Why C# doesn't allow inheritance of return type when implementing an Interface

Question

Is there any rational reason why the code below is not legal in C#?

class X: IA, IB
{
    public X test() // Compliation Error, saying that X is not IB
    {         


        

Answer 1

The signatures must match exactly to what the interface specifies. There's no reason you cannot return an instance of X from the method, but the method signature will have to use IB.

As for a rational reason.. it's probably preferable from a code readability point of view.

You could implement the interface explicitly, and provide an alternative signature that returns X that is not defined by the interface. If you know your IA is actually an X, you could use that instead.