C# Virtual Override

The following example was changed from the example of name hiding 01-55-02

Example 01-56-01

using System;

namespace TestVirtualOverride
{
    public class A
    {
        public int i = 1;
        public int j = 2;

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

    public class B : A
    {
        new public int j = 3;     // Add new here

        public override void test()    // Add new here
        {
            Console.WriteLine("B class");
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            A a = new A();
            Console.WriteLine("i={0}, j={1}", a.i, a.j);    // Output i=1, j=2
            a.test();                                       // Output A class
            B b = new B();
            Console.WriteLine("i={0}, j={1}", b.i, b.j);    // Output i=1, j=3
            b.test();                                       // Output B class
            Console.Read();
        }
    }
}

Output

i=1, j=2
A class
i=1, j=3
B class

Explanation

• Line 10: Define a virtual method test() in class A.
• Line 20: Define an override method test() in class B to override the method in line 10.

You can see we got the same results of Example 01-55-02 and Example 01-56-01. So the next, we'll give you an example to distinguish the difference between the method overriding and method hiding.

Any objects of a derived class can be assigned to the base class type instead of the derived class type. This is called Polymorphism which will be introduced later.

For example, Class B is a derived class of class A. We can create an instance of the class B and assign it to Class A type.

A a = new B();

But the following is not allowed.

B b = new A();

It's something like you can convert a int-typed data to a long-typed data implicitly but you cannot do it reversely.

Assume class C is a derived class of class B, the following declarations are all legal.

C c = new C();
B b = new C();
A a = new C();

Now we can describe the difference between Method Overriding and Method Hiding in the following example.

Example 01-56-02

using System;

namespace TestVirtualOverride
{
    public class A
    {
        public virtual void test1()
        {
            Console.WriteLine("Test 1 in A class");
        }

        public void test2()
        {
            Console.WriteLine("Test 2 in A class");
        }
    }

    public class B : A
    {
        public override void test1()
        {
            Console.WriteLine("Test 1 in B class");
        }

        new public void test2()
        {
            Console.WriteLine("Test 2 in B class");
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            A a = new B();
            a.test1();    // Test 1 in B class
            a.test2();    // Test 2 in A class
            B b = (B) a;
            b.test1();    // Test 1 in B class
            b.test2();    // Test 2 in B class
            Console.Read();
        }
    }
}

Output

Test 1 in B class
Test 2 in A class
Test 1 in B class
Test 2 in B class

Explanation

• Line 5-16: Define the class A with one virtual method test1() and a normal method test2().
• Line 18-29: Define the derived class B inheriting from class A.
• Line 20-23: Define the overriding method test1() with the keyword override in class B.
• Line 25-28: Define a new method test2() to hide the method with the same name in class A.
• Line 31-43: Testing class Program.
• Line 33-42: Main method.
• Line 35: Create an instance of B and assign it to class A.
• Line 36: Call the method test1 of class A. However, this is a virtual method and the overriding method in the most derived class will be invoked. The a hosts the instance of class B at run time so the method in class B will be invoked instead of that in class A.
• Line 37: Call the method test2 of class A. This method is not a virtual method so the method test2() in class A will be invoked and output "Test 2 in A class".
• Line 38: A cast is needed to explicitly convert an instance from a base class type to a derived class type. There is no run time error in the code because a is actually holding the instance of class B.
• Line 39: The method test1() in class B is invoked.
• Line 40: The method test2() in class B is invoked because it hides the method in class A and runs as a new method.