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• C# delegate or Func for 'all methods'?

  - by Michel

  Hi, i've read something about Func's and delegates and that they can help you to pass a method as a parameter. Now i have a cachingservice, and it has this declaration: public static void AddToCache<T>(T model, double millisecs, string cacheId) where T : class public static T GetFromCache<T>(string cacheId) where T : class So in a place where i want to cache some data, i check if it exists in the cache (with GetFromCache) and if not, get the data from somewhere, and the add it to the cache (with AddToCache) Now i want to extend the AddToCache method with a parameter, which is the class+method to call to get the data Then the declaration would be like this public static void AddToCache<T>(T model, double millisecs, string cacheId, Func/Delegate methode) where T : class Then this method could check wether the cache has data or not, and if not, get the data itself via the method it got provided. Then in the calling code i could say: AddToCache<Person>(p, 10000, "Person", new PersonService().GetPersonById(1)); AddToCache<Advert>(a, 100000, "Advert", new AdvertService().GetAdverts(3)); What i want to achieve is that the 'if cache is empty get data and add to cache' logic is placed on only one place. I hope this makes sense :) Oh, by the way, the question is: is this possible?

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• How am I able to create A List<T> containing a generic Interface?

  - by Conrad Clark

  I have a List which must contain IInteract Objects. But IInteract is a generic interface which requires 2 type arguments. My main idea is iterate through a list of Objects and "Interact" one with another if they didn't interact yet. So i have this object List<IObject> WorldObjects = new List<IObject>(); and this one: private List<IInteract> = new List<IInteract>(); Except I can't compile the last line because IInteract requires 2 type arguments. But I don't know what the arguments are until I add them. I could add interactions between Objects of Type A and A... or Objects of Type B and C. I want to create "Interaction" classes which do something with the "acting" object and the "target" object, but I want them to be independent from the objects... so I could add an Interaction between for instance... "SuperUltraClass" and... an "integer". Am I using the wrong approach?

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• What should I name my files with generic class definitions?

  - by Tomas Lycken

  I'm writing a couple of classes that all have generic type arguments, but I need to overload the classes because I need a different number of arguments in different scenarios. Basically, I have public class MyGenericClass<T> { ... } public class MyGenericClass<T, K> { ... } public class MyGenericClass<T, K, L> { ... } // it could go on forever, but it won't... I want them all in the same namespace, but in one source file per class. What should I name the files? Is there a best practice?

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• How can I define an anonymous generic Scala function?

  - by Geo

  Let's say I have this: val myAnon:(Option[String],String):String = (a:Option[String],def:String) => { return a.getOrElse(def) } Don't mind what the function does. Is there anyway of making it generic, so I can have an Option[T]?

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• Why does C# (4.0) not allow co- and contravariance in generic class types?

  - by Lars Corneliussen

  What is the reason for that limitation. Sure, one couldn't use fields, because they are allways read-write. But that can't be the answer, can it?

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• Can I Create A Generic Method of a Type of Interface?

  - by DaveDev

  Is it possible to create a generic method with a signature like public static string MyMethod<IMyTypeOfInterface>(object dataToPassToInterface) { // an instance of IMyTypeOfInterface knows how to handle // the data that is passed in } Would I have to instantiate the Interface with (T)Activator.CreateInstance();?

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• Generic Dictionary C#

  - by pm_2

  I have a class that inherits from a generic dictionary as follows: Class myClass : System.Collections.Generic.Dictionary<int, Object> I have added a list of values to this in a particular order, but I now wish to change that order. Is there any way (without removing and re-adding) that I could effectively re-index the values; so change the object at index 1 to now be at index 10 for example? For example, this doesn't work: myClass[1].Index = 10;

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• How to use List<T>.Find() on a simple collection that does not implement Find()?

  - by Bilal

  Hi, I want to use List.Find() on a simple collection that does not implement Find(). The naive way I thought of, is to just wrap it with a list and execute .Find(), like this: ICollection myCows = GetAllCowsFromFarm(); // whatever the collection impl. is... var steak = new List<Cow>(myCows).Find(moo => moo.Name == "La Vache qui Rit"); Now, 1st of all I'd like to know, C#-wise, what is the cost of this wrapping? Is it still faster to 'for' this collection the traditional way? Second, is there a better straightforward way elegantly use that .Find()? Cheers!

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• Can't Use Generic C# Class in Using Statement

  - by Eric J.

  I'm trying to use a generic class in a using statement but the compiler can't seem to treat it as implementing IDisposable. using System; using System.Collections.Generic; using System.Linq; using System.Text; using System.Data.Objects; namespace Sandbox { public sealed class UnitOfWorkScope<T> where T : ObjectContext, IDisposable, new() { public void Dispose() { } } public class MyObjectContext : ObjectContext, IDisposable { public MyObjectContext() : base("DummyConnectionString") { } #region IDisposable Members void IDisposable.Dispose() { throw new NotImplementedException(); } #endregion } public class Consumer { public void DoSomething() { using (new UnitOfWorkScope<MyObjectContext>()) { } } } } Compiler error is: Error 1 'Sandbox.UnitOfWorkScope<Sandbox.MyObjectContext>': type used in a using statement must be implicitly convertible to 'System.IDisposable' I implemented IDisposable on UnitOfWorkScope (and to see if that was the problem, also on MyObjectContext). What am I missing?

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• Shallow copying a list with downcasting

  - by Mohit

  I have the class herichary as follows CEntity----CNode---CElement I have a class Nodes : List<Cnode> and Class Elements : List<Element> Node class contain common item common across different project Element class has item specific to a project. I have to shallow copy the element list into the node list (basically down casting the elements to nodes)

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• Uses for static generic classes?

  - by Hightechrider

  What are the key uses of a Static Generic Class in C#? When should they be used? What examples best illustrate their usage? e.g. public static class Example<T> { public static ... } Since you can't define extension methods in them they appear to be somewhat limited in their utility. Web references on the topic are scarce so clearly there aren't a lot of people using them. Here's a couple:- http://ayende.com/Blog/archive/2005/10/05/StaticGenericClass.aspx http://stackoverflow.com/questions/686630/static-generic-class-as-dictionary

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• How to bind data from a view of type List<List<MyViewModelClass>>?

  - by Robert Koritnik

  I have a strong type view of type List<List<MyViewModelClass>> The outer list will always have two lists of List<MyViewModelClass>. For each of the two outer lists I want to display a group of checkboxes. Each set can have an arbitrary number of choices. My view model class looks similar to this: public class MyViewModelClass { public Area Area { get; set; } public bool IsGeneric { get; set; } public string Code { get; set; } public bool IsChecked { get; set; } } So the final view will look something like: Please select those that apply: First set of choices: x Option 1 x Option 2 x Option 3 etc. Second set of choices: x Second Option 1 x Second Option 2 x Second Option 3 x Second Option 4 etc. Checkboxes should display MyViewModelClass.Area.Name, and their value should be related to MyViewModelClass.Area.Id. Checked state is of course related to MyViewModel.IsChecked. Question I wonder how should I use Html.CheckBox() or Html.CheckBoxFor() helper to display my checkboxes? I have to get these values back to the server on a postback of course. If it makes things simpler, I could make a separate view model type like: public class Options { public List<MyViewModelClass> General { get; set; } public List<MyViewModelClass> Others { get; set; } }

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• How to modify TList<record> value?

  - by Astronavigator

  How to modify TList < record value ? type TTest = record a,b,c:Integer end; var List:TList<TTest>; A:TTest; P:Pointer; .... .... List[10] := A; <- OK List[10].a:=1; <- Here compiler error : Left side cannot be assined to P:=@List[10]; <- Error: Variable requied

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• How to declare a generic constraint that is a generic type

  - by HackedByChinese

  I have a two generic abstract types: Entity and Association. Let's say Entity looks like this: public class Entity<TId> { //... } and Association looks like this: public class Association<TEntity, TEntity2> { //... } How do I constrain Association so they can be of any Entity? I can accomplish it by the following: public class Association<TEntity, TId, TEntity2, TId2> where TEntity : Entity<TId> where TEntity2: Entity<TId2> { //... } This gets very tedious as more types derive from Association, because I have to keep passing down TId and TId2. Is there a simpler way to do this, besides just removing the constraint?

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• Generic Any/Attach/Add function for Entity Framework

  - by Matt Thrower

  Looking through my EF classes, they're littered with code that looks like this: if (_myContext.[EntityType].Any(d => d.RowId == dc.RowId)) { _myContext.[EntityType].Attach(dc); _myContext.Entry(dc).State = EntityState.Modified; } else { _myContext.[EntityType].Add(dc); } It's the same thing over and over, and is clearly itching to be handled by a generic function. However, I'm not sure how you'd go about handling the need for it to deal with a variety of unexpected entity types. A good example to get me started would be most appreciated.

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• Trying to put a generic MyObj<T, U> into an IList where U can be different between objects

  - by Sergio Romero

  I have the following class definition: public interface IItem{} public class FirstObject<T, U> : IItem { public U SomeProperty { get; private set; } } IList<IItem> myList = new List<IItem>(); I did it like this because U can be of different types. Now I want to iterate the list and get my items back, the problem is that I do not know how to cast them back to their original type so I can read the value of SomeProperty. Thanks for your help.

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• Using C# Type as generic

  - by I Clark

  I'm trying to create a generic list from a specific Type that is retrieved from elsewhere: Type listType; // Passed in to function, could be anything var list = _service.GetAll<listType>(); However I get a build error of: The type or namespace name 'listType' could not be found (are you missing a using directive or an assembly reference?) Is this even possible or am I setting foot onto C# 4 Dynamic territory? As a background: I want to automatically load all lists with data from the repository. The code below get's passed a Form Model whose properties are iterated for any IEnum (where T inherits from DomainEntity). I want to fill the list with objects of the Type the list made of from the repository. public void LoadLists(object model) { foreach (var property in model.GetType() .GetProperties(BindingFlags.Public | BindingFlags.Instance | BindingFlags.SetProperty)) { if (IsEnumerableOfNssEntities(property.PropertyType)) { var listType = property.PropertyType.GetGenericArguments()[0]; var list = _repository.Query<listType>().ToList(); property.SetValue(model, list, null); } } }

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• Traversing through an arbitrary dictionary tree structure in C#

  - by Rudism

  I am trying to write a recursive C# function that will operate on generic dictionaries of the form IDictionary<string, T> where T is either another IDictionary<string, T> or a string. My first failed attempt looked something like this: public string HandleDict(IDictionary<string, string> dict){ // handle the leaf-node here } public string HandleDict<T>(IDictionary<string, IDictionary<string, T>> dict){ // loop through children foreach(string key in dict.Keys){ HandleDict(dict[key]); } } I also tried variants of HandleDict<T>(IDictionary<string, T>) where T : IDictionary<string, T> but that also doesn't quite work. I know that what I want to do could be achieved through a custom class as the parameter instead of dictionaries, and that my attempts are conceptually flawed (there's no end to the recursion in the generic function). I would still like to know if there is actually a way to do what I want here using just generic IDictionaries.

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• List of objects plus a tag

  - by MC

  I want to store a list of objects, lets say of type Car, but with an additional 'tag' property eg a boolean True/False which does not belong on the Car class. What is the best way to accomplish this? I need to pass the result between methods.

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• .NET generic class instance - passing a variable data type

  - by FerretallicA

  As the title suggests, I'm tyring to pass a variable data type to a template class. Something like this: frmExample = New LookupForm(Of Models.MyClass) 'Works fine Dim SelectedType As Type = InstanceOfMyClass.GetType() 'Works fine frmExample = New LookupForm(Of SelectedType) 'Ba-bow! frmExample = New LookupForm(Of InstanceOfMyClass.GetType()) 'Ba-bow! LookupForm<Models.MyClass> frmExample; Type SelectedType = InstanceOfMyClass.GetType(); frmExample = new LookupForm<SelectedType.GetType()>(); //Ba-bow frmExample = new LookupForm<(Type)SelectedType>(); //Ba-bow I'm assuming it's something to do with the template being processed at compile time but even if I'm off the mark there, it wouldn't solve my problem anyway. I can't find any relevant information on using Reflection to instance template classes either. (How) can I create an instance of a dynamically typed repository at runtime?

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• Deserialize Stream to List<T> or any other type

  - by Sam

  Attempting to deserialize a stream to List<T> (or any other type) and am failing with the error: The type arguments for method 'Foo.Deserialize<T>(System.IO.Stream)' cannot be inferred from the usage. Try specifying the type arguments explicitly. This fails: public static T Deserialize<T>(this Stream stream) { BinaryFormatter bin = new BinaryFormatter(); return (T)bin.Deserialize(stream); } But this works: public static List<MyClass.MyStruct> Deserialize(this Stream stream) { BinaryFormatter bin = new BinaryFormatter(); return (List<MyClass.MyStruct>)bin.Deserialize(stream); } or: public static object Deserialize(this Stream stream) { BinaryFormatter bin = new BinaryFormatter(); return bin.Deserialize(stream); } Is it possible to do this without casting, e.g. (List<MyStruct>)stream.Deserialize()?

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• C# Calling Methods in Generic Classes

  - by aip.cd.aish

  I am extending the ImageBox control from EmguCV. The control's Image property can be set to anything implementing the IImage interface. All of the following implement this interface: Image<Bgr, Byte> Image<Ycc, Byte> Image<Hsv, Byte> Now I want to call the Draw method on the object of the above type (what ever it may be). The problem is when I access the Image property, the return type is IImage. IImage does not implement the Draw method, but all of the above do. I believe I can cast the object of type IImage to one of the above (the right one) and I can access the Draw method. But how do I know what the right one is? If you have a better way of doing this, please suggest that as well.

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• Is there a way to achieve covariance of generic types in C# 3.5?

  - by nullDev

  This has been introduced in C# 4.0, but is there a way to achieve this in c# 3.5? For e.g., consider the following code: class Base { } class Derived1 : Base { } class Derived2 : Base { } class User<T> where T : Base { } class User1 : User<Derived1> { } Now, I would like to have a list of User<T>, in which I can store User<Derived1> as well as User<Derived2>, but the following code fails to compile in C# 3.5: List<User<Base>> users = new List<User<Base>>(); users.Add(new User1()); Any ideas?

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• 'Lexical' scoping of type parameters in C#

  - by leppie

  I have 2 scenarios. This fails: class F<X> { public X X { get; set; } } error CS0102: The type 'F' already contains a definition for 'X' This works: class F<X> { class G { public X X { get; set; } } } The only logical explanation is that in the second snippet the type parameter X is out of scope, which is not true... Why should a type parameter affect my definitions in a type? IMO, for consistency, either both should work or neither should work. Any other ideas? PS: I call it 'lexical', but it probably is not not the correct term.

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• How to Work Around Limitations in Generic Type Constraints in C#?

  - by Jose

  Okay I'm looking for some input, I'm pretty sure this is not currently supported in .NET 3.5 but here goes. I want to require a generic type passed into my class to have a constructor like this: new(IDictionary<string,object>) so the class would look like this public MyClass<T> where T : new(IDictionary<string,object>) { T CreateObject(IDictionary<string,object> values) { return new T(values); } } But the compiler doesn't support this, it doesn't really know what I'm asking. Some of you might ask, why do you want to do this? Well I'm working on a pet project of an ORM so I get values from the DB and then create the object and load the values. I thought it would be cleaner to allow the object just create itself with the values I give it. As far as I can tell I have two options: 1) Use reflection(which I'm trying to avoid) to grab the PropertyInfo[] array and then use that to load the values. 2) require T to support an interface like so: public interface ILoadValues { void LoadValues(IDictionary values); } and then do this public MyClass<T> where T:new(),ILoadValues { T CreateObject(IDictionary<string,object> values) { T obj = new T(); obj.LoadValues(values); return obj; } } The problem I have with the interface I guess is philosophical, I don't really want to expose a public method for people to load the values. Using the constructor the idea was that if I had an object like this namespace DataSource.Data { public class User { protected internal User(IDictionary<string,object> values) { //Initialize } } } As long as the MyClass<T> was in the same assembly the constructor would be available. I personally think that the Type constraint in my opinion should ask (Do I have access to this constructor? I do, great!) Anyways any input is welcome.

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