fragile-subclass-problem/2/fragile.cs - A revised program with a new version of class A with a method M2.

Lecture 8 - slide 32 : 37 Program 3

// We have added M2 to class A.
// In addtion, M1 now calls M2.
// Does not compile.

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 void M2(){
    Console.WriteLine("M2 in new version of A");
  }

}

class B: A {
  
  // Compile-time error in C#:
  // 'B.M2()' hides inherited member 'A.M2()'.
  //     Use the new keyword if hiding was intended.
  public 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
             // if M2 is regarded as virtual.

    a.M2();  // Makes sense when M2 exists in class A.
             // Dangerous

    b.M2();  // Expects dangerous operation
  }
}