Question

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?

Answer 1

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.