlHelper method: @Html.DropDownList("myDropdown") which threw an exception: System.InvalidOperationException: There is no ViewData item of type 'IEnumerable<SelectListItem>' that has the key 'myDropdown'. That's funny--I made no indication I wanted to use ViewData. Why was it looking there? Just render an empty select list for me. When I populated the list with items, it worked, but I didn't want to do that: @Html.DropDownList("myDropdown", new List<SelectListItem>() { new SelectListItem() { Text = "", Value = "" } }) I removed this dummy item in JavaScript after the AJAX call, so this worked fine, but I shouldn't have to give it a list with a dummy item when what I really want is an empty select. A bit of research with JetBrains dotPeek (helpfully recommended by Scott Hanselman) revealed the problem. Html.DropDownList requires some sort of data to render or it throws an error. The documentation hints at this but doesn't make it very clear. Behind the scenes, it checks if you've provided the DropDownList method any data. If you haven't, it looks in ViewData. If it's not there, you get the exception above. In my case, the helper wasn't doing much for me anyway, so I reverted to writing the HTML by hand (I ain't scared), and amended my best practice: When an HTML control has an associated HtmlHelper method and you're populating that control with data on the initial view, use the HtmlHelper method instead of writing by hand.

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Class Design -- Multiple Calls from One Method or One Call from Multiple Methods?

- by Andrew

I've been working on some code recently that interfaces with a CMS we use and it's presented me with a question on class design that I think is applicable in a number of situations. Essentially, what I am doing is extracting information from the CMS and transforming this information into objects that I can use programatically for other purposes. This consists of two steps: Retrieve the data from the CMS (we have a DAL that I use, so this is essentially just specifying what data from the CMS I want--no connection logic or anything like that) Map the parsed data to my own [C#] objects There are basically two ways I can approach this: One call from multiple methods public void MainMethodWhereIDoStuff() { IEnumerable<MyObject> myObjects = GetMyObjects(); // Do other stuff with myObjects } private static IEnumerable<MyObject> GetMyObjects() { IEnumerable<CmsDataItem> cmsDataItems = GetCmsDataItems(); List<MyObject> mappedObjects = new List<MyObject>(); // do stuff to map the CmsDataItems to MyObjects return mappedObjects; } private static IEnumerable<CmsDataItem> GetCmsDataItems() { List<CmsDataItem> cmsDataItems = new List<CmsDataItem>(); // do stuff to get the CmsDataItems I want return cmsDataItems; } Multiple calls from one method public void MainMethodWhereIDoStuff() { IEnumerable<CmsDataItem> cmsDataItems = GetCmsDataItems(); IEnumerable<MyObject> myObjects = GetMyObjects(cmsDataItems); // do stuff with myObjects } private static IEnumerable<MyObject> GetMyObjects(IEnumerable<CmsDataItem> itemsToMap) { // ... } private static IEnumerable<CmsDataItem> GetCmsDataItems() { // ... } I am tempted to say that the latter is better than the former, as GetMyObjects does not depend on GetCmsDataItems, and it is explicit in the calling method the steps that are executed to retrieve the objects (I'm concerned that the first approach is kind of an object-oriented version of spaghetti code). On the other hand, the two helper methods are never going to be used outside of the class, so I'm not sure if it really matters whether one depends on the other. Furthermore, I like the fact that in the first approach the objects can be retrieved from one line-- most likely anyone working with the main method doesn't care how the objects are retrieved, they just need to retrieve the objects, and the "daisy chained" helper methods hide the exact steps needed to retrieve them (in practice, I actually have a few more methods but am still able to retrieve the object collection I want in one line). Is one of these methods right and the other wrong? Or is it simply a matter of preference or context dependent?

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Enumerable Interleave Extension Method

- by João Angelo

A recent stackoverflow question, which I didn’t bookmark and now I’m unable to find, inspired me to implement an extension method for Enumerable that allows to insert a constant element between each pair of elements in a sequence. Kind of what String.Join does for strings, but maintaining an enumerable as the return value. Having done the single element part I got a bit carried away and ended up expanding it adding overloads to support interleaving elements of another sequence and support for a predicate to control when interleaving takes place. I have to confess that I did this for fun and now I can’t think of any real usage scenario, nonetheless, it may prove useful for someone. First a simple example: var target = new string[] { "(", ")", "(", ")" }; var result = target.Interleave(".", (f, s) => f == "("); // Prints: (.)(.) Console.WriteLine(String.Join(string.Empty, result)); And now the untested but documented implementation: using System; using System.Collections; using System.Collections.Generic; using System.Linq; public static class EnumerableExtensions { /// <summary> /// Iterates infinitely over a constant element. /// </summary> /// <typeparam name="T"> /// The type of element in the sequence. /// </typeparam> private class InfiniteSequence<T> : IEnumerable<T>, IEnumerator<T> { public InfiniteSequence(T element) { this.Element = element; } public T Element { get; private set; } public IEnumerator<T> GetEnumerator() { return this; } IEnumerator IEnumerable.GetEnumerator() { return this; } T IEnumerator<T>.Current { get { return this.Element; } } void IDisposable.Dispose() { } object IEnumerator.Current { get { return this.Element; } } bool IEnumerator.MoveNext() { return true; } void IEnumerator.Reset() { } } /// <summary> /// Interleaves the specified <paramref name="element"/> between each pair of elements in the <paramref name="target"/> sequence. /// </summary> /// <typeparam name="T"> /// The type of elements in the sequence. /// </typeparam> /// <param name="target"> /// The target sequence to be interleaved. /// </param> /// <param name="element"> /// The element used to perform the interleave operation. /// </param> /// <exception cref="ArgumentNullException"> /// <paramref name="target"/> or <paramref name="element"/> is a null reference. /// </exception> /// <returns> /// The <paramref name="target"/> sequence interleaved with the specified <paramref name="element"/>. /// </returns> public static IEnumerable<T> Interleave<T>( this IEnumerable<T> target, T element) { if (target == null) throw new ArgumentNullException("target"); if (element == null) throw new ArgumentNullException("element"); return InterleaveInternal(target, new InfiniteSequence<T>(element), (f, s) => true); } /// <summary> /// Interleaves the specified <paramref name="element"/> between each pair of elements in the <paramref name="target"/> sequence. /// </summary> /// <remarks> /// The interleave operation is interrupted as soon as the <paramref name="target"/> sequence is exhausted; If the number of <paramref name="elements"/> to be interleaved are not enough to completely interleave the <paramref name="target"/> sequence then the remainder of the sequence is returned without being interleaved. /// </remarks> /// <typeparam name="T"> /// The type of elements in the sequence. /// </typeparam> /// <param name="target"> /// The target sequence to be interleaved. /// </param> /// <param name="elements"> /// The elements used to perform the interleave operation. /// </param> /// <exception cref="ArgumentNullException"> /// <paramref name="target"/> or <paramref name="element"/> is a null reference. /// </exception> /// <returns> /// The <paramref name="target"/> sequence interleaved with the specified <paramref name="elements"/>. /// </returns> public static IEnumerable<T> Interleave<T>( this IEnumerable<T> target, IEnumerable<T> elements) { if (target == null) throw new ArgumentNullException("target"); if (elements == null) throw new ArgumentNullException("elements"); return InterleaveInternal(target, elements, (f, s) => true); } /// <summary> /// Interleaves the specified <paramref name="element"/> between each pair of elements in the <paramref name="target"/> sequence that satisfy <paramref name="predicate"/>. /// </summary> /// <typeparam name="T"> /// The type of elements in the sequence. /// </typeparam> /// <param name="target"> /// The target sequence to be interleaved. /// </param> /// <param name="element"> /// The element used to perform the interleave operation. /// </param> /// <param name="predicate"> /// A predicate used to assert if interleaving should occur between two target elements. /// </param> /// <exception cref="ArgumentNullException"> /// <paramref name="target"/> or <paramref name="element"/> or <paramref name="predicate"/> is a null reference. /// </exception> /// <returns> /// The <paramref name="target"/> sequence interleaved with the specified <paramref name="element"/>. /// </returns> public static IEnumerable<T> Interleave<T>( this IEnumerable<T> target, T element, Func<T, T, bool> predicate) { if (target == null) throw new ArgumentNullException("target"); if (element == null) throw new ArgumentNullException("element"); if (predicate == null) throw new ArgumentNullException("predicate"); return InterleaveInternal(target, new InfiniteSequence<T>(element), predicate); } /// <summary> /// Interleaves the specified <paramref name="element"/> between each pair of elements in the <paramref name="target"/> sequence that satisfy <paramref name="predicate"/>. /// </summary> /// <remarks> /// The interleave operation is interrupted as soon as the <paramref name="target"/> sequence is exhausted; If the number of <paramref name="elements"/> to be interleaved are not enough to completely interleave the <paramref name="target"/> sequence then the remainder of the sequence is returned without being interleaved. /// </remarks> /// <typeparam name="T"> /// The type of elements in the sequence. /// </typeparam> /// <param name="target"> /// The target sequence to be interleaved. /// </param> /// <param name="elements"> /// The elements used to perform the interleave operation. /// </param> /// <param name="predicate"> /// A predicate used to assert if interleaving should occur between two target elements. /// </param> /// <exception cref="ArgumentNullException"> /// <paramref name="target"/> or <paramref name="element"/> or <paramref name="predicate"/> is a null reference. /// </exception> /// <returns> /// The <paramref name="target"/> sequence interleaved with the specified <paramref name="elements"/>. /// </returns> public static IEnumerable<T> Interleave<T>( this IEnumerable<T> target, IEnumerable<T> elements, Func<T, T, bool> predicate) { if (target == null) throw new ArgumentNullException("target"); if (elements == null) throw new ArgumentNullException("elements"); if (predicate == null) throw new ArgumentNullException("predicate"); return InterleaveInternal(target, elements, predicate); } private static IEnumerable<T> InterleaveInternal<T>( this IEnumerable<T> target, IEnumerable<T> elements, Func<T, T, bool> predicate) { var targetEnumerator = target.GetEnumerator(); if (targetEnumerator.MoveNext()) { var elementsEnumerator = elements.GetEnumerator(); while (true) { T first = targetEnumerator.Current; yield return first; if (!targetEnumerator.MoveNext()) yield break; T second = targetEnumerator.Current; bool interleave = true && predicate(first, second) && elementsEnumerator.MoveNext(); if (interleave) yield return elementsEnumerator.Current; } } } }

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If I cast an IQueryable as an IEnumerable then call a Linq extension method, which implementation gets called?

- by James Morcom

Considering the following code: IQueryable<T> queryable; // something to instantiate queryable var enumerable = (IEnumerable<T>) queryable; var filtered = enumerable.Where(i => i > 3); In the final line, which extension method gets called? Is it IEnumerable<T>.Where(...)? Or will IQueryable<T>.Where(...) be called because the actual implementation is still obviously a queryable? Presumably the ideal would be for the IQueryable version to be called, in the same way that normal polymorphism will always use the more specific override. In Visual Studio though when I right-click on the Where method and "Go to Definition" I'm taken to the IEnumerable version, which kind of makes sense from a visual point-of-view. My main concern is that if somewhere in my app I use Linq to NHibernate to get a Queryable, but I pass it around using an interface that uses the more general IEnumerable signature, I'll lose the wonders of deferred database execution!

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Can I split an IEnumerable into two by a boolean criteria without two queries?

- by SFun28

Folks, Is it possible to split an IEnumerable into two IEnumerables using LINQ and only a single query/linq statement? In this way, I would avoid iterating through the IEnumerable twice. For example, is it possible to combine the last two statements below so allValues is only traversed once? IEnumerable<MyObj> allValues = ... List<MyObj> trues = allValues.Where( val => val.SomeProp ).ToList(); List<MyObj> falses = allValues.Where( val => !val.SomeProp ).ToList();

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Why does calling IEnumerable<string>.Count() create an additional assembly dependency ?

- by Gishu

Assume this chain of dll references Tests.dll >> Automation.dll >> White.Core.dll with the following line of code in Tests.dll, where everything builds result.MissingPaths Now when I change this to result.MissingPaths.Count() I get the following build error for Tests.dll "White.UIItem is not defined in an assembly that is not referenced. You must add a reference to White.Core.dll." And I don't want to do that because it breaks my layering. Here is the type definition for result, which is in Automation.dll public class HasResult { public HasResult(IEnumerable<string> missingPaths ) { MissingPaths = missingPaths; } public IEnumerable<string> MissingPaths { get; set; } public bool AllExist { get { return !MissingPaths.Any(); } } } Down the call chain the input param to this ctor is created via (The TreeNode class is in White.Core.dll) assetPaths.Where(assetPath => !FindTreeNodeUsingCache(treeHandle, assetPath)); Why does this dependency leak when calling Count() on IEnumerable ? I then suspected that lazy evaluation was causing this (for some reason) - so I slotted in an ToArray() in the above line but didn't work. Update 2011 01 07: Curiouser and Curiouser! it won't build until I add a White.Core reference. So I add a reference and build it (in order to find the elusive dependency source). Open it up in Reflector and the only references listed are Automation, mscorlib, System.core and NUnit. So the compiler threw away the White reference as it was not needed. ILDASM also confirms that there is no White AssemblyRef entry. Any ideas on how to get to the bottom of this thing (primarily for 'now I wanna know why' reasons)? What are the chances that this is an VS2010/MSBuild bug? Update 2011 01 07 #2 As per Shimmy's suggestion, tried calling the method explcitly as an extension method Enumerable.Count(result.MissingPaths) and it stops cribbing (not sure why). However I moved some code around after that and now I'm getting the same issue at a different location using IEnumerable - this time reading and filtering lines out of a file on disk (totally unrelated to White). Seems like it's a 'symptom-fix'. var lines = File.ReadLines(aFilePath).ToArray(); once again, if I remove the ToArray() it compiles again - it seems that any method that causes the enumerable to be evaluated (ToArray, Count, ToList, etc.) causes this. Let me try and get a working tiny-app to demo this issue... Update 2011 01 07 #3 Phew! More information.. It turns out the problem is just in one source file - this file is LINQ-phobic. Any call to an Enumerable extension method has to be explicitly called out. The refactorings that I did caused a new method to be moved into this source file, which had some LINQ :) Still no clue as to why this class dislikes LINQ. using System; using System.Collections.Generic; using System.IO; using System.Linq; using G.S.OurAutomation.Constants; using G.S.OurAutomation.Framework; using NUnit.Framework; namespace G.S.AcceptanceTests { public abstract class ConfigureThingBase : OurTestFixture { .... private static IEnumerable<string> GetExpectedThingsFor(string param) { // even this won't compile - although it compiles fine in an adjoining source file in the same assembly //IEnumerable<string> s = new string[0]; //Console.WriteLine(s.Count()); // this is the line that is now causing a build failure // var expectedInfo = File.ReadLines(someCsvFilePath)) // .Where(line => !line.StartsWith("REM", StringComparison.InvariantCultureIgnoreCase)) // .Select(line => line.Replace("%PLACEHOLDER%", param)) // .ToArray(); // Unrolling the LINQ above removes the build error var expectedInfo = Enumerable.ToArray( Enumerable.Select( Enumerable.Where( File.ReadLines(someCsvFilePath)), line => !line.StartsWith("REM", StringComparison.InvariantCultureIgnoreCase)), line => line.Replace("%PLACEHOLDER%", param)));

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How do I refactor this IEnumerable<T> to be thread-safe?

- by DayOne

I am looking at Skeet's AtomicEnumerable but I'm not sure how to integrate it into my current IEnumerable exmaple below (http://msmvps.com/blogs/jon_skeet/archive/2009/10/23/iterating-atomically.aspx) Basically I want to foreach my blahs type in a thread-safe way. thanks public sealed class Blahs : IEnumerable<string> { private readonly IList<string> _data = new List<string>() { "blah1", "blah2", "blah3" }; public IEnumerator<string> GetEnumerator() { return _data.GetEnumerator(); } IEnumerator IEnumerable.GetEnumerator() { return GetEnumerator(); } }

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Is there a built-in way to compare IEnumerable<T> (by their elements)?

- by Cristi Diaconescu

I would like to compare lists of elements of a given type, to see which list is "bigger". new BuiltInComparer<IEnumerable<int>>().Compare( new[] {1,2}, new[] {1,2,3}) ...would return -1 new BuiltInComparer<IEnumerable<int>>().Compare( new[] {1,2,3}, new[] {1,2,3}) ...would return 0 etc Is there any such built in comparer?

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what is the basic role of Ienumerable object in .net?

- by Shailesh Jaiswal

I dont know anything about Ienumerable object.what is the role of Ienumarble interface in .net ? Please tell me whether it is in built? Is every class in .net automatically implements the Ienumarable interface ? why there is need to implement the ienumerable interface? what are the methods it contains & what roles they plays. Please explain in detail.

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Parallelism in .NET – Part 7, Some Differences between PLINQ and LINQ to Objects

- by Reed

In my previous post on Declarative Data Parallelism, I mentioned that PLINQ extends LINQ to Objects to support parallel operations. Although nearly all of the same operations are supported, there are some differences between PLINQ and LINQ to Objects. By introducing Parallelism to our declarative model, we add some extra complexity. This, in turn, adds some extra requirements that must be addressed. In order to illustrate the main differences, and why they exist, let’s begin by discussing some differences in how the two technologies operate, and look at the underlying types involved in LINQ to Objects and PLINQ . LINQ to Objects is mainly built upon a single class: Enumerable. The Enumerable class is a static class that defines a large set of extension methods, nearly all of which work upon an IEnumerable<T>. Many of these methods return a new IEnumerable<T>, allowing the methods to be chained together into a fluent style interface. This is what allows us to write statements that chain together, and lead to the nice declarative programming model of LINQ: double min = collection .Where(item => item.SomeProperty > 6 && item.SomeProperty < 24) .Min(item => item.PerformComputation()); .csharpcode, .csharpcode pre { font-size: small; color: black; font-family: consolas, "Courier New", courier, monospace; background-color: #ffffff; /*white-space: pre;*/ } .csharpcode pre { margin: 0em; } .csharpcode .rem { color: #008000; } .csharpcode .kwrd { color: #0000ff; } .csharpcode .str { color: #006080; } .csharpcode .op { color: #0000c0; } .csharpcode .preproc { color: #cc6633; } .csharpcode .asp { background-color: #ffff00; } .csharpcode .html { color: #800000; } .csharpcode .attr { color: #ff0000; } .csharpcode .alt { background-color: #f4f4f4; width: 100%; margin: 0em; } .csharpcode .lnum { color: #606060; } Other LINQ variants work in a similar fashion. For example, most data-oriented LINQ providers are built upon an implementation of IQueryable<T>, which allows the database provider to turn a LINQ statement into an underlying SQL query, to be performed directly on the remote database. PLINQ is similar, but instead of being built upon the Enumerable class, most of PLINQ is built upon a new static class: ParallelEnumerable. When using PLINQ, you typically begin with any collection which implements IEnumerable<T>, and convert it to a new type using an extension method defined on ParallelEnumerable: AsParallel(). This method takes any IEnumerable<T>, and converts it into a ParallelQuery<T>, the core class for PLINQ. There is a similar ParallelQuery class for working with non-generic IEnumerable implementations. This brings us to our first subtle, but important difference between PLINQ and LINQ – PLINQ always works upon specific types, which must be explicitly created. Typically, the type you’ll use with PLINQ is ParallelQuery<T>, but it can sometimes be a ParallelQuery or an OrderedParallelQuery<T>. Instead of dealing with an interface, implemented by an unknown class, we’re dealing with a specific class type. This works seamlessly from a usage standpoint – ParallelQuery<T> implements IEnumerable<T>, so you can always “switch back” to an IEnumerable<T>. The difference only arises at the beginning of our parallelization. When we’re using LINQ, and we want to process a normal collection via PLINQ, we need to explicitly convert the collection into a ParallelQuery<T> by calling AsParallel(). There is an important consideration here – AsParallel() does not need to be called on your specific collection, but rather any IEnumerable<T>. This allows you to place it anywhere in the chain of methods involved in a LINQ statement, not just at the beginning. This can be useful if you have an operation which will not parallelize well or is not thread safe. For example, the following is perfectly valid, and similar to our previous examples: double min = collection .AsParallel() .Select(item => item.SomeOperation()) .Where(item => item.SomeProperty > 6 && item.SomeProperty < 24) .Min(item => item.PerformComputation()); However, if SomeOperation() is not thread safe, we could just as easily do: double min = collection .Select(item => item.SomeOperation()) .AsParallel() .Where(item => item.SomeProperty > 6 && item.SomeProperty < 24) .Min(item => item.PerformComputation()); In this case, we’re using standard LINQ to Objects for the Select(…) method, then converting the results of that map routine to a ParallelQuery<T>, and processing our filter (the Where method) and our aggregation (the Min method) in parallel. PLINQ also provides us with a way to convert a ParallelQuery<T> back into a standard IEnumerable<T>, forcing sequential processing via standard LINQ to Objects. If SomeOperation() was thread-safe, but PerformComputation() was not thread-safe, we would need to handle this by using the AsEnumerable() method: double min = collection .AsParallel() .Select(item => item.SomeOperation()) .Where(item => item.SomeProperty > 6 && item.SomeProperty < 24) .AsEnumerable() .Min(item => item.PerformComputation()); Here, we’re converting our collection into a ParallelQuery<T>, doing our map operation (the Select(…) method) and our filtering in parallel, then converting the collection back into a standard IEnumerable<T>, which causes our aggregation via Min() to be performed sequentially. This could also be written as two statements, as well, which would allow us to use the language integrated syntax for the first portion: var tempCollection = from item in collection.AsParallel() let e = item.SomeOperation() where (e.SomeProperty > 6 && e.SomeProperty < 24) select e; double min = tempCollection.AsEnumerable().Min(item => item.PerformComputation()); This allows us to use the standard LINQ style language integrated query syntax, but control whether it’s performed in parallel or serial by adding AsParallel() and AsEnumerable() appropriately. The second important difference between PLINQ and LINQ deals with order preservation. PLINQ, by default, does not preserve the order of of source collection. This is by design. In order to process a collection in parallel, the system needs to naturally deal with multiple elements at the same time. Maintaining the original ordering of the sequence adds overhead, which is, in many cases, unnecessary. Therefore, by default, the system is allowed to completely change the order of your sequence during processing. If you are doing a standard query operation, this is usually not an issue. However, there are times when keeping a specific ordering in place is important. If this is required, you can explicitly request the ordering be preserved throughout all operations done on a ParallelQuery<T> by using the AsOrdered() extension method. This will cause our sequence ordering to be preserved. For example, suppose we wanted to take a collection, perform an expensive operation which converts it to a new type, and display the first 100 elements. In LINQ to Objects, our code might look something like: // Using IEnumerable<SourceClass> collection IEnumerable<ResultClass> results = collection .Select(e => e.CreateResult()) .Take(100); If we just converted this to a parallel query naively, like so: IEnumerable<ResultClass> results = collection .AsParallel() .Select(e => e.CreateResult()) .Take(100); We could very easily get a very different, and non-reproducable, set of results, since the ordering of elements in the input collection is not preserved. To get the same results as our original query, we need to use: IEnumerable<ResultClass> results = collection .AsParallel() .AsOrdered() .Select(e => e.CreateResult()) .Take(100); This requests that PLINQ process our sequence in a way that verifies that our resulting collection is ordered as if it were processed serially. This will cause our query to run slower, since there is overhead involved in maintaining the ordering. However, in this case, it is required, since the ordering is required for correctness. PLINQ is incredibly useful. It allows us to easily take nearly any LINQ to Objects query and run it in parallel, using the same methods and syntax we’ve used previously. There are some important differences in operation that must be considered, however – it is not a free pass to parallelize everything. When using PLINQ in order to parallelize your routines declaratively, the same guideline I mentioned before still applies: Parallelization is something that should be handled with care and forethought, added by design, and not just introduced casually.

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Why is Func<T> ambiguous with Func<IEnumerable<T>>?

- by Matt Hamilton

This one's got me flummoxed, so I thought I'd ask here in the hope that a C# guru can explain it to me. Why does this code generate an error? class Program { static void Main(string[] args) { Foo(X); // the error is on this line } static String X() { return "Test"; } static void Foo(Func<IEnumerable<String>> x) { } static void Foo(Func<String> x) { } } The error in question: Error 1 The call is ambiguous between the following methods or properties: 'ConsoleApplication1.Program.Foo(System.Func<System.Collections.Generic.IEnumerable<string>>)' and 'ConsoleApplication1.Program.Foo(System.Func<string>)' C:\Users\mabster\AppData\Local\Temporary Projects\ConsoleApplication1\Program.cs 12 13 ConsoleApplication1 It doesn't matter what type I use - if you replace the "String" declarations with "int" in that code you'll get the same sort of error. It's like the compiler can't tell the difference between Func<T> and Func<IEnumerable<T>>. Can someone shed some light on this?

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How can I access IEnumerable<T> extension methods on my custom subclass of BindingList<T>?

- by Dan

I have a custom subclass of BindingList<T> that I want to execute a LINQ query over using the handy extension methods. For example: public int GetSum(MyList<T> list) { return list.Sum(x => x.Value); } But the compiler complains that it can't resolve Sum because it doesn't recognize list as an IEnumerable<T>, which it obviously is, because this works: public int GetSum(MyList<T> list) { return ((IEnumerable<T>)list).Sum(x => x.Value); } Anyone have a clever way I can avoid the ugly and unecessary cast?

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The ViewData item that has the key 'MY KEY' is of type 'System.String' but must be of type 'IEnumerable<SelectListItem>'.

- by JBibbs

I am trying to populate a dropdown list from a database mapped with Linq 2 SQL, using ASP.NET MVC 2, and keep getting this error. I am so confused because I am declaring a variable with the type IEnumerable<SelectListItem> on the second line, but the error makes me think this is not the case. I feel like this should be very simple, but I am struggling. Any help is appreciated. Here are the interesting bits of my controller: public ActionResult Create() { var db = new DB(); IEnumerable<SelectListItem> basetypes = db.Basetypes.Select(b => new SelectListItem { Value = b.basetype, Text = b.basetype }); ViewData["basetype"] = basetypes; return View(); } And here are the interesting bits of my view: <div class="editor-label"> <%: Html.LabelFor(model => model.basetype) %> </div> <div class="editor-field"> <%: Html.DropDownList("basetype") %> <%: Html.ValidationMessageFor(model => model.basetype) %> </div> Here is the Post action when submitting the Form // POST: /Meals/Create [HttpPost] public ActionResult Create(Meal meal) { if (ModelState.IsValid) { try { // TODO: Add insert logic here var db = new DB(); db.Meals.InsertOnSubmit(meal); db.SubmitChanges(); return RedirectToAction("Index"); } catch { return View(meal); } } else { return View(meal); } } Thanks.

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How can I use Generics to create a way of making an IEnumerable from an enum?

- by Samantha J

Given an enum like this: public enum City { London = 1, Liverpool = 20, Leeds = 25 } public enum House { OneFloor = 1, TwoFloors = 2 } I am using the following code to give me an IEnumerable: City[] values = (City[])Enum.GetValues(typeof(City)); var valuesWithNames = from value in values select new { value = (int)value, name = value.ToString() }; The code works very good however I have to do this for quite a lot of enums. Is there a way I could create a generic way of doing this?

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What is the most efficient way to convert to binary and back in C#?

- by Saad Imran.

I'm trying to write a general purpose socket server for a game I'm working on. I know I could very well use already built servers like SmartFox and Photon, but I wan't to go through the pain of creating one myself for learning purposes. I've come up with a BSON inspired protocol to convert the the basic data types, their arrays, and a special GSObject to binary and arrange them in a way so that it can be put back together into object form on the client end. At the core, the conversion methods utilize the .Net BitConverter class to convert the basic data types to binary. Anyways, the problem is performance, if I loop 50,000 times and convert my GSObject to binary each time it takes about 5500ms (the resulting byte[] is just 192 bytes per conversion). I think think this would be way too slow for an MMO that sends 5-10 position updates per second with a 1000 concurrent users. Yes, I know it's unlikely that a game will have a 1000 users on at the same time, but like I said earlier this is supposed to be a learning process for me, I want to go out of my way and build something that scales well and can handle at least a few thousand users. So yea, if anyone's aware of other conversion techniques or sees where I'm loosing performance I would appreciate the help. GSBitConverter.cs This is the main conversion class, it adds extension methods to main datatypes to convert to the binary format. It uses the BitConverter class to convert the base types. I've shown only the code to convert integer and integer arrays, but the rest of the method are pretty much replicas of those two, they just overload the type. public static class GSBitConverter { public static byte[] ToGSBinary(this short value) { return BitConverter.GetBytes(value); } public static byte[] ToGSBinary(this IEnumerable<short> value) { List<byte> bytes = new List<byte>(); short length = (short)value.Count(); bytes.AddRange(length.ToGSBinary()); for (int i = 0; i < length; i++) bytes.AddRange(value.ElementAt(i).ToGSBinary()); return bytes.ToArray(); } public static byte[] ToGSBinary(this bool value); public static byte[] ToGSBinary(this IEnumerable<bool> value); public static byte[] ToGSBinary(this IEnumerable<byte> value); public static byte[] ToGSBinary(this int value); public static byte[] ToGSBinary(this IEnumerable<int> value); public static byte[] ToGSBinary(this long value); public static byte[] ToGSBinary(this IEnumerable<long> value); public static byte[] ToGSBinary(this float value); public static byte[] ToGSBinary(this IEnumerable<float> value); public static byte[] ToGSBinary(this double value); public static byte[] ToGSBinary(this IEnumerable<double> value); public static byte[] ToGSBinary(this string value); public static byte[] ToGSBinary(this IEnumerable<string> value); public static string GetHexDump(this IEnumerable<byte> value); } Program.cs Here's the the object that I'm converting to binary in a loop. class Program { static void Main(string[] args) { GSObject obj = new GSObject(); obj.AttachShort("smallInt", 15); obj.AttachInt("medInt", 120700); obj.AttachLong("bigInt", 10900800700); obj.AttachDouble("doubleVal", Math.PI); obj.AttachStringArray("muppetNames", new string[] { "Kermit", "Fozzy", "Piggy", "Animal", "Gonzo" }); GSObject apple = new GSObject(); apple.AttachString("name", "Apple"); apple.AttachString("color", "red"); apple.AttachBool("inStock", true); apple.AttachFloat("price", (float)1.5); GSObject lemon = new GSObject(); apple.AttachString("name", "Lemon"); apple.AttachString("color", "yellow"); apple.AttachBool("inStock", false); apple.AttachFloat("price", (float)0.8); GSObject apricoat = new GSObject(); apple.AttachString("name", "Apricoat"); apple.AttachString("color", "orange"); apple.AttachBool("inStock", true); apple.AttachFloat("price", (float)1.9); GSObject kiwi = new GSObject(); apple.AttachString("name", "Kiwi"); apple.AttachString("color", "green"); apple.AttachBool("inStock", true); apple.AttachFloat("price", (float)2.3); GSArray fruits = new GSArray(); fruits.AddGSObject(apple); fruits.AddGSObject(lemon); fruits.AddGSObject(apricoat); fruits.AddGSObject(kiwi); obj.AttachGSArray("fruits", fruits); Stopwatch w1 = Stopwatch.StartNew(); for (int i = 0; i < 50000; i++) { byte[] b = obj.ToGSBinary(); } w1.Stop(); Console.WriteLine(BitConverter.IsLittleEndian ? "Little Endian" : "Big Endian"); Console.WriteLine(w1.ElapsedMilliseconds + "ms"); } Here's the code for some of my other classes that are used in the code above. Most of it is repetitive. GSObject GSArray GSWrappedObject

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BindingList<T> and reflection!

- by Aren B

Background Working in .NET 2.0 Here, reflecting lists in general. I was originally using t.IsAssignableFrom(typeof(IEnumerable)) to detect if a Property I was traversing supported the IEnumerable Interface. (And thus I could cast the object to it safely) However this code was not evaluating to True when the object is a BindingList<T>. Next I tried to use t.IsSubclassOf(typeof(IEnumerable)) and didn't have any luck either. Code /// <summary> /// Reflects an enumerable (not a list, bad name should be fixed later maybe?) /// </summary> /// <param name="o">The Object the property resides on.</param> /// <param name="p">The Property We're reflecting on</param> /// <param name="rla">The Attribute tagged to this property</param> public void ReflectList(object o, PropertyInfo p, ReflectedListAttribute rla) { Type t = p.PropertyType; //if (t.IsAssignableFrom(typeof(IEnumerable))) if (t.IsSubclassOf(typeof(IEnumerable))) { IEnumerable e = p.GetValue(o, null) as IEnumerable; int count = 0; if (e != null) { foreach (object lo in e) { if (count >= rla.MaxRows) break; ReflectObject(lo, count); count++; } } } } The Intent I want to basically tag lists i want to reflect through with the ReflectedListAttribute and call this function on the properties that has it. (Already Working) Once inside this function, given the object the property resides on, and the PropertyInfo related, get the value of the property, cast it to an IEnumerable (assuming it's possible) and then iterate through each child and call ReflectObject(...) on the child with the count variable.

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Upload ICS (calandar) file to my server

- by IEnumerable

I want to upload my ICS file to my server, well this is the easy part. What I really want is to be able to share this *.ics file with 2-3 people so they can edit. I have currently uploaded the file on the www/ (root) mydomain.com.au/myfile.ics of my server and added it to my Email/Cal client. Was all looking ok until I tried to save changes. Can someone please direct me to some documentation on how I do this properly. The easy solution would be to upload to google, but I would rather learn how to manage the file myself. Thank you

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Writing Unit Tests for ASP.NET Web API Controller

- by shiju

In this blog post, I will write unit tests for a ASP.NET Web API controller in the EFMVC reference application. Let me introduce the EFMVC app, If you haven't heard about EFMVC. EFMVC is a simple app, developed as a reference implementation for demonstrating ASP.NET MVC, EF Code First, ASP.NET Web API, Domain-Driven Design (DDD), Test-Driven Development (DDD). The current version is built with ASP.NET MVC 4, EF Code First 5, ASP.NET Web API, Autofac, AutoMapper, Nunit and Moq. All unit tests were written with Nunit and Moq. You can download the latest version of the reference app from http://efmvc.codeplex.com/ Unit Test for HTTP Get Let’s write a unit test class for verifying the behaviour of a ASP.NET Web API controller named CategoryController. Let’s define mock implementation for Repository class, and a Command Bus that is used for executing write operations. [TestFixture] public class CategoryApiControllerTest { private Mock<ICategoryRepository> categoryRepository; private Mock<ICommandBus> commandBus; [SetUp] public void SetUp() { categoryRepository = new Mock<ICategoryRepository>(); commandBus = new Mock<ICommandBus>(); } The code block below provides the unit test for a HTTP Get operation. [Test] public void Get_All_Returns_AllCategory() { // Arrange IEnumerable<CategoryWithExpense> fakeCategories = GetCategories(); categoryRepository.Setup(x => x.GetCategoryWithExpenses()).Returns(fakeCategories); CategoryController controller = new CategoryController(commandBus.Object, categoryRepository.Object) { Request = new HttpRequestMessage() { Properties = { { HttpPropertyKeys.HttpConfigurationKey, new HttpConfiguration() } } } }; // Act var categories = controller.Get(); // Assert Assert.IsNotNull(categories, "Result is null"); Assert.IsInstanceOf(typeof(IEnumerable<CategoryWithExpense>),categories, "Wrong Model"); Assert.AreEqual(3, categories.Count(), "Got wrong number of Categories"); } The GetCategories method is provided below: private static IEnumerable<CategoryWithExpense> GetCategories() { IEnumerable<CategoryWithExpense> fakeCategories = new List<CategoryWithExpense> { new CategoryWithExpense {CategoryId=1, CategoryName = "Test1", Description="Test1Desc", TotalExpenses=1000}, new CategoryWithExpense {CategoryId=2, CategoryName = "Test2", Description="Test2Desc",TotalExpenses=2000}, new CategoryWithExpense { CategoryId=3, CategoryName = "Test3", Description="Test3Desc",TotalExpenses=3000} }.AsEnumerable(); return fakeCategories; } In the unit test method Get_All_Returns_AllCategory, we specify setup on the mocked type ICategoryrepository, for a call to GetCategoryWithExpenses method returns dummy data. We create an instance of the ApiController, where we have specified the Request property of the ApiController since the Request property is used to create a new HttpResponseMessage that will provide the appropriate HTTP status code along with response content data. Unit Tests are using for specifying the behaviour of components so that we have specified that Get operation will use the model type IEnumerable<CategoryWithExpense> for sending the Content data. The implementation of HTTP Get in the CategoryController is provided below: public IQueryable<CategoryWithExpense> Get() { var categories = categoryRepository.GetCategoryWithExpenses().AsQueryable(); return categories; } Unit Test for HTTP Post The following are the behaviours we are going to implement for the HTTP Post: A successful HTTP Post operation should return HTTP status code Created An empty Category should return HTTP status code BadRequest A successful HTTP Post operation should provide correct Location header information in the response for the newly created resource. Writing unit test for HTTP Post is required more information than we write for HTTP Get. In the HTTP Post implementation, we will call to Url.Link for specifying the header Location of Response as shown in below code block. var response = Request.CreateResponse(HttpStatusCode.Created, category); string uri = Url.Link("DefaultApi", new { id = category.CategoryId }); response.Headers.Location = new Uri(uri); return response; While we are executing Url.Link from unit tests, we have to specify HttpRouteData information from the unit test method. Otherwise, Url.Link will get a null value. The code block below shows the unit tests for specifying the behaviours for the HTTP Post operation. [Test] public void Post_Category_Returns_CreatedStatusCode() { // Arrange commandBus.Setup(c => c.Submit(It.IsAny<CreateOrUpdateCategoryCommand>())).Returns(new CommandResult(true)); Mapper.CreateMap<CategoryFormModel, CreateOrUpdateCategoryCommand>(); var httpConfiguration = new HttpConfiguration(); WebApiConfig.Register(httpConfiguration); var httpRouteData = new HttpRouteData(httpConfiguration.Routes["DefaultApi"], new HttpRouteValueDictionary { { "controller", "category" } }); var controller = new CategoryController(commandBus.Object, categoryRepository.Object) { Request = new HttpRequestMessage(HttpMethod.Post, "http://localhost/api/category/") { Properties = { { HttpPropertyKeys.HttpConfigurationKey, httpConfiguration }, { HttpPropertyKeys.HttpRouteDataKey, httpRouteData } } } }; // Act CategoryModel category = new CategoryModel(); category.CategoryId = 1; category.CategoryName = "Mock Category"; var response = controller.Post(category); // Assert Assert.AreEqual(HttpStatusCode.Created, response.StatusCode); var newCategory = JsonConvert.DeserializeObject<CategoryModel>(response.Content.ReadAsStringAsync().Result); Assert.AreEqual(string.Format("http://localhost/api/category/{0}", newCategory.CategoryId), response.Headers.Location.ToString()); } [Test] public void Post_EmptyCategory_Returns_BadRequestStatusCode() { // Arrange commandBus.Setup(c => c.Submit(It.IsAny<CreateOrUpdateCategoryCommand>())).Returns(new CommandResult(true)); Mapper.CreateMap<CategoryFormModel, CreateOrUpdateCategoryCommand>(); var httpConfiguration = new HttpConfiguration(); WebApiConfig.Register(httpConfiguration); var httpRouteData = new HttpRouteData(httpConfiguration.Routes["DefaultApi"], new HttpRouteValueDictionary { { "controller", "category" } }); var controller = new CategoryController(commandBus.Object, categoryRepository.Object) { Request = new HttpRequestMessage(HttpMethod.Post, "http://localhost/api/category/") { Properties = { { HttpPropertyKeys.HttpConfigurationKey, httpConfiguration }, { HttpPropertyKeys.HttpRouteDataKey, httpRouteData } } } }; // Act CategoryModel category = new CategoryModel(); category.CategoryId = 0; category.CategoryName = ""; // The ASP.NET pipeline doesn't run, so validation don't run. controller.ModelState.AddModelError("", "mock error message"); var response = controller.Post(category); // Assert Assert.AreEqual(HttpStatusCode.BadRequest, response.StatusCode); } In the above code block, we have written two unit methods, Post_Category_Returns_CreatedStatusCode and Post_EmptyCategory_Returns_BadRequestStatusCode. The unit test method Post_Category_Returns_CreatedStatusCode verifies the behaviour 1 and 3, that we have defined in the beginning of the section “Unit Test for HTTP Post”. The unit test method Post_EmptyCategory_Returns_BadRequestStatusCode verifies the behaviour 2. For extracting the data from response, we call Content.ReadAsStringAsync().Result of HttpResponseMessage object and deserializeit it with Json Convertor. The implementation of HTTP Post in the CategoryController is provided below: // POST /api/category public HttpResponseMessage Post(CategoryModel category) { if (ModelState.IsValid) { var command = new CreateOrUpdateCategoryCommand(category.CategoryId, category.CategoryName, category.Description); var result = commandBus.Submit(command); if (result.Success) { var response = Request.CreateResponse(HttpStatusCode.Created, category); string uri = Url.Link("DefaultApi", new { id = category.CategoryId }); response.Headers.Location = new Uri(uri); return response; } } else { return Request.CreateErrorResponse(HttpStatusCode.BadRequest, ModelState); } throw new HttpResponseException(HttpStatusCode.BadRequest); } The unit test implementation for HTTP Put and HTTP Delete are very similar to the unit test we have written for HTTP Get. The complete unit tests for the CategoryController is given below: [TestFixture] public class CategoryApiControllerTest { private Mock<ICategoryRepository> categoryRepository; private Mock<ICommandBus> commandBus; [SetUp] public void SetUp() { categoryRepository = new Mock<ICategoryRepository>(); commandBus = new Mock<ICommandBus>(); } [Test] public void Get_All_Returns_AllCategory() { // Arrange IEnumerable<CategoryWithExpense> fakeCategories = GetCategories(); categoryRepository.Setup(x => x.GetCategoryWithExpenses()).Returns(fakeCategories); CategoryController controller = new CategoryController(commandBus.Object, categoryRepository.Object) { Request = new HttpRequestMessage() { Properties = { { HttpPropertyKeys.HttpConfigurationKey, new HttpConfiguration() } } } }; // Act var categories = controller.Get(); // Assert Assert.IsNotNull(categories, "Result is null"); Assert.IsInstanceOf(typeof(IEnumerable<CategoryWithExpense>),categories, "Wrong Model"); Assert.AreEqual(3, categories.Count(), "Got wrong number of Categories"); } [Test] public void Get_CorrectCategoryId_Returns_Category() { // Arrange IEnumerable<CategoryWithExpense> fakeCategories = GetCategories(); categoryRepository.Setup(x => x.GetCategoryWithExpenses()).Returns(fakeCategories); CategoryController controller = new CategoryController(commandBus.Object, categoryRepository.Object) { Request = new HttpRequestMessage() { Properties = { { HttpPropertyKeys.HttpConfigurationKey, new HttpConfiguration() } } } }; // Act var response = controller.Get(1); // Assert Assert.AreEqual(HttpStatusCode.OK, response.StatusCode); var category = JsonConvert.DeserializeObject<CategoryWithExpense>(response.Content.ReadAsStringAsync().Result); Assert.AreEqual(1, category.CategoryId, "Got wrong number of Categories"); } [Test] public void Get_InValidCategoryId_Returns_NotFound() { // Arrange IEnumerable<CategoryWithExpense> fakeCategories = GetCategories(); categoryRepository.Setup(x => x.GetCategoryWithExpenses()).Returns(fakeCategories); CategoryController controller = new CategoryController(commandBus.Object, categoryRepository.Object) { Request = new HttpRequestMessage() { Properties = { { HttpPropertyKeys.HttpConfigurationKey, new HttpConfiguration() } } } }; // Act var response = controller.Get(5); // Assert Assert.AreEqual(HttpStatusCode.NotFound, response.StatusCode); } [Test] public void Post_Category_Returns_CreatedStatusCode() { // Arrange commandBus.Setup(c => c.Submit(It.IsAny<CreateOrUpdateCategoryCommand>())).Returns(new CommandResult(true)); Mapper.CreateMap<CategoryFormModel, CreateOrUpdateCategoryCommand>(); var httpConfiguration = new HttpConfiguration(); WebApiConfig.Register(httpConfiguration); var httpRouteData = new HttpRouteData(httpConfiguration.Routes["DefaultApi"], new HttpRouteValueDictionary { { "controller", "category" } }); var controller = new CategoryController(commandBus.Object, categoryRepository.Object) { Request = new HttpRequestMessage(HttpMethod.Post, "http://localhost/api/category/") { Properties = { { HttpPropertyKeys.HttpConfigurationKey, httpConfiguration }, { HttpPropertyKeys.HttpRouteDataKey, httpRouteData } } } }; // Act CategoryModel category = new CategoryModel(); category.CategoryId = 1; category.CategoryName = "Mock Category"; var response = controller.Post(category); // Assert Assert.AreEqual(HttpStatusCode.Created, response.StatusCode); var newCategory = JsonConvert.DeserializeObject<CategoryModel>(response.Content.ReadAsStringAsync().Result); Assert.AreEqual(string.Format("http://localhost/api/category/{0}", newCategory.CategoryId), response.Headers.Location.ToString()); } [Test] public void Post_EmptyCategory_Returns_BadRequestStatusCode() { // Arrange commandBus.Setup(c => c.Submit(It.IsAny<CreateOrUpdateCategoryCommand>())).Returns(new CommandResult(true)); Mapper.CreateMap<CategoryFormModel, CreateOrUpdateCategoryCommand>(); var httpConfiguration = new HttpConfiguration(); WebApiConfig.Register(httpConfiguration); var httpRouteData = new HttpRouteData(httpConfiguration.Routes["DefaultApi"], new HttpRouteValueDictionary { { "controller", "category" } }); var controller = new CategoryController(commandBus.Object, categoryRepository.Object) { Request = new HttpRequestMessage(HttpMethod.Post, "http://localhost/api/category/") { Properties = { { HttpPropertyKeys.HttpConfigurationKey, httpConfiguration }, { HttpPropertyKeys.HttpRouteDataKey, httpRouteData } } } }; // Act CategoryModel category = new CategoryModel(); category.CategoryId = 0; category.CategoryName = ""; // The ASP.NET pipeline doesn't run, so validation don't run. controller.ModelState.AddModelError("", "mock error message"); var response = controller.Post(category); // Assert Assert.AreEqual(HttpStatusCode.BadRequest, response.StatusCode); } [Test] public void Put_Category_Returns_OKStatusCode() { // Arrange commandBus.Setup(c => c.Submit(It.IsAny<CreateOrUpdateCategoryCommand>())).Returns(new CommandResult(true)); Mapper.CreateMap<CategoryFormModel, CreateOrUpdateCategoryCommand>(); CategoryController controller = new CategoryController(commandBus.Object, categoryRepository.Object) { Request = new HttpRequestMessage() { Properties = { { HttpPropertyKeys.HttpConfigurationKey, new HttpConfiguration() } } } }; // Act CategoryModel category = new CategoryModel(); category.CategoryId = 1; category.CategoryName = "Mock Category"; var response = controller.Put(category.CategoryId,category); // Assert Assert.AreEqual(HttpStatusCode.OK, response.StatusCode); } [Test] public void Delete_Category_Returns_NoContentStatusCode() { // Arrange commandBus.Setup(c => c.Submit(It.IsAny<DeleteCategoryCommand >())).Returns(new CommandResult(true)); CategoryController controller = new CategoryController(commandBus.Object, categoryRepository.Object) { Request = new HttpRequestMessage() { Properties = { { HttpPropertyKeys.HttpConfigurationKey, new HttpConfiguration() } } } }; // Act var response = controller.Delete(1); // Assert Assert.AreEqual(HttpStatusCode.NoContent, response.StatusCode); } private static IEnumerable<CategoryWithExpense> GetCategories() { IEnumerable<CategoryWithExpense> fakeCategories = new List<CategoryWithExpense> { new CategoryWithExpense {CategoryId=1, CategoryName = "Test1", Description="Test1Desc", TotalExpenses=1000}, new CategoryWithExpense {CategoryId=2, CategoryName = "Test2", Description="Test2Desc",TotalExpenses=2000}, new CategoryWithExpense { CategoryId=3, CategoryName = "Test3", Description="Test3Desc",TotalExpenses=3000} }.AsEnumerable(); return fakeCategories; } } The complete implementation for the Api Controller, CategoryController is given below: public class CategoryController : ApiController { private readonly ICommandBus commandBus; private readonly ICategoryRepository categoryRepository; public CategoryController(ICommandBus commandBus, ICategoryRepository categoryRepository) { this.commandBus = commandBus; this.categoryRepository = categoryRepository; } public IQueryable<CategoryWithExpense> Get() { var categories = categoryRepository.GetCategoryWithExpenses().AsQueryable(); return categories; } // GET /api/category/5 public HttpResponseMessage Get(int id) { var category = categoryRepository.GetCategoryWithExpenses().Where(c => c.CategoryId == id).SingleOrDefault(); if (category == null) { return Request.CreateResponse(HttpStatusCode.NotFound); } return Request.CreateResponse(HttpStatusCode.OK, category); } // POST /api/category public HttpResponseMessage Post(CategoryModel category) { if (ModelState.IsValid) { var command = new CreateOrUpdateCategoryCommand(category.CategoryId, category.CategoryName, category.Description); var result = commandBus.Submit(command); if (result.Success) { var response = Request.CreateResponse(HttpStatusCode.Created, category); string uri = Url.Link("DefaultApi", new { id = category.CategoryId }); response.Headers.Location = new Uri(uri); return response; } } else { return Request.CreateErrorResponse(HttpStatusCode.BadRequest, ModelState); } throw new HttpResponseException(HttpStatusCode.BadRequest); } // PUT /api/category/5 public HttpResponseMessage Put(int id, CategoryModel category) { if (ModelState.IsValid) { var command = new CreateOrUpdateCategoryCommand(category.CategoryId, category.CategoryName, category.Description); var result = commandBus.Submit(command); return Request.CreateResponse(HttpStatusCode.OK, category); } else { return Request.CreateErrorResponse(HttpStatusCode.BadRequest, ModelState); } throw new HttpResponseException(HttpStatusCode.BadRequest); } // DELETE /api/category/5 public HttpResponseMessage Delete(int id) { var command = new DeleteCategoryCommand { CategoryId = id }; var result = commandBus.Submit(command); if (result.Success) { return new HttpResponseMessage(HttpStatusCode.NoContent); } throw new HttpResponseException(HttpStatusCode.BadRequest); } } Source Code The EFMVC app can download from http://efmvc.codeplex.com/ . The unit test project can be found from the project EFMVC.Tests and Web API project can be found from EFMVC.Web.API.

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Use Extension method to write cleaner code

- by Fredrik N

This blog post will show you step by step to refactoring some code to be more readable (at least what I think). Patrik Löwnedahl gave me some of the ideas when we where talking about making code much cleaner. The following is an simple application that will have a list of movies (Normal and Transfer). The task of the application is to calculate the total sum of each movie and also display the price of each movie. class Program { enum MovieType { Normal, Transfer } static void Main(string[] args) { var movies = GetMovies(); int totalPriceOfNormalMovie = 0; int totalPriceOfTransferMovie = 0; foreach (var movie in movies) { if (movie == MovieType.Normal) { totalPriceOfNormalMovie += 2; Console.WriteLine("$2"); } else if (movie == MovieType.Transfer) { totalPriceOfTransferMovie += 3; Console.WriteLine("$3"); } } } private static IEnumerable<MovieType> GetMovies() { return new List<MovieType>() { MovieType.Normal, MovieType.Transfer, MovieType.Normal }; } } .csharpcode, .csharpcode pre { font-size: small; color: black; font-family: consolas, "Courier New", courier, monospace; background-color: #ffffff; /*white-space: pre;*/ } .csharpcode pre { margin: 0em; } .csharpcode .rem { color: #008000; } .csharpcode .kwrd { color: #0000ff; } .csharpcode .str { color: #006080; } .csharpcode .op { color: #0000c0; } .csharpcode .preproc { color: #cc6633; } .csharpcode .asp { background-color: #ffff00; } .csharpcode .html { color: #800000; } .csharpcode .attr { color: #ff0000; } .csharpcode .alt { background-color: #f4f4f4; width: 100%; margin: 0em; } .csharpcode .lnum { color: #606060; } In the code above I’m using an enum, a good way to add types (isn’t it ;)). I also use one foreach loop to calculate the price, the loop has a condition statement to check what kind of movie is added to the list of movies. I want to reuse the foreach only to increase performance and let it do two things (isn’t that smart of me?! ;)). First of all I can admit, I’m not a big fan of enum. Enum often results in ugly condition statements and can be hard to maintain (if a new type is added we need to check all the code in our app to see if we use the enum somewhere else). I don’t often care about pre-optimizations when it comes to write code (of course I have performance in mind). I rather prefer to use two foreach to let them do one things instead of two. So based on what I don’t like and Martin Fowler’s Refactoring catalog, I’m going to refactoring this code to what I will call a more elegant and cleaner code. First of all I’m going to use Split Loop to make sure the foreach will do one thing not two, it will results in two foreach (Don’t care about performance here, if the results will results in bad performance, you can refactoring later, but computers are so fast to day, so iterating through a list is not often so time consuming.) Note: The foreach actually do four things, will come to is later. var movies = GetMovies(); int totalPriceOfNormalMovie = 0; int totalPriceOfTransferMovie = 0; foreach (var movie in movies) { if (movie == MovieType.Normal) { totalPriceOfNormalMovie += 2; Console.WriteLine("$2"); } } foreach (var movie in movies) { if (movie == MovieType.Transfer) { totalPriceOfTransferMovie += 3; Console.WriteLine("$3"); } } .csharpcode, .csharpcode pre { font-size: small; color: black; font-family: consolas, "Courier New", courier, monospace; background-color: #ffffff; /*white-space: pre;*/ } .csharpcode pre { margin: 0em; } .csharpcode .rem { color: #008000; } .csharpcode .kwrd { color: #0000ff; } .csharpcode .str { color: #006080; } .csharpcode .op { color: #0000c0; } .csharpcode .preproc { color: #cc6633; } .csharpcode .asp { background-color: #ffff00; } .csharpcode .html { color: #800000; } .csharpcode .attr { color: #ff0000; } .csharpcode .alt { background-color: #f4f4f4; width: 100%; margin: 0em; } .csharpcode .lnum { color: #606060; } To remove the condition statement we can use the Where extension method added to the IEnumerable<T> and is located in the System.Linq namespace: foreach (var movie in movies.Where( m => m == MovieType.Normal)) { totalPriceOfNormalMovie += 2; Console.WriteLine("$2"); } foreach (var movie in movies.Where( m => m == MovieType.Transfer)) { totalPriceOfTransferMovie += 3; Console.WriteLine("$3"); } .csharpcode, .csharpcode pre { font-size: small; color: black; font-family: consolas, "Courier New", courier, monospace; background-color: #ffffff; /*white-space: pre;*/ } .csharpcode pre { margin: 0em; } .csharpcode .rem { color: #008000; } .csharpcode .kwrd { color: #0000ff; } .csharpcode .str { color: #006080; } .csharpcode .op { color: #0000c0; } .csharpcode .preproc { color: #cc6633; } .csharpcode .asp { background-color: #ffff00; } .csharpcode .html { color: #800000; } .csharpcode .attr { color: #ff0000; } .csharpcode .alt { background-color: #f4f4f4; width: 100%; margin: 0em; } .csharpcode .lnum { color: #606060; } The above code will still do two things, calculate the total price, and display the price of the movie. I will not take care of it at the moment, instead I will focus on the enum and try to remove them. One way to remove enum is by using the Replace Conditional with Polymorphism. So I will create two classes, one base class called Movie, and one called MovieTransfer. The Movie class will have a property called Price, the Movie will now hold the price: public class Movie { public virtual int Price { get { return 2; } } } public class MovieTransfer : Movie { public override int Price { get { return 3; } } } .csharpcode, .csharpcode pre { font-size: small; color: black; font-family: consolas, "Courier New", courier, monospace; background-color: #ffffff; /*white-space: pre;*/ } .csharpcode pre { margin: 0em; } .csharpcode .rem { color: #008000; } .csharpcode .kwrd { color: #0000ff; } .csharpcode .str { color: #006080; } .csharpcode .op { color: #0000c0; } .csharpcode .preproc { color: #cc6633; } .csharpcode .asp { background-color: #ffff00; } .csharpcode .html { color: #800000; } .csharpcode .attr { color: #ff0000; } .csharpcode .alt { background-color: #f4f4f4; width: 100%; margin: 0em; } .csharpcode .lnum { color: #606060; } The following code has no enum and will use the new Movie classes instead: class Program { static void Main(string[] args) { var movies = GetMovies(); int totalPriceOfNormalMovie = 0; int totalPriceOfTransferMovie = 0; foreach (var movie in movies.Where( m => m is Movie)) { totalPriceOfNormalMovie += movie.Price; Console.WriteLine(movie.Price); } foreach (var movie in movies.Where( m => m is MovieTransfer)) { totalPriceOfTransferMovie += movie.Price; Console.WriteLine(movie.Price); } } private static IEnumerable<Movie> GetMovies() { return new List<Movie>() { new Movie(), new MovieTransfer(), new Movie() }; } } .csharpcode, .csharpcode pre { font-size: small; color: black; font-family: consolas, "Courier New", courier, monospace; background-color: #ffffff; /*white-space: pre;*/ } .csharpcode pre { margin: 0em; } .csharpcode .rem { color: #008000; } .csharpcode .kwrd { color: #0000ff; } .csharpcode .str { color: #006080; } .csharpcode .op { color: #0000c0; } .csharpcode .preproc { color: #cc6633; } .csharpcode .asp { background-color: #ffff00; } .csharpcode .html { color: #800000; } .csharpcode .attr { color: #ff0000; } .csharpcode .alt { background-color: #f4f4f4; width: 100%; margin: 0em; } .csharpcode .lnum { color: #606060; } If you take a look at the foreach now, you can see it still actually do two things, calculate the price and display the price. We can do some more refactoring here by using the Sum extension method to calculate the total price of the movies: static void Main(string[] args) { var movies = GetMovies(); int totalPriceOfNormalMovie = movies.Where(m => m is Movie) .Sum(m => m.Price); int totalPriceOfTransferMovie = movies.Where(m => m is MovieTransfer) .Sum(m => m.Price); foreach (var movie in movies.Where( m => m is Movie)) Console.WriteLine(movie.Price); foreach (var movie in movies.Where( m => m is MovieTransfer)) Console.WriteLine(movie.Price); } .csharpcode, .csharpcode pre { font-size: small; color: black; font-family: consolas, "Courier New", courier, monospace; background-color: #ffffff; /*white-space: pre;*/ } .csharpcode pre { margin: 0em; } .csharpcode .rem { color: #008000; } .csharpcode .kwrd { color: #0000ff; } .csharpcode .str { color: #006080; } .csharpcode .op { color: #0000c0; } .csharpcode .preproc { color: #cc6633; } .csharpcode .asp { background-color: #ffff00; } .csharpcode .html { color: #800000; } .csharpcode .attr { color: #ff0000; } .csharpcode .alt { background-color: #f4f4f4; width: 100%; margin: 0em; } .csharpcode .lnum { color: #606060; } Now when the Movie object will hold the price, there is no need to use two separate foreach to display the price of the movies in the list, so we can use only one instead: foreach (var movie in movies) Console.WriteLine(movie.Price); .csharpcode, .csharpcode pre { font-size: small; color: black; font-family: consolas, "Courier New", courier, monospace; background-color: #ffffff; /*white-space: pre;*/ } .csharpcode pre { margin: 0em; } .csharpcode .rem { color: #008000; } .csharpcode .kwrd { color: #0000ff; } .csharpcode .str { color: #006080; } .csharpcode .op { color: #0000c0; } .csharpcode .preproc { color: #cc6633; } .csharpcode .asp { background-color: #ffff00; } .csharpcode .html { color: #800000; } .csharpcode .attr { color: #ff0000; } .csharpcode .alt { background-color: #f4f4f4; width: 100%; margin: 0em; } .csharpcode .lnum { color: #606060; } If we want to increase the Maintainability index we can use the Extract Method to move the Sum of the prices into two separate methods. The name of the method will explain what we are doing: static void Main(string[] args) { var movies = GetMovies(); int totalPriceOfMovie = TotalPriceOfMovie(movies); int totalPriceOfTransferMovie = TotalPriceOfMovieTransfer(movies); foreach (var movie in movies) Console.WriteLine(movie.Price); } private static int TotalPriceOfMovieTransfer(IEnumerable<Movie> movies) { return movies.Where(m => m is MovieTransfer) .Sum(m => m.Price); } private static int TotalPriceOfMovie(IEnumerable<Movie> movies) { return movies.Where(m => m is Movie) .Sum(m => m.Price); } .csharpcode, .csharpcode pre { font-size: small; color: black; font-family: consolas, "Courier New", courier, monospace; background-color: #ffffff; /*white-space: pre;*/ } .csharpcode pre { margin: 0em; } .csharpcode .rem { color: #008000; } .csharpcode .kwrd { color: #0000ff; } .csharpcode .str { color: #006080; } .csharpcode .op { color: #0000c0; } .csharpcode .preproc { color: #cc6633; } .csharpcode .asp { background-color: #ffff00; } .csharpcode .html { color: #800000; } .csharpcode .attr { color: #ff0000; } .csharpcode .alt { background-color: #f4f4f4; width: 100%; margin: 0em; } .csharpcode .lnum { color: #606060; } Now to the last thing, I love the ForEach method of the List<T>, but the IEnumerable<T> doesn’t have it, so I created my own ForEach extension, here is the code of the ForEach extension method: public static class LoopExtensions { public static void ForEach<T>(this IEnumerable<T> values, Action<T> action) { Contract.Requires(values != null); Contract.Requires(action != null); foreach (var v in values) action(v); } } .csharpcode, .csharpcode pre { font-size: small; color: black; font-family: consolas, "Courier New", courier, monospace; background-color: #ffffff; /*white-space: pre;*/ } .csharpcode pre { margin: 0em; } .csharpcode .rem { color: #008000; } .csharpcode .kwrd { color: #0000ff; } .csharpcode .str { color: #006080; } .csharpcode .op { color: #0000c0; } .csharpcode .preproc { color: #cc6633; } .csharpcode .asp { background-color: #ffff00; } .csharpcode .html { color: #800000; } .csharpcode .attr { color: #ff0000; } .csharpcode .alt { background-color: #f4f4f4; width: 100%; margin: 0em; } .csharpcode .lnum { color: #606060; } I will now replace the foreach by using this ForEach method: static void Main(string[] args) { var movies = GetMovies(); int totalPriceOfMovie = TotalPriceOfMovie(movies); int totalPriceOfTransferMovie = TotalPriceOfMovieTransfer(movies); movies.ForEach(m => Console.WriteLine(m.Price)); } .csharpcode, .csharpcode pre { font-size: small; color: black; font-family: consolas, "Courier New", courier, monospace; background-color: #ffffff; /*white-space: pre;*/ } .csharpcode pre { margin: 0em; } .csharpcode .rem { color: #008000; } .csharpcode .kwrd { color: #0000ff; } .csharpcode .str { color: #006080; } .csharpcode .op { color: #0000c0; } .csharpcode .preproc { color: #cc6633; } .csharpcode .asp { background-color: #ffff00; } .csharpcode .html { color: #800000; } .csharpcode .attr { color: #ff0000; } .csharpcode .alt { background-color: #f4f4f4; width: 100%; margin: 0em; } .csharpcode .lnum { color: #606060; } The ForEach on the movies will now display the price of the movie, but maybe we want to display the name of the movie etc, so we can use Extract Method by moving the lamdba expression into a method instead, and let the method explains what we are displaying: movies.ForEach(DisplayMovieInfo); private static void DisplayMovieInfo(Movie movie) { Console.WriteLine(movie.Price); } .csharpcode, .csharpcode pre { font-size: small; color: black; font-family: consolas, "Courier New", courier, monospace; background-color: #ffffff; /*white-space: pre;*/ } .csharpcode pre { margin: 0em; } .csharpcode .rem { color: #008000; } .csharpcode .kwrd { color: #0000ff; } .csharpcode .str { color: #006080; } .csharpcode .op { color: #0000c0; } .csharpcode .preproc { color: #cc6633; } .csharpcode .asp { background-color: #ffff00; } .csharpcode .html { color: #800000; } .csharpcode .attr { color: #ff0000; } .csharpcode .alt { background-color: #f4f4f4; width: 100%; margin: 0em; } .csharpcode .lnum { color: #606060; } Now the refactoring is done! Here is the complete code: class Program { static void Main(string[] args) { var movies = GetMovies(); int totalPriceOfMovie = TotalPriceOfMovie(movies); int totalPriceOfTransferMovie = TotalPriceOfMovieTransfer(movies); movies.ForEach(DisplayMovieInfo); } private static void DisplayMovieInfo(Movie movie) { Console.WriteLine(movie.Price); } private static int TotalPriceOfMovieTransfer(IEnumerable<Movie> movies) { return movies.Where(m => m is MovieTransfer) .Sum(m => m.Price); } private static int TotalPriceOfMovie(IEnumerable<Movie> movies) { return movies.Where(m => m is Movie) .Sum(m => m.Price); } private static IEnumerable<Movie> GetMovies() { return new List<Movie>() { new Movie(), new MovieTransfer(), new Movie() }; } } public class Movie { public virtual int Price { get { return 2; } } } public class MovieTransfer : Movie { public override int Price { get { return 3; } } } pulbic static class LoopExtensions { public static void ForEach<T>(this IEnumerable<T> values, Action<T> action) { Contract.Requires(values != null); Contract.Requires(action != null); foreach (var v in values) action(v); } } .csharpcode, .csharpcode pre { font-size: small; color: black; font-family: consolas, "Courier New", courier, monospace; background-color: #ffffff; /*white-space: pre;*/ } .csharpcode pre { margin: 0em; } .csharpcode .rem { color: #008000; } .csharpcode .kwrd { color: #0000ff; } .csharpcode .str { color: #006080; } .csharpcode .op { color: #0000c0; } .csharpcode .preproc { color: #cc6633; } .csharpcode .asp { background-color: #ffff00; } .csharpcode .html { color: #800000; } .csharpcode .attr { color: #ff0000; } .csharpcode .alt { background-color: #f4f4f4; width: 100%; margin: 0em; } .csharpcode .lnum { color: #606060; } I think the new code is much cleaner than the first one, and I love the ForEach extension on the IEnumerable<T>, I can use it for different kind of things, for example: movies.Where(m => m is Movie) .ForEach(DoSomething); .csharpcode, .csharpcode pre { font-size: small; color: black; font-family: consolas, "Courier New", courier, monospace; background-color: #ffffff; /*white-space: pre;*/ } .csharpcode pre { margin: 0em; } .csharpcode .rem { color: #008000; } .csharpcode .kwrd { color: #0000ff; } .csharpcode .str { color: #006080; } .csharpcode .op { color: #0000c0; } .csharpcode .preproc { color: #cc6633; } .csharpcode .asp { background-color: #ffff00; } .csharpcode .html { color: #800000; } .csharpcode .attr { color: #ff0000; } .csharpcode .alt { background-color: #f4f4f4; width: 100%; margin: 0em; } .csharpcode .lnum { color: #606060; } By using the Where and ForEach extension method, some if statements can be removed and will make the code much cleaner. But the beauty is in the eye of the beholder. What would you have done different, what do you think will make the first example in the blog post look much cleaner than my results, comments are welcome! If you want to know when I will publish a new blog post, you can follow me on twitter: http://www.twitter.com/fredrikn

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LINQ-to-SQL IN/Contains() for Nullable<T>

- by Craig Walker

I want to generate this SQL statement in LINQ: select * from Foo where Value in ( 1, 2, 3 ) The tricky bit seems to be that Value is a column that allows nulls. The equivalent LINQ code would seem to be: IEnumerable<Foo> foos = MyDataContext.Foos; IEnumerable<int> values = GetMyValues(); var myFoos = from foo in foos where values.Contains(foo.Value) select foo; This, of course, doesn't compile, since foo.Value is an int? and values is typed to int. I've tried this: IEnumerable<Foo> foos = MyDataContext.Foos; IEnumerable<int> values = GetMyValues(); IEnumerable<int?> nullables = values.Select( value => new Nullable<int>(value)); var myFoos = from foo in foos where nullables.Contains(foo.Value) select foo; ...and this: IEnumerable<Foo> foos = MyDataContext.Foos; IEnumerable<int> values = GetMyValues(); var myFoos = from foo in foos where values.Contains(foo.Value.Value) select foo; Both of these versions give me the results I expect, but they do not generate the SQL I want. It appears that they're generating full-table results and then doing the Contains() filtering in-memory (ie: in plain LINQ, without -to-SQL); there's no IN clause in the DataContext log. Is there a way to generate a SQL IN for Nullable types?

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Using LINQ in generic collections

- by Hugo S Ferreira

Hi, Please consider the following snippet from an implementation of the Interpreter pattern: public override object Execute(Interpreter interpreter, object ctx) { var list = ctx as IEnumerable<string>; return (list != null) ? list.FirstOrDefault() : null; } What about if I want to use the same function for integers? public override object Execute(Interpreter interpreter, object ctx) { var list = ctx as IEnumerable<string>; if (list != null) return list.FirstOrDefault(); var list = ctx as IEnumerable<int>; return (list != null) ? list.FirstOrDefault() : null; } What I wanted was something like: public override object Execute(Interpreter interpreter, object ctx) { var list = ctx as IEnumerable; return (list != null) ? list.FirstOrDefault() : null; } But Linq doesn't act on IEnumerables. Instead, to get to this solution, I would be forced to write something like: public override object Execute(Interpreter interpreter, object ctx) { var list = ctx as IEnumerable; if (list != null) foreach(var i in list) { yield return i; return; } return null; } Or use a generic method: public override T Execute<T>(Interpreter interpreter, object ctx) { var list = ctx as IEnumerable<T>; return (list != null) ? list.FirstOrDefault() : null; } Which would break the Interpreter pattern (as it was implemented in this system). Covariance would also fail (at least in C#3), though would it work, it would be the exact behavior I wanted: public override object Execute(Interpreter interpreter, object ctx) { var list = ctx as IEnumerable<object>; return (list != null) ? list.FirstOrDefault() : null; } So, my question is: what's the best way to achieve the intended behavior? Thanks :-)

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What is the difference between these two linq implementations?

- by Mahesh Velaga

I was going through Jon Skeet's Reimplemnting Linq to Objects series. In the implementation of where article, I found the following snippets, but I don't get what is the advantage that we are gettting by splitting the original method into two. Original Method: // Naive validation - broken! public static IEnumerable<TSource> Where<TSource>( this IEnumerable<TSource> source, Func<TSource, bool> predicate) { if (source == null) { throw new ArgumentNullException("source"); } if (predicate == null) { throw new ArgumentNullException("predicate"); } foreach (TSource item in source) { if (predicate(item)) { yield return item; } } } Refactored Method: public static IEnumerable<TSource> Where<TSource>( this IEnumerable<TSource> source, Func<TSource, bool> predicate) { if (source == null) { throw new ArgumentNullException("source"); } if (predicate == null) { throw new ArgumentNullException("predicate"); } return WhereImpl(source, predicate); } private static IEnumerable<TSource> WhereImpl<TSource>( this IEnumerable<TSource> source, Func<TSource, bool> predicate) { foreach (TSource item in source) { if (predicate(item)) { yield return item; } } } Jon says - Its for eager validation and then defferring for the rest of the part. But, I don't get it. Could some one please explain it in a little more detail, whats the difference between these 2 functions and why will the validations be performed in one and not in the other eagerly? Conclusion/Solution: I got confused due to my lack of understanding on which functions are determined to be iterator-generators. I assumed that, it is based on signature of a method like IEnumerable<T>. But, based on the answers, now I get it, a method is an iterator-generator if it uses yield statements.

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ArgumentNullException when accessing a FormView instance

- by David

Background: I have an ASP.NET page which has a numebr of user controls within it. There are 2 user controls which are of interest. I need to display either one of them or neither of them, depending on the record selected previously. In the user controls I need to set properties of some controls which are in a FormView. So in my user control code-behind I have a number of properties which look something like this: Private ReadOnly Property phSectionReports() As PlaceHolder Get Return fvConfirmationReport.FindControl("phSectionReports") End Get End Property The problem: I am having problems with this Property. Sometimes it is returning Nothing/Null and sometimes it is throwing a NullArgumentException with the message "Value cannot be null. Parameter name: container". The exception is coming from trying to reference the fvConfirmationReport variable. fvConfirmationReport is the ID of my FormView in the page itself. So I am really after things to look for and if any ideas what sort of conditions (e.g stage in page cycle, etc.) might lead to this? An example stack trace is included below. ASP.NET 3.5 SP1, VB.NET Thanks, StackTrace: at System.Web.UI.DataBinder.GetPropertyValue(Object container, String propName) at System.Web.UI.WebControls.GridView.CreateChildControls(IEnumerable dataSource, Boolean dataBinding) at System.Web.UI.WebControls.CompositeDataBoundControl.PerformDataBinding(IEnumerable data) at System.Web.UI.WebControls.GridView.PerformDataBinding(IEnumerable data) at System.Web.UI.WebControls.DataBoundControl.OnDataSourceViewSelectCallback(IEnumerable data) at System.Web.UI.DataSourceView.Select(DataSourceSelectArguments arguments, DataSourceViewSelectCallback callback) at System.Web.UI.WebControls.DataBoundControl.PerformSelect() at System.Web.UI.WebControls.BaseDataBoundControl.DataBind() at System.Web.UI.WebControls.GridView.DataBind() at System.Web.UI.Control.DataBindChildren() at System.Web.UI.Control.DataBind(Boolean raiseOnDataBinding) ...snip... at System.Web.UI.Control.DataBind() at System.Web.UI.Control.DataBindChildren() at System.Web.UI.Control.DataBind(Boolean raiseOnDataBinding) at System.Web.UI.WebControls.FormView.CreateChildControls(IEnumerable dataSource, Boolean dataBinding) at System.Web.UI.WebControls.CompositeDataBoundControl.PerformDataBinding(IEnumerable data) at System.Web.UI.WebControls.FormView.PerformDataBinding(IEnumerable data) at System.Web.UI.WebControls.DataBoundControl.OnDataSourceViewSelectCallback(IEnumerable data) at System.Web.UI.DataSourceView.Select(DataSourceSelectArguments arguments, DataSourceViewSelectCallback callback) at System.Web.UI.WebControls.DataBoundControl.PerformSelect() at System.Web.UI.WebControls.BaseDataBoundControl.DataBind() at System.Web.UI.WebControls.FormView.DataBind() at System.Web.UI.WebControls.BaseDataBoundControl.EnsureDataBound() at System.Web.UI.WebControls.FormView.EnsureDataBound() at System.Web.UI.WebControls.CompositeDataBoundControl.CreateChildControls() at System.Web.UI.Control.EnsureChildControls() at System.Web.UI.Control.FindControl(String id, Int32 pathOffset) at System.Web.UI.Control.FindControl(String id) at App_UserControls_xxx_ucConfirmationForm.get_phSectionReports() in ucConfirmationForm.ascx.vb:line 343 at App_UserControls_xxx_ucConfirmationForm.Page_Load(Object sender, EventArgs e) in ucConfirmationForm.ascx.vb:line 412 at System.Web.UI.Control.OnLoad(EventArgs e) at System.Web.UI.Control.LoadRecursive() ...snip... at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

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