Virtual, Override and new Keyword in C#

Consider following class Hierarchy:

In this example, we will consider three classes which are TestA, TestB and TestC.

namespace OOPS_Concept
{
    class TestA
    {
        public void display() { Console.WriteLine("TestA - display()"); }
    }

    class TestB : TestA
    {
        public void display() { Console.WriteLine("TestB - display()"); }
    }

    class Test
    {
        static void Main(string[] args)
        {
            TestA a;
            TestB b;

            a = new TestA();
            b = new TestB();
            a.display();  // TestA - display()
            b.display();  // TestB - display()

            a = new TestB();
            a.display();  // TestA - display()

            Console.Read();
        }
    }
}

output :

TestA – display()
TestB – display()
TestA – display()

Program will compile and run successfully but

• The Problem of above code is that, third output should be “TestB –display()” because the variable a have the object of B.
• …Test.cs(15,21): warning CS0108: ‘OOPS_Concept.TestB.display()’ hides inherited member ‘OOPS_Concept.TestA.display()’. Use the new keyword if hiding was intended.

Lets resolve the problem 1.

Keyword Virtual and Override
In C#, if you like to override the parent class method then you must mark the parent method by keyword “Virtual“ and method in derived class which intended to override the parent method should be marked by keyword “override“

Note:

1. If parent method is marked by keyword “Virtual” but child is not marked by keyword “override”, then program will compile but the parent method will not be overrided.
2. If Child method is marked by keyword “override” but parent method is not marked by keyword “virtual” then program will not compile. It will give following error :
   ‘OOPS_Concept.TestB.display()’: cannot override inherited member ‘OOPS_Concept.TestA.display()’ because it is not marked virtual, abstract, or override.

Example with keyword Virtual and override:

namespace OOPS_Concept
{
    class TestA
    {
        public virtual void display() { Console.WriteLine("TestA - display()"); }
    }

    class TestB : TestA
    {
        public override void display() { Console.WriteLine("TestB - display()"); }
    }

    class Test
    {
        static void Main(string[] args)
        {
            TestA a;
            TestB b;

            a = new TestA();
            b = new TestB();
            a.display();  // TestA - display()
            b.display();  // TestB - display()

            a = new TestB();
            a.display();  // TestA - display()

            Console.Read();
        }
    }
}

output:

TestA – display()
TestB – display()
TestB – display()

Method Hiding – Keyword new:
As discussed earlier, second point in output of first program was “Compiler generated warning message”.
Because c# also support Method Hiding. To mark method as “hiding” use keyword “new” in derived class .
Keyword “new” can be used with keyword “virtual” also.
Quick Note :
If keyword “override” is used in derive class then its override the parent method.
If Keyword “new” is used in derive class then derive method hided by parent method. As shown in below program:

namespace OOPS_Concept
{
    class TestA
    {
        public void display() { Console.WriteLine("TestA - display()"); }
    }

    class TestB : TestA
    {
        public virtual new void display() { Console.WriteLine("TestB - display()"); }
        //Instead of virtual new, new can also be written.
    }

    class TestC : TestB
    {
        public new void display() { Console.WriteLine("TestC - display()"); }
    }

    class Test
    {
        static void Main(string[] args)
        {
            TestB b;
            TestC c;

            b = new TestB();
            b.display();  // TestB - display()

            b = new TestC();
            b.display();  // TestB - display() instead of TestC - display()

            Console.Read();
        }
    }
}

Output:

TestB – display()
TestB – display()