CODEWITHSUNDEEP
c#casting
rgx71
rgx71

Reputation: 857

Casting classes in C#

I have a question, why in this code, One() method is execute from class B and Two() method is executed from class A? I know that is doing casting, but I don't understand the way is working. By the way, any good link or book with this kind of tricks will be much appreciated. Than you.

class Program
{
    static void Main(string[] args)
    {
        B b = new B();
        A a = (A)b;

        a.One(); 
        a.Two(); 
    }
}

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

public class B : A
{
    public override void One()
    {
        Console.WriteLine("B One");
    }
    public new void Two()
    {
        Console.WriteLine("B Two");
    }
}

Upvotes: 3

Views: 163

Answers (2)

Sando
Sando

Reputation: 667

The Two() method is not virtual in class A (so it does not allow it to be overridden, almost for the same reason you had to make B.Two() as new - otherwise the compiler would have complained).

So when you dont have a virtual method in the base class it is not meant to be overridden. Thus when you cast object of type B to type A the call to Two() method is bound to the method table of type A and not type B.

You can read more about it here:

http://msdn.microsoft.com/en-us/library/6fawty39(v=vs.80).aspx http://www.akadia.com/services/dotnet_polymorphism.html

Upvotes: 0

Jim D'Angelo
Jim D'Angelo

Reputation: 3952

It is because Two() is not a virtual method. The only time Two() will be called from class B is if you are specifically looking at an instance of B. Class A doesn't have a lookup table for a virtual method when calling Two() so nobody knows to look elsewhere for a different method.

You can see more details in my answer to this question.

Upvotes: 5

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