Question

I’m trying to do this:

interface IA
{
}

class A : IA
{
}

class Foo<T> where T: IA
{
}

class Program
{
    static void Main( string[] args )
    {
        Foo<A> fooA = new Foo<A>();
        Foo<IA> fooIA = fooA as Foo<IA>;
    }
}

However, the cast from Foo<A> to Foo<IA> does not compile. I recall seeing covariance issues like this when casting between List<T>‘s, but I didn’t think it applied to simple generics like this.

What is a good work around to getting this cast to work? How do I solve this problem?

Answer 1

All generic classes are invariant. Interfaces (and delegates) on the other hand can support co- and contra-variance, but only in the cases where it’s possible safely. And they need to opt-in explicitly.

For example via IFoo<out T> or IFoo<in T>