Usage of Virtual keyword & Overriding using C#

Question

The book on "C# 3.0- The Complete Reference" (p.295)says:

The program creates a base class called Base and two derived classes, called Derived1 andDerived2. Base declares a method called Who( ), and the derived classes override it. Inside the Main( )method, objects of type Base, Derived1, and Derived2 are declared. Also, a reference of type Base, called baseRef, is declared. The program then assigns a reference to each type of object to baseRef and uses that reference to call Who( ). As the output shows, the version of Who( )executed is determined by the type of object being referred to at the time of the call, not by the class type of baseRef

Now the point is that ,is it necessary that we should use a basereference and assign to it the object of each derived class in order to take advantage of method overriding and virtual keyword as i already tested that it worked correctly too ,instead when i didn't use any basereference.

Here you go:

class Base
{
    // Create virtual method in the base class.  
    public virtual void Who()
    {
        Console.WriteLine("Who() in Base");
    }
}
class Derived1 : Base
{
    // Override Who() in a derived class. 
    public override void Who()
    {
        Console.WriteLine("Who() in Derived1");
    }
}

class Derived2 : Base
{
    // Override Who() again in another derived class. 
    public override void Who()
    {
        Console.WriteLine("Who() in Derived2");
    }
}

class Program
{
    static void Main(string[] args)
    {
        Base baseOb = new Base();
        Derived1 dOb1 = new Derived1();
        Derived2 dOb2 = new Derived2();

        Base baseRef; // a base-class reference 

        baseRef = baseOb;
        baseRef.Who();

        baseRef = dOb1;
        baseRef.Who();

        baseRef = dOb2;
        baseRef.Who();

        //but this works too the same

          baseOb.Who();
          dob1.Who();
          dob2.Who();



    }
}

}

Be brave enough to answer it rather than downvote !!!

Answer 1

The point that they are trying to make here is to clarify that if you do Base baseRef = new Derived1() that the behaviour used when you are overriding virtual methods is that of the derived class. IF you didn't know how it should work you might think that if your reference is of type Base that it will use the method on that class. This is of course wrong and you know that but it is nice to demonstrate it to people.

That behaviour does exist if you use the new keyword instead of override. Try changing your class to this and run it again:

class Derived2 : Base
{
    // Override Who() again in another derived class. 
    public new void Who()
    {
        Console.WriteLine("Who() in Derived2");
    }
}

Now you will see that your final block works exactly as before but that when BaseRef is dOb2 that it actually calls the method on the base class. This is because override replaces the virtual method call whereas new says that you have a different method in the derived class and the way to call the one on the base class is like that. http://msdn.microsoft.com/en-us/library/435f1dw2.aspx talks about this in detail.

It should be noted that you very very rarely want to use the new modifier so don't dwell on this too long.