Question

I’m facing a bit of problem with the following casting:

    class A 
    { 
    }

    class B : A 
    {
    }

    class C<T> where T : A
    {
        protected T property { get; set; }
    }

    class D : C<B> 
    {
    }

    class MainClass
    {
        public static void Main (string[] args)
        {
            C<A> x = new D();
            // Error CS0029: Cannot implicitly convert type `SampleApp.D' to `SampleApp.C<SampleApp.A>' (CS0029) (SampleApp)
        }
    }

I don’t understand why this is failing since D is wider than C<A> since it implements C<B>, and B : A. Any workarounds?

Answer 1

If you can use C# 4.0, you can write the following code.

class A { }
class B : A {}

interface IC<out T> {}
class C<T> :IC<T> where T : A { protected T property { get; set; }  }

class D : C<B> {}

class MainClass {
    public static void Main()
    {
        IC<A> x = new D();
    }
}