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  • Does my GetEnumerator cause a deadlock?

    - by Scott Chamberlain
    I am starting to write my first parallel applications. This partitioner will enumerate over a IDataReader pulling chunkSize records at a time from the data-source. TLDR; version private object _Lock = new object(); public IEnumerator GetEnumerator() { var infoSource = myInforSource.GetEnumerator(); //Will this cause a deadlock if two threads lock (_Lock) //use the enumator at the same time? { while (infoSource.MoveNext()) { yield return infoSource.Current; } } } full code protected class DataSourcePartitioner<object[]> : System.Collections.Concurrent.Partitioner<object[]> { private readonly System.Data.IDataReader _Input; private readonly int _ChunkSize; public DataSourcePartitioner(System.Data.IDataReader input, int chunkSize = 10000) : base() { if (chunkSize < 1) throw new ArgumentOutOfRangeException("chunkSize"); _Input = input; _ChunkSize = chunkSize; } public override bool SupportsDynamicPartitions { get { return true; } } public override IList<IEnumerator<object[]>> GetPartitions(int partitionCount) { var dynamicPartitions = GetDynamicPartitions(); var partitions = new IEnumerator<object[]>[partitionCount]; for (int i = 0; i < partitionCount; i++) { partitions[i] = dynamicPartitions.GetEnumerator(); } return partitions; } public override IEnumerable<object[]> GetDynamicPartitions() { return new ListDynamicPartitions(_Input, _ChunkSize); } private class ListDynamicPartitions : IEnumerable<object[]> { private System.Data.IDataReader _Input; int _ChunkSize; private object _ChunkLock = new object(); public ListDynamicPartitions(System.Data.IDataReader input, int chunkSize) { _Input = input; _ChunkSize = chunkSize; } public IEnumerator<object[]> GetEnumerator() { while (true) { List<object[]> chunk = new List<object[]>(_ChunkSize); lock(_Input) { for (int i = 0; i < _ChunkSize; ++i) { if (!_Input.Read()) break; var values = new object[_Input.FieldCount]; _Input.GetValues(values); chunk.Add(values); } if (chunk.Count == 0) yield break; } var chunkEnumerator = chunk.GetEnumerator(); lock(_ChunkLock) //Will this cause a deadlock? { while (chunkEnumerator.MoveNext()) { yield return chunkEnumerator.Current; } } } } IEnumerator IEnumerable.GetEnumerator() { return ((IEnumerable<object[]>)this).GetEnumerator(); } } } I wanted IEnumerable object it passed back to be thread safe (the MSDN example was so I am assuming PLINQ and TPL could need it) will the lock on _ChunkLock near the bottom help provide thread safety or will it cause a deadlock? From the documentation I could not tell if the lock would be released on the yeld return. Also if there is built in functionality to .net that will do what I am trying to do I would much rather use that. And if you find any other problems with the code I would appreciate it.

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  • Synchronizing thread communication?

    - by Roger Alsing
    Just for the heck of it I'm trying to emulate how JRuby generators work using threads in C#. Also, I'm fully aware that C# haas built in support for yield return, I'm just toying around a bit. I guess it's some sort of poor mans coroutines by keeping multiple callstacks alive using threads. (even though none of the callstacks should execute at the same time) The idea is like this: The consumer thread requests a value The worker thread provides a value and yields back to the consumer thread Repeat untill worker thread is done So, what would be the correct way of doing the following? //example class Program { static void Main(string[] args) { ThreadedEnumerator<string> enumerator = new ThreadedEnumerator<string>(); enumerator.Init(() => { for (int i = 1; i < 100; i++) { enumerator.Yield(i.ToString()); } }); foreach (var item in enumerator) { Console.WriteLine(item); }; Console.ReadLine(); } } //naive threaded enumerator public class ThreadedEnumerator<T> : IEnumerator<T>, IEnumerable<T> { private Thread enumeratorThread; private T current; private bool hasMore = true; private bool isStarted = false; AutoResetEvent enumeratorEvent = new AutoResetEvent(false); AutoResetEvent consumerEvent = new AutoResetEvent(false); public void Yield(T item) { //wait for consumer to request a value consumerEvent.WaitOne(); //assign the value current = item; //signal that we have yielded the requested enumeratorEvent.Set(); } public void Init(Action userAction) { Action WrappedAction = () => { userAction(); consumerEvent.WaitOne(); enumeratorEvent.Set(); hasMore = false; }; ThreadStart ts = new ThreadStart(WrappedAction); enumeratorThread = new Thread(ts); enumeratorThread.IsBackground = true; isStarted = false; } public T Current { get { return current; } } public void Dispose() { enumeratorThread.Abort(); } object System.Collections.IEnumerator.Current { get { return Current; } } public bool MoveNext() { if (!isStarted) { isStarted = true; enumeratorThread.Start(); } //signal that we are ready to receive a value consumerEvent.Set(); //wait for the enumerator to yield enumeratorEvent.WaitOne(); return hasMore; } public void Reset() { throw new NotImplementedException(); } public IEnumerator<T> GetEnumerator() { return this; } System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator() { return this; } } Ideas?

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  • Does my TPL partitioner cause a deadlock?

    - by Scott Chamberlain
    I am starting to write my first parallel applications. This partitioner will enumerate over a IDataReader pulling chunkSize records at a time from the data-source. protected class DataSourcePartitioner<object[]> : System.Collections.Concurrent.Partitioner<object[]> { private readonly System.Data.IDataReader _Input; private readonly int _ChunkSize; public DataSourcePartitioner(System.Data.IDataReader input, int chunkSize = 10000) : base() { if (chunkSize < 1) throw new ArgumentOutOfRangeException("chunkSize"); _Input = input; _ChunkSize = chunkSize; } public override bool SupportsDynamicPartitions { get { return true; } } public override IList<IEnumerator<object[]>> GetPartitions(int partitionCount) { var dynamicPartitions = GetDynamicPartitions(); var partitions = new IEnumerator<object[]>[partitionCount]; for (int i = 0; i < partitionCount; i++) { partitions[i] = dynamicPartitions.GetEnumerator(); } return partitions; } public override IEnumerable<object[]> GetDynamicPartitions() { return new ListDynamicPartitions(_Input, _ChunkSize); } private class ListDynamicPartitions : IEnumerable<object[]> { private System.Data.IDataReader _Input; int _ChunkSize; private object _ChunkLock = new object(); public ListDynamicPartitions(System.Data.IDataReader input, int chunkSize) { _Input = input; _ChunkSize = chunkSize; } public IEnumerator<object[]> GetEnumerator() { while (true) { List<object[]> chunk = new List<object[]>(_ChunkSize); lock(_Input) { for (int i = 0; i < _ChunkSize; ++i) { if (!_Input.Read()) break; var values = new object[_Input.FieldCount]; _Input.GetValues(values); chunk.Add(values); } if (chunk.Count == 0) yield break; } var chunkEnumerator = chunk.GetEnumerator(); lock(_ChunkLock) //Will this cause a deadlock? { while (chunkEnumerator.MoveNext()) { yield return chunkEnumerator.Current; } } } } IEnumerator IEnumerable.GetEnumerator() { return ((IEnumerable<object[]>)this).GetEnumerator(); } } } I wanted IEnumerable object it passed back to be thread safe (the .Net example was so I am assuming PLINQ and TPL could need it) will the lock on _ChunkLock near the bottom help provide thread safety or will it cause a deadlock? From the documentation I could not tell if the lock would be released on the yeld return. Also if there is built in functionality to .net that will do what I am trying to do I would much rather use that. And if you find any other problems with the code I would appreciate it.

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  • We've completed the first iteration

    - by CliveT
    There are a lot of features in C# that are implemented by the compiler and not by the underlying platform. One such feature is a lambda expression. Since local variables cannot be accessed once the current method activation finishes, the compiler has to go out of its way to generate a new class which acts as a home for any variable whose lifetime needs to be extended past the activation of the procedure. Take the following example:     Random generator = new Random();     Func func = () = generator.Next(10); In this case, the compiler generates a new class called c_DisplayClass1 which is marked with the CompilerGenerated attribute. [CompilerGenerated] private sealed class c__DisplayClass1 {     // Fields     public Random generator;     // Methods     public int b__0()     {         return this.generator.Next(10);     } } Two quick comments on this: (i)    A display was the means that compilers for languages like Algol recorded the various lexical contours of the nested procedure activations on the stack. I imagine that this is what has led to the name. (ii)    It is a shame that the same attribute is used to mark all compiler generated classes as it makes it hard to figure out what they are being used for. Indeed, you could imagine optimisations that the runtime could perform if it knew that classes corresponded to certain high level concepts. We can see that the local variable generator has been turned into a field in the class, and the body of the lambda expression has been turned into a method of the new class. The code that builds the Func object simply constructs an instance of this class and initialises the fields to their initial values.     c__DisplayClass1 class2 = new c__DisplayClass1();     class2.generator = new Random();     Func func = new Func(class2.b__0); Reflector already contains code to spot this pattern of code and reproduce the form containing the lambda expression, so this is example is correctly decompiled. The use of compiler generated code is even more spectacular in the case of iterators. C# introduced the idea of a method that could automatically store its state between calls, so that it can pick up where it left off. The code can express the logical flow with yield return and yield break denoting places where the method should return a particular value and be prepared to resume.         {             yield return 1;             yield return 2;             yield return 3;         } Of course, there was already a .NET pattern for expressing the idea of returning a sequence of values with the computation proceeding lazily (in the sense that the work for the next value is executed on demand). This is expressed by the IEnumerable interface with its Current property for fetching the current value and the MoveNext method for forcing the computation of the next value. The sequence is terminated when this method returns false. The C# compiler links these two ideas together so that an IEnumerator returning method using the yield keyword causes the compiler to produce the implementation of an Iterator. Take the following piece of code.         IEnumerable GetItems()         {             yield return 1;             yield return 2;             yield return 3;         } The compiler implements this by defining a new class that implements a state machine. This has an integer state that records which yield point we should go to if we are resumed. It also has a field that records the Current value of the enumerator and a field for recording the thread. This latter value is used for optimising the creation of iterator instances. [CompilerGenerated] private sealed class d__0 : IEnumerable, IEnumerable, IEnumerator, IEnumerator, IDisposable {     // Fields     private int 1__state;     private int 2__current;     public Program 4__this;     private int l__initialThreadId; The body gets converted into the code to construct and initialize this new class. private IEnumerable GetItems() {     d__0 d__ = new d__0(-2);     d__.4__this = this;     return d__; } When the class is constructed we set the state, which was passed through as -2 and the current thread. public d__0(int 1__state) {     this.1__state = 1__state;     this.l__initialThreadId = Thread.CurrentThread.ManagedThreadId; } The state needs to be set to 0 to represent a valid enumerator and this is done in the GetEnumerator method which optimises for the usual case where the returned enumerator is only used once. IEnumerator IEnumerable.GetEnumerator() {     if ((Thread.CurrentThread.ManagedThreadId == this.l__initialThreadId)               && (this.1__state == -2))     {         this.1__state = 0;         return this;     } The state machine itself is implemented inside the MoveNext method. private bool MoveNext() {     switch (this.1__state)     {         case 0:             this.1__state = -1;             this.2__current = 1;             this.1__state = 1;             return true;         case 1:             this.1__state = -1;             this.2__current = 2;             this.1__state = 2;             return true;         case 2:             this.1__state = -1;             this.2__current = 3;             this.1__state = 3;             return true;         case 3:             this.1__state = -1;             break;     }     return false; } At each stage, the current value of the state is used to determine how far we got, and then we generate the next value which we return after recording the next state. Finally we return false from the MoveNext to signify the end of the sequence. Of course, that example was really simple. The original method body didn't have any local variables. Any local variables need to live between the calls to MoveNext and so they need to be transformed into fields in much the same way that we did in the case of the lambda expression. More complicated MoveNext methods are required to deal with resources that need to be disposed when the iterator finishes, and sometimes the compiler uses a temporary variable to hold the return value. Why all of this explanation? We've implemented the de-compilation of iterators in the current EAP version of Reflector (7). This contrasts with previous version where all you could do was look at the MoveNext method and try to figure out the control flow. There's a fair amount of things we have to do. We have to spot the use of a CompilerGenerated class which implements the Enumerator pattern. We need to go to the class and figure out the fields corresponding to the local variables. We then need to go to the MoveNext method and try to break it into the various possible states and spot the state transitions. We can then take these pieces and put them back together into an object model that uses yield return to show the transition points. After that Reflector can carry on optimising using its usual optimisations. The pattern matching is currently a little too sensitive to changes in the code generation, and we only do a limited analysis of the MoveNext method to determine use of the compiler generated fields. In some ways, it is a pity that iterators are compiled away and there is no metadata that reflects the original intent. Without it, we are always going to dependent on our knowledge of the compiler's implementation. For example, we have noticed that the Async CTP changes the way that iterators are code generated, so we'll have to do some more work to support that. However, with that warning in place, we seem to do a reasonable job of decompiling the iterators that are built into the framework. Hopefully, the EAP will give us a chance to find examples where we don't spot the pattern correctly or regenerate the wrong code, and we can improve things. Please give it a go, and report any problems.

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  • Manually iterating over a selection of XML elements (C#, XDocument)

    - by user316117
    What is the “best practice” way of manually iterating (i.e., one at a time with a “next” button) over a set of XElements in my XDocument? Say I select the set of elements I want thusly: var elems = from XElement el in m_xDoc.Descendants() where (el.Name.LocalName.ToString() == "q_a") select el; I can use an IEnumerator to iterate over them, i.e., IEnumerator m_iter; But when I get to the end and I want to wrap around to the beginning if I call Reset() on it, it throws a NotSupportedException. That’s because, as the Microsoft C# 2.0 Specification under chapter 22 "Iterators" says "Note that enumerator objects do not support the IEnumerator.Reset method. Invoking this method causes a System.NotSupportedException to be thrown ." So what IS the right way of doing this? And what if I also want to have bidirectional iteration, i.e., a “back” button, too? Someone on a Microsoft discussion forum said I shouldn’t be using IEnumerable directly anyway. He said there was a way to do what I want with LINQ but I didn’t understand what. Someone else suggested dumping the XElements into a List with ToList(), which I think would work, but I wasn’t sure it was “best practice”. Thanks in advance for any suggestions!

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  • ASP.NET MVC: Converting business objects to select list items

    - by DigiMortal
    Some of our business classes are used to fill dropdown boxes or select lists. And often you have some base class for all your business classes. In this posting I will show you how to use base business class to write extension method that converts collection of business objects to ASP.NET MVC select list items without writing a lot of code. BusinessBase, BaseEntity and other base classes I prefer to have some base class for all my business classes so I can easily use them regardless of their type in contexts I need. NB! Some guys say that it is good idea to have base class for all your business classes and they also suggest you to have mappings done same way in database. Other guys say that it is good to have base class but you don’t have to have one master table in database that contains identities of all your business objects. It is up to you how and what you prefer to do but whatever you do – think and analyze first, please. :) To keep things maximally simple I will use very primitive base class in this example. This class has only Id property and that’s it. public class BaseEntity {     public virtual long Id { get; set; } } Now we have Id in base class and we have one more question to solve – how to better visualize our business objects? To users ID is not enough, they want something more informative. We can define some abstract property that all classes must implement. But there is also another option we can use – overriding ToString() method in our business classes. public class Product : BaseEntity {     public virtual string SKU { get; set; }     public virtual string Name { get; set; }       public override string ToString()     {         if (string.IsNullOrEmpty(Name))             return base.ToString();           return Name;     } } Although you can add more functionality and properties to your base class we are at point where we have what we needed: identity and human readable presentation of business objects. Writing list items converter Now we can write method that creates list items for us. public static class BaseEntityExtensions {            public static IEnumerable<SelectListItem> ToSelectListItems<T>         (this IList<T> baseEntities) where T : BaseEntity     {         return ToSelectListItems((IEnumerator<BaseEntity>)                    baseEntities.GetEnumerator());     }       public static IEnumerable<SelectListItem> ToSelectListItems         (this IEnumerator<BaseEntity> baseEntities)     {         var items = new HashSet<SelectListItem>();           while (baseEntities.MoveNext())         {             var item = new SelectListItem();             var entity = baseEntities.Current;               item.Value = entity.Id.ToString();             item.Text = entity.ToString();               items.Add(item);         }           return items;     } } You can see here to overloads of same method. One works with List<T> and the other with IEnumerator<BaseEntity>. Although mostly my repositories return IList<T> when querying data there are always situations where I can use more abstract types and interfaces. Using extension methods in code In your code you can use ToSelectListItems() extension methods like shown on following code fragment. ... var model = new MyFormModel(); model.Statuses = _myRepository.ListStatuses().ToSelectListItems(); ... You can call this method on all your business classes that extend your base entity. Wanna have some fun with this code? Write overload for extension method that accepts selected item ID.

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  • What might cause this ExecutionEngineException?

    - by Qwertie
    I am trying to use Reflection.Emit to generate a wrapper class in a dynamic assembly. Automatic wrapper generation is part of a new open-source library I'm writing called "GoInterfaces". The wrapper class implements IEnumerable<string> and wraps List<string>. In C# terms, all it does is this: class List1_7931B0B4_79328AA0 : IEnumerable<string> { private readonly List<string> _obj; public List1_7931B0B4_79328AA0(List<string> obj) { this._obj = obj; } IEnumerator IEnumerable.GetEnumerator() { return this._obj.GetEnumerator(); } public sealed IEnumerator<string> GetEnumerator() { return this._obj.GetEnumerator(); } } However, when I try to call the GetEnumerator() method on my wrapper class, I get ExecutionEngineException. So I saved my dynamic assembly to a DLL and used ildasm on it. Is there anything wrong with the following code? .class public auto ansi sealed List`1_7931B0B4_79328AA0 extends [mscorlib]System.Object implements [mscorlib]System.Collections.Generic.IEnumerable`1<string>, [Loyc.Runtime]Loyc.Runtime.IGoInterfaceWrapper { .field private initonly class [mscorlib]System.Collections.Generic.List`1<string> _obj .method public hidebysig virtual final instance class [mscorlib]System.Collections.Generic.IEnumerator`1<string> GetEnumerator() cil managed { // Code size 12 (0xc) .maxstack 1 IL_0000: ldarg.0 IL_0001: ldfld class [mscorlib]System.Collections.Generic.List`1<string> List`1_7931B0B4_79328AA0::_obj IL_0006: call instance valuetype [mscorlib]System.Collections.Generic.List`1/Enumerator<!0> class [mscorlib]System.Collections.Generic.List`1<string>::GetEnumerator() IL_000b: ret } // end of method List`1_7931B0B4_79328AA0::GetEnumerator .method public hidebysig virtual final instance class [mscorlib]System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator() cil managed { .override [mscorlib]System.Collections.IEnumerable::GetEnumerator // Code size 12 (0xc) .maxstack 1 IL_0000: ldarg.0 IL_0001: ldfld class [mscorlib]System.Collections.Generic.List`1<string> List`1_7931B0B4_79328AA0::_obj IL_0006: call instance valuetype [mscorlib]System.Collections.Generic.List`1/Enumerator<!0> class [mscorlib]System.Collections.Generic.List`1<string>::GetEnumerator() IL_000b: ret } // end of method List`1_7931B0B4_79328AA0::System.Collections.IEnumerable.GetEnumerator ... I have a test suite that wraps all sorts of different things, including interfaces derived from other interfaces, and multiple interface methods with identical signatures. It's only when I try to wrap IEnumerable<T> that this problem occurs. I'd be happy to send the source code (2 *.cs files, no dependencies) if anyone would like.

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  • C# 4.0 'dynamic' and foreach statement

    - by ControlFlow
    Not long time before I've discovered, that new dynamic keyword doesn't work well with the C#'s foreach statement: using System; sealed class Foo { public struct FooEnumerator { int value; public bool MoveNext() { return true; } public int Current { get { return value++; } } } public FooEnumerator GetEnumerator() { return new FooEnumerator(); } static void Main() { foreach (int x in new Foo()) { Console.WriteLine(x); if (x >= 100) break; } foreach (int x in (dynamic)new Foo()) { // :) Console.WriteLine(x); if (x >= 100) break; } } } I've expected that iterating over the dynamic variable should work completely as if the type of collection variable is known at compile time. I've discovered that the second loop actually is looked like this when is compiled: foreach (object x in (IEnumerable) /* dynamic cast */ (object) new Foo()) { ... } and every access to the x variable results with the dynamic lookup/cast so C# ignores that I've specify the correct x's type in the foreach statement - that was a bit surprising for me... And also, C# compiler completely ignores that collection from dynamically typed variable may implements IEnumerable<T> interface! The full foreach statement behavior is described in the C# 4.0 specification 8.8.4 The foreach statement article. But... It's perfectly possible to implement the same behavior at runtime! It's possible to add an extra CSharpBinderFlags.ForEachCast flag, correct the emmited code to looks like: foreach (int x in (IEnumerable<int>) /* dynamic cast with the CSharpBinderFlags.ForEachCast flag */ (object) new Foo()) { ... } And add some extra logic to CSharpConvertBinder: Wrap IEnumerable collections and IEnumerator's to IEnumerable<T>/IEnumerator<T>. Wrap collections doesn't implementing Ienumerable<T>/IEnumerator<T> to implement this interfaces. So today foreach statement iterates over dynamic completely different from iterating over statically known collection variable and completely ignores the type information, specified by user. All that results with the different iteration behavior (IEnumarble<T>-implementing collections is being iterated as only IEnumerable-implementing) and more than 150x slowdown when iterating over dynamic. Simple fix will results a much better performance: foreach (int x in (IEnumerable<int>) dynamicVariable) { But why I should write code like this? It's very nicely to see that sometimes C# 4.0 dynamic works completely the same if the type will be known at compile-time, but it's very sadly to see that dynamic works completely different where IT CAN works the same as statically typed code. So my question is: why foreach over dynamic works different from foreach over anything else?

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  • Most efficient algorithm for merging sorted IEnumerable<T>

    - by franck
    Hello, I have several huge sorted enumerable sequences that I want to merge. Theses lists are manipulated as IEnumerable but are already sorted. Since input lists are sorted, it should be possible to merge them in one trip, without re-sorting anything. I would like to keep the defered execution behavior. I tried to write a naive algorithm which do that (see below). However, it looks pretty ugly and I'm sure it can be optimized. It may exist a more academical algorithm... IEnumerable<T> MergeOrderedLists<T, TOrder>(IEnumerable<IEnumerable<T>> orderedlists, Func<T, TOrder> orderBy) { var enumerators = orderedlists.ToDictionary(l => l.GetEnumerator(), l => default(T)); IEnumerator<T> tag = null; var firstRun = true; while (true) { var toRemove = new List<IEnumerator<T>>(); var toAdd = new List<KeyValuePair<IEnumerator<T>, T>>(); foreach (var pair in enumerators.Where(pair => firstRun || tag == pair.Key)) { if (pair.Key.MoveNext()) toAdd.Add(pair); else toRemove.Add(pair.Key); } foreach (var enumerator in toRemove) enumerators.Remove(enumerator); foreach (var pair in toAdd) enumerators[pair.Key] = pair.Key.Current; if (enumerators.Count == 0) yield break; var min = enumerators.OrderBy(t => orderBy(t.Value)).FirstOrDefault(); tag = min.Key; yield return min.Value; firstRun = false; } } The method can be used like that: // Person lists are already sorted by age MergeOrderedLists(orderedList, p => p.Age); assuming the following Person class exists somewhere: public class Person { public int Age { get; set; } } Duplicates should be conserved, we don't care about their order in the new sequence. Do you see any obvious optimization I could use?

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  • Wanted a tool for decompiling obfuscated .NET code

    - by Shrike
    Hello. I need a tool to decompile obfuscated .NET code. Yes, I know about Reflector and its plugins (FileDisassemble, FileGenerator). But they create VS project which won't compile. For an example the decompiled code contains: private sealed class d__0 : IEnumerator, IEnumerator, IDisposable { private int <1__state; private int <2__current; I need a tool which could rename automatically such name into readable (read "compilable") form. Thnx.

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  • Custom Collection Implementing IEnumerable

    - by Burnzy
    I know that technically, an Interface is used for reading and not writting or editing however, I want to add an add and addrange function to the following class, here is what I currently have which is not working public class HrefCollection : IEnumerable<Href> { private IEnumerable<Href> hrefs; public IEnumerable<Href> Add( Href href ) { yield return href; } public IEnumerable<Href> AddRange( List<Href> hrefs ) { foreach( Href href in hrefs ) { yield return href; } } public IEnumerator<Href> GetEnumerator() { return hrefs.GetEnumerator(); } System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator() { return hrefs.GetEnumerator(); } } I'm not quite sure how to associate the yield return with the private list. Thanks for your help!

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  • IEnumerable<T> ToArray usage, is it a copy or a pointer?

    - by Daniel
    I am parsing an arbitrary length byte array that is going to be passed around to a few different layers of parsing. Each parser creates a Header and a Packet payload just like any ordinary encapsulation. And my problem lies in how the encapsulation holds its packet byte array payload. Say i have a 100 byte array, and it has 3 levels of encapsulation. 3 packet objects will be created and i want to set the payload of these packets to the corresponding position in the byte array of the packet. For example lets say the payload size is 20 for all levels, then imagine it has a public byte[] Payload on each object. However the problem is that this byte[] Payload is a copy of the original 100 bytes. So i'm going to end up with 160 bytes in memory instead of 100. If it were in c++ i could just easily use a pointer however i'm writing this in c#. So i created the following class: public class PayloadSegment<T> : IEnumerable<T> { public readonly T[] Array; public readonly int Offset; public readonly int Count; public PayloadSegment(T[] array, int offset, int count) { this.Array = array; this.Offset = offset; this.Count = count; } public T this[int index] { get { if (index < 0 || index >= this.Count) throw new IndexOutOfRangeException(); else return Array[Offset + index]; } set { if (index < 0 || index >= this.Count) throw new IndexOutOfRangeException(); else Array[Offset + index] = value; } } public IEnumerator<T> GetEnumerator() { for (int i = Offset; i < Offset + Count; i++) yield return Array[i]; } System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator() { IEnumerator<T> enumerator = this.GetEnumerator(); while (enumerator.MoveNext()) { yield return enumerator.Current; } } } This way i can simply reference a position inside the original byte array but use positional indexing. However if i do something like: PayloadSegment<byte> something = new PayloadSegment<byte>(someArray, 5, 10); byte[] somethingArray = something.ToArray(); Will the somethingArray be a copy of the bytes, or a reference to the original PayloadSegment which in turn is a reference to the original byte array? Sorry it was hard to word this lol _<

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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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  • Reinventing the Paged IEnumerable, Weigert Style!

    - by adweigert
    I am pretty sure someone else has done this, I've seen variations as PagedList<T>, but this is my style of a paged IEnumerable collection. I just store a reference to the collection and generate the paged data when the enumerator is needed, so you could technically add to a list that I'm referencing and the properties and results would be adjusted accordingly. I don't mind reinventing the wheel when I can add some of my own personal flare ... // Extension method for easy use public static PagedEnumerable AsPaged(this IEnumerable collection, int currentPage = 1, int pageSize = 0) { Contract.Requires(collection != null); Contract.Assume(currentPage >= 1); Contract.Assume(pageSize >= 0); return new PagedEnumerable(collection, currentPage, pageSize); } public class PagedEnumerable : IEnumerable { public PagedEnumerable(IEnumerable collection, int currentPage = 1, int pageSize = 0) { Contract.Requires(collection != null); Contract.Assume(currentPage >= 1); Contract.Assume(pageSize >= 0); this.collection = collection; this.PageSize = pageSize; this.CurrentPage = currentPage; } IEnumerable collection; int currentPage; public int CurrentPage { get { if (this.currentPage > this.TotalPages) { return this.TotalPages; } return this.currentPage; } set { if (value < 1) { this.currentPage = 1; } else if (value > this.TotalPages) { this.currentPage = this.TotalPages; } else { this.currentPage = value; } } } int pageSize; public int PageSize { get { if (this.pageSize == 0) { return this.collection.Count(); } return this.pageSize; } set { this.pageSize = (value < 0) ? 0 : value; } } public int TotalPages { get { return (int)Math.Ceiling(this.collection.Count() / (double)this.PageSize); } } public IEnumerator GetEnumerator() { var pageSize = this.PageSize; var currentPage = this.CurrentPage; var startCount = (currentPage - 1) * pageSize; return this.collection.Skip(startCount).Take(pageSize).GetEnumerator(); } IEnumerator IEnumerable.GetEnumerator() { return this.GetEnumerator(); } }

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  • Why enumerator structs are a really bad idea

    If you've ever poked around the .NET class libraries in Reflector, you probably would have noticed that the generic collection classes all have implementations of their IEnumerator as a struct rather than a class. As you will see, this design decision has some rather unfortunate side effects......Did you know that DotNetSlackers also publishes .net articles written by top known .net Authors? We already have over 80 articles in several categories including Silverlight. Take a look: here.

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  • Unity GUI not in build, but works fine in editor

    - by Darren
    I have: GUITexture attached to an object A script that has GUIStyles created for the Textfield and Buttons that are created in OnGUI(). This script is attached to the same object in number 1 3 GUIText objects each separate from the above. A script that enables the GUITexture and the script in number 1 and 2 respectively This is how it is supposed to work: When I cross the finish line, number 4 script enables number 1 GUITexture component and number 2 script component. The script component uses one of number 3's GUIText objects to show you your best lap time, and also makes a GUI.Textfield for name entry and 2 GUI.Buttons for "Submit" and "Skip". If you hit "Submit" the script will submit the time. No matter which button you press, The remaining 2 GUIText objects from number 3 will show you the top 10 best times. For some reason, when I run it in editor, everything works 100%, but when I'm in different kinds of builds, the results vary. When I am in a webplayer, The GUITexture and the textfield and buttons appear, but the textfield and buttons are plain and have no evidence of GUIStyles. When I click one of the buttons, the score gets submitted but I do not get the fastest times showing. When I am in a standalone build, the GUITexture shows up, but nothing else does. If I remove the GUIStyle parameter of the GUI.Textfield and GUI.Button, they show up. Why am I getting these variations and how can I fix it? Code below: void Start () { Names.text = ""; Times.text = ""; YourBestTime.text = "Your Best Lap: " + bestTime + "\nEnter your name:"; //StartCoroutine(GetTimes("Test")); } void Update() { if (!ShowButtons && !GettingTimes) { StartCoroutine(GetTimes()); GettingTimes = true; } } IEnumerator GetTimes () { Debug.Log("Getting times"); YourBestTime.text = "Loading Best Lap Times"; WWW times_get = new WWW(GetTimesUrl); yield return times_get; WWW names_get = new WWW(GetNamesUrl); yield return names_get; if(times_get.error != null || names_get.error != null) { print("There was an error retrieiving the data: " + names_get.error + times_get.error); } else { Times.text = times_get.text; Names.text = names_get.text; YourBestTime.text = "Your Best Lap: " + bestTime; } } IEnumerator PostLapTime (string Name, string LapTime) { string hash= MD5.Md5Sum(Name + LapTime + secretKey); string bestTime_url = SubmitTimeUrl + "&Name=" + WWW.EscapeURL(Name) + "&LapTime=" + LapTime + "&hash=" + hash; Debug.Log (bestTime_url); // Post the URL to the site and create a download object to get the result. WWW hs_post = new WWW(bestTime_url); //label = "Submitting..."; yield return hs_post; // Wait until the download is done if (hs_post.error != null) { print("There was an error posting the lap time: " + hs_post.error); //label = "Error: " + hs_post.error; //show = false; } else { Debug.Log("Posted: " + hs_post.text); ShowButtons = false; PostingTime = false; } } void OnGUI() { if (ShowButtons) { //makes text box nameString = GUI.TextField( new Rect((Screen.width/2)-111, (Screen.height/2)-130, 222, 25), nameString, 20, TextboxStyle); if (GUI.Button( new Rect( (Screen.width/2-74.0f), (Screen.height/2)- 90, 64, 32), "Submit", ButtonStyle)) { //SUBMIT TIME if (nameString == "") { nameString = "Player"; } if (!PostingTime) { StartCoroutine(PostLapTime(nameString, bestTime)); PostingTime = true; } } else if (GUI.Button( new Rect( (Screen.width/2+10.0f), (Screen.height/2)- 90, 64, 32), "Skip", ButtonStyle)) { ShowButtons = false; } } } }

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  • Strings in .NET are Enumerable

    - by Scott Dorman
    It seems like there is always some confusion concerning strings in .NET. This is both from developers who are new to the Framework and those that have been working with it for quite some time. Strings in the .NET Framework are represented by the System.String class, which encapsulates the data manipulation, sorting, and searching methods you most commonly perform on string data. In the .NET Framework, you can use System.String (which is the actual type name or the language alias (for C#, string). They are equivalent so use whichever naming convention you prefer but be consistent. Common usage (and my preference) is to use the language alias (string) when referring to the data type and String (the actual type name) when accessing the static members of the class. Many mainstream programming languages (like C and C++) treat strings as a null terminated array of characters. The .NET Framework, however, treats strings as an immutable sequence of Unicode characters which cannot be modified after it has been created. Because strings are immutable, all operations which modify the string contents are actually creating new string instances and returning those. They never modify the original string data. There is one important word in the preceding paragraph which many people tend to miss: sequence. In .NET, strings are treated as a sequence…in fact, they are treated as an enumerable sequence. This can be verified if you look at the class declaration for System.String, as seen below: // Summary:// Represents text as a series of Unicode characters.public sealed class String : IEnumerable, IComparable, IComparable<string>, IEquatable<string> The first interface that String implements is IEnumerable, which has the following definition: // Summary:// Exposes the enumerator, which supports a simple iteration over a non-generic// collection.public interface IEnumerable{ // Summary: // Returns an enumerator that iterates through a collection. // // Returns: // An System.Collections.IEnumerator object that can be used to iterate through // the collection. IEnumerator GetEnumerator();} As a side note, System.Array also implements IEnumerable. Why is that important to know? Simply put, it means that any operation you can perform on an array can also be performed on a string. This allows you to write code such as the following: string s = "The quick brown fox";foreach (var c in s){ System.Diagnostics.Debug.WriteLine(c);}for (int i = 0; i < s.Length; i++){ System.Diagnostics.Debug.WriteLine(s[i]);} If you executed those lines of code in a running application, you would see the following output in the Visual Studio Output window: In the case of a string, these enumerable or array operations return a char (System.Char) rather than a string. That might lead you to believe that you can get around the string immutability restriction by simply treating strings as an array and assigning a new character to a specific index location inside the string, like this: string s = "The quick brown fox";s[2] = 'a';   However, if you were to write such code, the compiler will promptly tell you that you can’t do it: This preserves the notion that strings are immutable and cannot be changed once they are created. (Incidentally, there is no built in way to replace a single character like this. It can be done but it would require converting the string to a character array, changing the appropriate indexed location, and then creating a new string.)

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  • c# find nearest match to array of doubles

    - by Scott
    Given the code below, how do I compare a List of objects's values with a test value? I'm building a geolocation application. I'll be passing in longitude and latitude and would like to have the service answer back with the location closest to those values. I started down the path of converting to a string, and formatting the values down to two decimal places, but that seemed a bit too ghetto, and I'm looking for a more elegant solution. Any help would be great. Thanks, Scott public class Location : IEnumerable { public string label { get; set; } public double lat { get; set; } public double lon { get; set; } //Implement IEnumerable public IEnumerator GetEnumerator() { return (IEnumerator)this; } } [HandleError] public class HomeController : Controller { private List<Location> myList = new List<Location> { new Location { label="Atlanta Midtown", lon=33.657674, lat=-84.423130}, new Location { label="Atlanta Airport", lon=33.794151, lat=-84.387228}, new Location { label="Stamford, CT", lon=41.053758, lat=-73.530979}, ... } public static int Main(String[] args) { string inLat = "-80.987654"; double dblInLat = double.Parse(inLat); // here's where I would like to find the closest location to the inLat // once I figure out this, I'll implement the Longitude, and I'll be set }

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  • Using LinqExtender to make OData feed fails

    - by BurningIce
    A pretty simple question, has anyone here tried to make a OData feed based on a IQueryable created with LinqExtender? I have created a simple Linq-provider that supports Where, Select, OrderBy and Take and wanted to expose it as an OData Feed. I keep getting an error though, and the Exception is a NullReference with the following StackTrace at System.Data.Services.Serializers.Serializer.GetObjectKey(Object resource, IDataServiceProvider provider, String containerName) at System.Data.Services.Serializers.Serializer.GetUri(Object resource, IDataServiceProvider provider, ResourceContainer container, Uri absoluteServiceUri) at System.Data.Services.Serializers.SyndicationSerializer.WriteEntryElement(IExpandedResult expanded, Object element, Type expectedType, Uri absoluteUri, String relativeUri, SyndicationItem target) at System.Data.Services.Serializers.SyndicationSerializer.<DeferredFeedItems>d__0.MoveNext() at System.ServiceModel.Syndication.Atom10FeedFormatter.WriteItems(XmlWriter writer, IEnumerable`1 items, Uri feedBaseUri) at System.ServiceModel.Syndication.Atom10FeedFormatter.WriteFeedTo(XmlWriter writer, SyndicationFeed feed, Boolean isSourceFeed) at System.ServiceModel.Syndication.Atom10FeedFormatter.WriteFeed(XmlWriter writer) at System.ServiceModel.Syndication.Atom10FeedFormatter.WriteTo(XmlWriter writer) at System.Data.Services.Serializers.SyndicationSerializer.WriteTopLevelElements(IExpandedResult expanded, IEnumerator elements, Boolean hasMoved) at System.Data.Services.Serializers.Serializer.WriteRequest(IEnumerator queryResults, Boolean hasMoved) at System.Data.Services.ResponseBodyWriter.Write(Stream stream) I've kinda narrowed it down to a issue where LinqExtender wraps every returned object, so that my object actually inherits itself - thats at least how it looks like in the debugger. These two queries are basicly the same. The first is the legacy-api where the OrderBy and Select is regular Linq to Objects. The second query is a "real" linq-provider made with LinqExtender. var db = CalendarDataProvider.GetCalendarEntriesByDate(DateTime.Now, DateTime.Now.AddMonths(1), Guid.Empty) .OrderBy(o => o.Title) .Select(o => new ODataCalendarEntry(o)); var query = new ODataCalendarEntryQuery() .Where(o => o.Start > DateTime.Now && o.End < DateTime.Now.AddMonths(1)) .OrderBy(o => o.Title); When returning db for the OData feed everything is fine, but returning query throws a NullRefenceException. I've tried all kind of tricks and even tried to project all the data into a new object like this, but still the same error return query.Select(o => new ODataCalendarEntry { Title = o.Title, Start = o.Start, End = o.End, Name = o.Name });

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  • VSTO Word ContentControls, Y U No have Name property?

    - by System.Cats.Lol
    When you add a VSTO (not Word native) content control, you specify the name: controls.AddContentControl(wordRange, "foo", wdType); Where controls is the VSTO (extended) Document.Controls collection. You can later look up the control by name: ContentControl myContentControl = controls["foo"]; So why in the world is there no Name property for ContentControl? (or ContentControlBase, or any of the other derivatives). I'm implementing a wrapper class for the Document.Controls property that lets you add or iterate the content controls. When iterating the underlying Document.Controls, there's no way to look up the name of each control. (We need it to return an instance of our ContentControl wrapper). So currently I'm doing this in our ContentControls wrapper class: public IEnumerator<IContentControl> GetEnumerator() { System.Collections.IEnumerator en = this.wordControls.GetEnumerator(); while (en.MoveNext()) { // VSTO Document.Controls includes all managed controls, not just // VSTO ContentControls; return only those. if (en.Current is Microsoft.Office.Tools.Word.ContentControl) { // The control's name isn't stored with the control, only when it was added, // so use a placeholder name for the wrapper. yield return new ContentControl("Unknown", (Microsoft.Office.Tools.Word.ContentControl)en.Current); } } } I'd prefer to not have to resort to keeping a map of names-to-wrapper-objects in our ContentControls object. Can anyone tell me how to get the control's name (the name parameter that was passed to Controls.Add()?

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  • How do I create a graph from this datastructure?

    - by Shawn Mclean
    I took this data structure from this A* tutorial: public interface IHasNeighbours<N> { IEnumerable<N> Neighbours { get; } } public class Path<TNode> : IEnumerable<TNode> { public TNode LastStep { get; private set; } public Path<TNode> PreviousSteps { get; private set; } public double TotalCost { get; private set; } private Path(TNode lastStep, Path<TNode> previousSteps, double totalCost) { LastStep = lastStep; PreviousSteps = previousSteps; TotalCost = totalCost; } public Path(TNode start) : this(start, null, 0) { } public Path<TNode> AddStep(TNode step, double stepCost) { return new Path<TNode>(step, this, TotalCost + stepCost); } public IEnumerator<TNode> GetEnumerator() { for (Path<TNode> p = this; p != null; p = p.PreviousSteps) yield return p.LastStep; } IEnumerator IEnumerable.GetEnumerator() { return this.GetEnumerator(); } } I have no idea how to create a simple graph with. How do I add something like the following undirected graph using C#: Basically I'd like to know how to connect nodes. I have my own datastructures that I can already determine the neighbors and the distance. I'd now like to convert that into this posted datastructure so I can run it through the AStar algorithm. I was seeking something more like: Path<EdgeNode> startGraphNode = new Path<EdgeNode>(tempStartNode); startGraphNode.AddNeighbor(someOtherNode, distance);

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  • WCF data services (OData), query with inheritance limitation?

    - by Mathieu Hétu
    Project: WCF Data service using internally EF4 CTP5 Code-First approach. I configured entities with inheritance (TPH). See previous question on this topic: Previous question about multiple entities- same table The mapping works well, and unit test over EF4 confirms that queries runs smoothly. My entities looks like this: ContactBase (abstract) Customer (inherits from ContactBase), this entity has also several Navigation properties toward other entities Resource (inherits from ContactBase) I have configured a discriminator, so both Customer and Resource map to the same table. Again, everythings works fine on the Ef4 point of view (unit tests all greens!) However, when exposing this DBContext over WCF Data services, I get: - CustomerBases sets exposed (Customers and Resources sets seems hidden, is it by design?) - When I query over Odata on Customers, I get this error: Navigation Properties are not supported on derived entity types. Entity Set 'ContactBases' has a instance of type 'CodeFirstNamespace.Customer', which is an derived entity type and has navigation properties. Please remove all the navigation properties from type 'CodeFirstNamespace.Customer'. Stacktrace: at System.Data.Services.Serializers.SyndicationSerializer.WriteObjectProperties(IExpandedResult expanded, Object customObject, ResourceType resourceType, Uri absoluteUri, String relativeUri, SyndicationItem item, DictionaryContent content, EpmSourcePathSegment currentSourceRoot) at System.Data.Services.Serializers.SyndicationSerializer.WriteEntryElement(IExpandedResult expanded, Object element, ResourceType expectedType, Uri absoluteUri, String relativeUri, SyndicationItem target) at System.Data.Services.Serializers.SyndicationSerializer.<DeferredFeedItems>d__b.MoveNext() at System.ServiceModel.Syndication.Atom10FeedFormatter.WriteItems(XmlWriter writer, IEnumerable`1 items, Uri feedBaseUri) at System.ServiceModel.Syndication.Atom10FeedFormatter.WriteFeedTo(XmlWriter writer, SyndicationFeed feed, Boolean isSourceFeed) at System.ServiceModel.Syndication.Atom10FeedFormatter.WriteFeed(XmlWriter writer) at System.ServiceModel.Syndication.Atom10FeedFormatter.WriteTo(XmlWriter writer) at System.Data.Services.Serializers.SyndicationSerializer.WriteTopLevelElements(IExpandedResult expanded, IEnumerator elements, Boolean hasMoved) at System.Data.Services.Serializers.Serializer.WriteRequest(IEnumerator queryResults, Boolean hasMoved) at System.Data.Services.ResponseBodyWriter.Write(Stream stream) Seems like a limitation of WCF Data services... is it? Not much documentation can be found on the web about WCF Data services (OData) and inheritance specifications. How can I overpass this exception? I need these navigation properties on derived entities, and inheritance seems the only way to provide mapping of 2 entites on the same table with Ef4 CTP5... Any thoughts?

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  • Recursive Iterators

    - by soandos
    I am having some trouble making an iterator that can traverse the following type of data structure. I have a class called Expression, which has one data member, a List<object>. This list can have any number of children, and some of those children might be other Expression objects. I want to traverse this structure, and print out every non-list object (but I do want to print out the elements of the list of course), but before entering a list, I want to return "begin nest" and after I just exited a list, I want to return "end nest". I was able to do this if I ignored the class wherever possible, and just had List<object> objects with List<object> items if I wanted a subExpression, but I would rather do away with this, and instead have an Expressions as the sublists (it would make it easier to do operations on the object. I am aware that I could use extension methods on the List<object> but it would not be appropriate (who wants an Evaluate method on their list that takes no arguments?). The code that I used to generate the origonal iterator (that works) is: public IEnumerator GetEnumerator(){ return theIterator(expr).GetEnumerator(); } private IEnumerable theIterator(object root) { if ((root is List<object>)){ yield return " begin nest "; foreach (var item in (List<object>)root){ foreach (var item2 in theIterator(item)){ yield return item2; } } yield return " end nest "; } else yield return root; } A type swap of List<object> for expression did not work, and lead to a stackOverflow error. How should the iterator be implemented? Update: Here is the swapped code: public IEnumerator GetEnumerator() { return this.GetEnumerator(); } private IEnumerable theIterator(object root) { if ((root is Expression)) { yield return " begin nest "; foreach (var item in (Expression)root) { foreach (var item2 in theIterator(item)) yield return item2; } yield return " end nest "; } else yield return root; }

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  • find nearest match to array of doubles

    - by Scott
    Given the code below, how do I compare a List of objects's values with a test value? I'm building a geolocation application. I'll be passing in longitude and latitude and would like to have the service answer back with the location closest to those values. I started down the path of converting to a string, and formatting the values down to two decimal places, but that seemed a bit too ghetto, and I'm looking for a more elegant solution. public class Location : IEnumerable { public string label { get; set; } public double lat { get; set; } public double lon { get; set; } //Implement IEnumerable public IEnumerator GetEnumerator() { return (IEnumerator)this; } } [HandleError] public class HomeController : Controller { private List<Location> myList = new List<Location> { new Location { label="Atlanta Midtown", lon=33.657674, lat=-84.423130}, new Location { label="Atlanta Airport", lon=33.794151, lat=-84.387228}, new Location { label="Stamford, CT", lon=41.053758, lat=-73.530979}, ... } public static int Main(String[] args) { string inLat = "-80.987654"; double dblInLat = double.Parse(inLat); // here's where I would like to find the closest location to the inLat // once I figure out this, I'll implement the Longitude, and I'll be set }

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  • C# 4.0: Covariance And Contravariance In Generics

    - by Paulo Morgado
    C# 4.0 (and .NET 4.0) introduced covariance and contravariance to generic interfaces and delegates. But what is this variance thing? According to Wikipedia, in multilinear algebra and tensor analysis, covariance and contravariance describe how the quantitative description of certain geometrical or physical entities changes when passing from one coordinate system to another.(*) But what does this have to do with C# or .NET? In type theory, a the type T is greater (>) than type S if S is a subtype (derives from) T, which means that there is a quantitative description for types in a type hierarchy. So, how does covariance and contravariance apply to C# (and .NET) generic types? In C# (and .NET), variance applies to generic type parameters and not to the resulting generic type. A generic type parameter is: covariant if the ordering of the generic types follows the ordering of the generic type parameters: Generic<T> = Generic<S> for T = S. contravariant if the ordering of the generic types is reversed from the ordering of the generic type parameters: Generic<T> = Generic<S> for T = S. invariant if neither of the above apply. If this definition is applied to arrays, we can see that arrays have always been covariant because this is valid code: object[] objectArray = new string[] { "string 1", "string 2" }; objectArray[0] = "string 3"; objectArray[1] = new object(); However, when we try to run this code, the second assignment will throw an ArrayTypeMismatchException. Although the compiler was fooled into thinking this was valid code because an object is being assigned to an element of an array of object, at run time, there is always a type check to guarantee that the runtime type of the definition of the elements of the array is greater or equal to the instance being assigned to the element. In the above example, because the runtime type of the array is array of string, the first assignment of array elements is valid because string = string and the second is invalid because string = object. This leads to the conclusion that, although arrays have always been covariant, they are not safely covariant – code that compiles is not guaranteed to run without errors. In C#, the way to define that a generic type parameter as covariant is using the out generic modifier: public interface IEnumerable<out T> { IEnumerator<T> GetEnumerator(); } public interface IEnumerator<out T> { T Current { get; } bool MoveNext(); } Notice the convenient use the pre-existing out keyword. Besides the benefit of not having to remember a new hypothetic covariant keyword, out is easier to remember because it defines that the generic type parameter can only appear in output positions — read-only properties and method return values. In a similar way, the way to define a type parameter as contravariant is using the in generic modifier: public interface IComparer<in T> { int Compare(T x, T y); } Once again, the use of the pre-existing in keyword makes it easier to remember that the generic type parameter can only be used in input positions — write-only properties and method non ref and non out parameters. Because covariance and contravariance apply only to the generic type parameters, a generic type definition can have both covariant and contravariant generic type parameters in its definition: public delegate TResult Func<in T, out TResult>(T arg); A generic type parameter that is not marked covariant (out) or contravariant (in) is invariant. All the types in the .NET Framework where variance could be applied to its generic type parameters have been modified to take advantage of this new feature. In summary, the rules for variance in C# (and .NET) are: Variance in type parameters are restricted to generic interface and generic delegate types. A generic interface or generic delegate type can have both covariant and contravariant type parameters. Variance applies only to reference types; if you specify a value type for a variant type parameter, that type parameter is invariant for the resulting constructed type. Variance does not apply to delegate combination. That is, given two delegates of types Action<Derived> and Action<Base>, you cannot combine the second delegate with the first although the result would be type safe. Variance allows the second delegate to be assigned to a variable of type Action<Derived>, but delegates can combine only if their types match exactly. If you want to learn more about variance in C# (and .NET), you can always read: Covariance and Contravariance in Generics — MSDN Library Exact rules for variance validity — Eric Lippert Events get a little overhaul in C# 4, Afterward: Effective Events — Chris Burrows Note: Because variance is a feature of .NET 4.0 and not only of C# 4.0, all this also applies to Visual Basic 10.

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