how to make new C return other static type than C

[deed]

interface I
{
     void setX(int x);
     int getX();
}

class C : I
{
     int x, y;

     void setX(int x) { this.x = x; }
     int getX()       { return x;   }
     void setY(int y) { this.y = y  }
     int getY()       { return y;   }
}


void main()
{
     auto obj = new C; // Want new C to instantiate obj with 
static type I.
     obj.setX(3);      // Ok, sets x to 3.
     obj.getY();       // Error, not specified in the interface.
}

- Is it possible to enforce, from within the class, a certain 
static interface type or ancestor type when instantiating with 
new? If so, how is it done?
- Also, is it possible for a class to implement multiple 
interfaces and choose static type among them when instantiated, 
based on static class state or arguments passed to the 
constructor?