Understanding virtual, override and new keyword in C#

C# Keywords: An Overview

Simple class inheritance

In this introductory part, I will explain with a demo example why we need to use new, virtual, and override keywords. Let's consider the below class hierarchy with classes A, B, and C. A is the super/base class, B is derived from class A and C is derived from class B. Also see the example of how the inheritance concept works before understanding virtual, override, and new keywords.

Simple class inheritance: Example

Let's see in theC# compiler, the example of when we need a new keyword.

 
using System;
namespace Polymorphism
{
 class A
 {
 public void Test() { Console.WriteLine("A::Test()"); }
 }
 class B : A { }
 class C : B { }
 class Program
 {
 static void Main(string[] args)
 {
 A a = new A();
 a.Test(); 
 B b = new B();
 b.Test(); 
 C c = new C();
 c.Test();
 Console.ReadKey();
 }
 }
}
   

Try it Yourself >>

Output:

A::Test()
A::Test()
A::Test()

Suppose a method Test() is declared in the base class A and classes B or C have no methods as shown above. Here conceptually, a derived class is a specialization of the base class. Because of this, it is possible to inherit fields and methods from one class to another class easily. In short, it is mainly useful for code reusability.

Read More - C# Interview Questions And Answers

Warning: Use the new keyword if hiding was intended:

 
using System;
namespace Polymorphism
{
 class A
 {
 public void Test() { Console.WriteLine("A::Test()"); }
 }

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

 class C : B
 {
 public void Test() { Console.WriteLine("C::Test()"); }
 }

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

 a.Test(); 
 b.Test();
 c.Test(); 

 a = new B();
 a.Test(); 

 b = new C();
 b.Test(); // 

 Console.ReadKey();
 }
 }
}

  

Output:

A::Test()
B::Test()
C::Test()
A::Test()
B::Test()

Now, Suppose you have the Test() method in all the classes A, B, and C as shown above program. The output will be as shown in the above window. Also when you run the above program, it will run successfully and give the O/P without getting any error. But It gives a warning as shown below:

• Warning: 'Polymorphism.B.Test()' hides inherited member 'Polymorphism.A.Test()'. Use the new keyword if hiding was intended.

The solution to this problem is already shown in the warning. Just read it carefully. Yes, you got that, To remove that warning we need to use the NEW keyword.

New Keyword (Method Hiding)

As you have seen in the above example the compiler generates the warnings. The great solution is you have to hide the base class method from the derived class here just declare the derived class method with the "new" keyword. Hence, the above code can be re-written as :

 using System;
namespace Polymorphism
{
 class A
 {
 public void Test() { Console.WriteLine("A::Test()"); }
 }

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

 class C : B
 {
 public new void Test() { Console.WriteLine("C::Test()"); }
 }

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

 a.Test(); 
 b.Test(); 
 c.Test(); 

 a = new B();
 a.Test();

 b = new C();
 b.Test(); 

 Console.ReadKey();
 }
 }
}
 

Output:

A::Test()
B::Test()
C::Test()
A::Test()
B::Test()

Virtual and Override keywords (Method Overriding)

Virtual keyword:

Override keyword:

Virtual and Override keyword Example:

 using System;
namespace Polymorphism
{
 class A
 {
 public virtual void Test() { Console.WriteLine("A::Test()"); }
 }

 class B : A
 {
 public override void Test() { Console.WriteLine("B::Test()"); }
 }
 
 class C : B
 {
 public override void Test() { Console.WriteLine("C::Test()"); }
 }
 
 class Program
 {
 static void Main(string[] args)
 {
 
 A a = new A();
 B b = new B();
 C c = new C();
 a.Test();
 b.Test(); 
 c.Test(); 
 
 a = new B();
 a.Test();
 
 b = new C();
 b.Test(); 

 Console.ReadKey();
 }
 }
}

A::Test()
B::Test()
C::Test()
B::Test()
C::Test()

 
using System;
namespace Polymorphism
{
 class A
 {
 public void Test() { Console.WriteLine("A::Test()"); }
 }

 class B : A
 {
 public new virtual void Test() { Console.WriteLine("B::Test()"); }
 }

 class C : B
 {
 public override void Test() { Console.WriteLine("C::Test()"); }
 }

 class Program
 {
 static void Main(string[] args)
 {

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

 a.Test(); 
 b.Test(); 
 c.Test(); 

 a = new B();
 a.Test(); 

 b = new C();
 b.Test();

 Console.ReadKey();
 }
 }
}
  

A::Test()
B::Test()
C::Test()
A::Test()
C::Test()

Unlock the next level of C#

• Introduction to C#: A Beginner's Guide
• Objects and Classes in C#: Examples and Differences
• Data Types in C# with Examples: Value and Reference Data Type
• Conditional Statements in C#: if, if..else, Nested if, if-else-if
• Scope of Variables in C#: Class Level, Method Level, and Block Level Scope OOPS Concepts

Summary

I have briefly explained these keywords, which are very important for beginners in both C# and OOPS. If you have a good understanding of OOPS concepts, you can win in any object-oriented language. So study OOPS concepts first then go for C#. Also, I would like to have feedback from my blog readers. Your valuable feedback, questions, or comments about this article are always welcome. And don't forget to enroll in the C-sharp programming course. Enjoy coding...!

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