Question

I’ve some issues with the following piece of code. I would like to explicit a string to an object, this is working perfectly fine, however, if this object is part of a generic class, this is failing with the following error exception: “Unable to cast object of type ‘System.String’ to type ‘test.B'”. Even though I’ve overloaded the the method.

using System;
using System.Collections.Generic;

namespace test {
    class Program {
        static void Main(string [] args) {
            // These two cast perfectly fine.
            B x = (B) "abc";
            C y = (C) "def";

            A <B> a = new A<B>();
            a.b();
            A <C> b = new A<C>();
            b.b();
        }
    }

    class A<T> {
        public List <T> a = new List<T>();

        public void b() {
            // Unable to cast object of type 'System.String' to type 'test.B'
            this.a.Add ((T) (object) "abc"); 
            this.a.Add ((T) (object) "def");
            this.a.Add ((T) (object) "ghi");
        }
    }

    class B {
        public string b;

        public static explicit operator B(string a) {
            B x = new B();
            x.b = a;
            return x;
        }
    }

    class C {
        public string c;

        public static explicit operator C(string a) {
            C x = new C();
            x.c = a;
            return x;
        }
    }
}

It would be awesome if somone could explain to me why this isn’t casting properly.

Thanks

Answer 1

Conversion operators only apply when the type is known statically; after all, generic methods need to use the exact same IL for every T – so it can’t call your operator in some cases, and a type-check in others.

Additionally, because you’ve explicitly cast to object, it will never be used; a cast from object is always a simple unbox or type check.

An evil fix would be (and I don’t like this):

        this.a.Add((T)(dynamic)"abc");
        this.a.Add((T)(dynamic)"def");
        this.a.Add((T)(dynamic)"ghi");

which defers the resolution to runtime. It works, but I would need to wash my eyes after that. More generally, though: operators and generics do not play nicely – so: try not to use that combination in your API. I really wouldn’t use the above, personally!