fragile-subclass-problem/3/fragile.cs - The revised version with method A.M2 being virtual.

Lecture 8 - slide 32 : 37 Program 4

// Dangerous program.
// M2 is virtual in A and overridden in B.
// Compiles and runs
// Default Java semantics.

using System;

// New version of A
class A {

  public void M1(){
    Console.WriteLine("Method 1");
    this.M2();
  }

  // New method in this version.
  // Same name as the dangerous operation in subclass B
  public  virtual  void M2(){
    Console.WriteLine("M2 in new version of A");
  }

}

class B: A {

  public override void M2(){
    Console.WriteLine("Dangerous Method 2");
  }
}

class Client{
  
  public static void Main(){
    A a = new B();
    B b = new B();

    a.M1();  // Nothing dangerous expected
             // Will, however, call the dangerous operation
             // because M2 is virtual.

    a.M2();  // Makes sense when M2 exists in class A.
             // Calls the dangerous method

    b.M2();  // Calls the dangerous method.
             // This is expected, however.
  }
}