class A {}
class B: A{}

class Client{
  public static void Main (){
                 // Static type    Dynamic type
    A x;         //    A             -
    B y;         //    B             -

    x = new A(); //    A             A   TRIVIAL
    y = new B(); //    B             B   TRIVIAL

    x = y;       //    A             B   OK - TYPICAL

    y = new A(); //    B             A   Compile time ERROR
                 //                      Cannot implicitly convert type 'A' to 'B'.
    y = x;       //    B             B   Compile time ERROR !
                 //                      Cannot implicitly convert type 'A' to 'B'.
  }
}