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

You could use explicit interface implementation to avoid the problem.

class  X : IA, IB
{
  public X test()
  {
    return this;
  }

  IB IA.test()
  {
    return this;
  }
}

interface IA
{
  IB test();
}

interface IB
{
}