C# 0 The Complete Reference

PART I

C h a p t e r   1 5 :  

D e l e g a t e s ,   E v e n t s ,   a n d   L a m b d a   E x p r e s s i o n s  

407

PART IPART I

  static X IncrA(X obj) {

    X temp = new X();

    temp.Val = obj.Val + 1;

    return temp;

  }

  // This method returns Y and has a Y parameter.

  static Y IncrB(Y obj) {

    Y temp = new Y();

    temp.Val = obj.Val + 1;

    return temp;

  }

  static void Main() {

    Y Yob = new Y();

    // In this case, the parameter to IncrA

    // is X and the parameter to ChangeIt is Y.

    // Because of contravariance, the following

    // line is OK.

    ChangeIt change = IncrA;

    X Xob = change(Yob);

    Console.WriteLine("Xob: " + Xob.Val);

    // In the next case, the return type of

    // IncrB is Y and the return type of

    // ChangeIt is X. Because of covariance,

    // the following line is OK.

    change = IncrB;

    Yob = (Y) change(Yob);

    Console.WriteLine("Yob: " + Yob.Val);

  }

}

The output from the program is shown here:

Xob: 1

Yob: 1

In the program, notice that class