Casting problem in C# generic method

c#, casting, collections, generics, methods

Solution

I don't get what you are trying to do with that code.

If you want to create a Collection of Things where you could add any type of class derived from Thing, ThingCollection should not have a Typename: it's supposed to be a collection for concrete types.

E.g, implementing A ThingCollection this way:

public class ThingCollection : List<Thing> {}

now you can do

ThingCollection tc = new ThingCollection();
tc.Add(new ThingA());
tc.Add(new ThingB());
tc.Add(new ThingC());

Assuming of course that ThingA, ThingB and ThingC inherits from Thing.

Or maybe you want to filter derived types of Things with the GetThings() i.e. you want that a call to GetThings() returns a ThingCollection.

Problem

I'm having some trouble with a generic method I'm writing. It has the following signature; ``` public static ThingCollection<T> GetThings<T>(...) where T : Thing ``` There are several classes; ThingA, ThingB and ThingC that inherit from Thing; and I want to be able to have code something like this in the method. ``` var things = new ThingCollection<T>(); if (typeof(T) == typeof(Thing)) foreach (var item in someCollection) things.Add((T)new Thing(...)); else if (typeof(T) == typeof(ThingA)) foreach (var item in someCollection) things.Add((T)new ThingA(...)); else if (typeof(T) == typeof(ThingB)) foreach (var item in someCollection) things.Add((T)new ThingB(...)); else if (typeof(T) == typeof(ThingC)) foreach (var item in someCollection) things.Add((T)new ThingC(...)); else throw new Exception("Cannot return things of type " + typeof(T).ToString()); return things; ``` The problem is that I get a best overloaded method match has invalid arguments error if I don't cast the new objects. Adding the T casts as shown above is fine for the new Thing() but reports Cannot convert type 'ThingA' to 'T' for the other new calls. Intellisense indicates that T is a Thing but I don't understand why I can't cast the other objects to Thing, as they inherit from it. Perhaps this is not the right way to be doing what I'm trying to do. Am I on the right track? Perhaps missing some small nuance, or should I be doing something else entirely?

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