Question

  class A {
      public virtual void Foo()
           { // do something
            }
    }

  class B:A {
      public void Foo() {
       }
    }

What’s the meaning of this? Can anyone give me an example? Up till now i thought that it’s mandatory to use the “override” keyword if you use virtual
Thanks

Answer 1

If you use such declaration you will just “hide” the Foo method of the A class using instance of the B class. When you cast this instance to the A and call Foo the method of A class will be called.

If you will use override in the class B you will use Foo method of the B class if you create instance of the B and then cast to A.

class A
{
    public virtual void Foo()
    {
        Console.WriteLine("A Foo");
    }
}

class B : A
{
    public void Foo()
    {
        Console.WriteLine("B Foo");
    }
}

B b = new B();
b.Foo(); // call 'B Foo'
A a = (A)b;
a.Foo(); // call 'A Foo'

With override:

B b = new B();
b.Foo(); // call 'B Foo'
A a = (A)b;
a.Foo(); // call 'B Foo'

For details you can Google a little. For example this post: Polymorphism, Method Hiding and Overriding in C#