Question

In Visual Basic, I want to create an interface that includes a function that returns an object of the implementing class. That is

public interface I
  function maker as ???
end interface

public class X
  implements I
  function maker as X
    return new X()
  end function
end class

public class Y
  implements I
  function maker as Y
    return new Y()
  end function
end class

Is there a way to say that? I suppose I could always say that maker returns an I rather than an X or a Y, but then callers would have to cast it. I thought of defining the interface as

public interface I(of ii as I)

But the whole point in doing this was so I could create a generic class that uses an of I, and if I say that, then the compiler insists on an infinite regression of I(of I(of I...

Answer 1

Not exactly, but you can do this:

Public Interface I(Of T)
    Function Maker() As T
End Interface

Public Class X
    Implements I(Of X)

    Public Function Maker() As X Implements I.Maker
        Return New X()
    End Function
End Class

Or, like this:

Public Interface I
    Function Maker() As I
End Interface

Public Class X
    Implements I

    Public Function Maker() As I
        Return New X()
    End Function
End Class

Neither of these options force the derived class to return an instance of their own type, but they allow you to implement it that way.