Casting return value to a generic type

Question

Suppose we have an interface with a single generic method:

public interface IExtender
{
    T GetValue<T>(string tag);
}

and a simple implementation A of it that returns instances of two different types (B and C) depending on the "tag" parameter:

public class A : IExtender
{
    public T GetValue<T>(string tag)
    {
        if (typeof(T) == typeof(B) && tag == null)
            return (T)(object) new B();
        if (typeof(T) == typeof(C) && tag == "foo")
            return (T)(object) new C();
        return default(T);
    }
}

is it possible to avoid the double cast (T)(object)? Or, is there a way to tell the compiler "hey, I am sure that this cast won't fail at runtime, just let me do it without first casting to object!"

Answer 1

Or, is there a way to tell the compiler "hey, I am sure that this cast won't fail at runtime, just let me do it without first casting to object!"

No, the language is deliberately designed to prevent this. Eric Lippert blogged about this recently. I agree it's annoying, but it does make a certain kind of sense.

To be honest, "generic" methods like this are usually a bit of a design smell. If a method has to have special cases for various different types, you should at least consider using separate methods instead. (GetB, GetC)