Question

I’m trying to understand why this cast doesn’t work:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace CastTest {

    class Foo {}

    // I want to use this kind of like a typedef, to avoid writing List<Foo> everywhere.
    class FooList : List<Foo> {}

    class Program {
        static void Main(string[] args) {
            FooList list = (FooList) Program.GetFooList();
        }

        // Suppose this is some library method, and i don't have control over the return type
        static List<Foo> GetFooList() {
            return new List<Foo>();
        }
    }
}

This generates a runtime error:

InvalidCastException: Unable to cast object of type ‘System.Collections.Generic.List`1[CastTest.Foo]’ to type ‘CastTest.FooList’.

Can anyone explain why this doesn’t work, and whether I can get around this somehow?

Answer 1

You cannot automatically cast from a parent class to a derived class: just because the object you’re attempting to cast is a List<Foo> doesn’t necessarily make it a FooList.

Check out:

http://social.msdn.microsoft.com/Forums/is/csharplanguage/thread/f7362ba9-48cd-49eb-9c65-d91355a3daee

You could write an operator that converts a List<Foo> to a FooList, perhaps using AddRange to populate the values, such as:

class FooList
{
 public explicit operator FooList(List<Foo> arg)
 {
   FooList result = new FooList();
   result.AddRange(arg);
   return result;
  }
}

Also, it may be better to just use a using alias:
http://msdn.microsoft.com/en-us/library/sf0df423(v=vs.80).aspx

using FooList = List<Foo>;

This way you’re not actually passing around needless derived classes.