Understanding
virtual, override and new keyword in C#
Polymorphism is the concepts of OOPS which
includes method overriding and method overloading. Virtual and Override keyword
are used for method overriding and new keyword is used for method hiding. Let's
have look on these keywords in C# and try to understand each importance.
Simple Class
Inheritance
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.
Class A ------> Class B -------> Class C
Test() Test() Test()
If a method Test() is declared in the base
class A and classes B or C has no methods as shown below.
1.
using System;
2.
namespace Polymorphism
3.
{
4.
class A
5.
{
6.
public void Test() { Console.WriteLine("A::Test()"); }
7.
}
8.
9.
class B : A { }
10.
11.
class C : B { }
12.
13.
class Program
14.
{
15.
static void Main(string[] args)
16.
{
17.
A a = new A();
18.
a.Test(); // output
--> "A::Test()"
19.
20.
B b = new B();
21.
b.Test(); // output
--> "A::Test()"
22.
23.
C c = new C();
24.
c.Test(); // output
--> "A::Test()"
25.
26.
Console.ReadKey();
27.
28.
}
29.
}
30.
}
Suppose you have Test() method in all the
classes A, B, C as shown below:
1.
using System;
2.
namespace Polymorphism
3.
{
4.
class A
5.
{
6.
public void Test() { Console.WriteLine("A::Test()"); }
7.
}
8.
9.
class B : A
10.
{
11.
public void Test() { Console.WriteLine("B::Test()"); }
12.
}
13.
14.
class C : B
15.
{
16.
public void Test() { Console.WriteLine("C::Test()"); }
17.
}
18.
19.
class Program
20.
{
21.
static void Main(string[] args)
22.
{
23.
24.
A a = new A();
25.
B b = new B();
26.
C c = new C();
27.
28.
a.Test(); // output
--> "A::Test()"
29.
b.Test(); // output
--> "B::Test()"
30.
c.Test(); // output
--> "C::Test()"
31.
32.
a = new B();
33.
a.Test(); // output
--> "A::Test()"
34.
35.
b = new C();
36.
b.Test(); // output
--> "B::Test()"
37.
38.
Console.ReadKey();
39.
}
40.
}
41.
}
When you will run the above program, it will
run successfully and gives the O/P. But this program will show the two warnings
as shown below:
1.
'Polymorphism.B.Test()'
hides inherited member 'Polymorphism.A.Test()'. Use the new keyword if hiding
was intended.
2.
'Polymorphism.C.Test()'
hides inherited member 'Polymorphism.B.Test()'. Use the new keyword if hiding
was intended.
Method Hiding (new
keyword)
As you have seen in the above example the
compiler generate the warnings since C# also supports method hiding. For hiding
the base class method from derived class simply declare the derived class
method with new keyword. Hence above code can be re-written as :
1.
using System;
2.
namespace Polymorphism
3.
{
4.
class A
5.
{
6.
public void Test() { Console.WriteLine("A::Test()"); }
7.
}
8.
9.
class B : A
10.
{
11.
public new void Test() { Console.WriteLine("B::Test()"); }
12.
}
13.
14.
class C : B
15.
{
16.
public new void Test() { Console.WriteLine("C::Test()"); }
17.
}
18.
19.
class Program
20.
{
21.
static void Main(string[] args)
22.
{
23.
24.
A a = new A();
25.
B b = new B();
26.
C c = new C();
27.
28.
a.Test(); // output
--> "A::Test()"
29.
b.Test(); // output
--> "B::Test()"
30.
c.Test(); // output
--> "C::Test()"
31.
32.
a = new B();
33.
a.Test(); // output
--> "A::Test()"
34.
35.
b = new C();
36.
b.Test(); // output
--> "B::Test()"
37.
38.
Console.ReadKey();
39.
}
40.
}
41.
}
Moreover, if you are expecting the fourth and
fifth output should be "B::Foo()" and "C::Foo()" since the
objects a and b are referenced by the object of B and C respectively then you
have to re-write the above code for Method Overriding.
Method Overriding
(virtual and override keyword)
In C#, for overriding the base class method in
derived class, you have to declare base class method as virtual and derived class method as override as shown below:
1.
using System;
2.
namespace Polymorphism
3.
{
4.
class A
5.
{
6.
public virtual void Test() { Console.WriteLine("A::Test()"); }
7.
}
8.
9.
class B : A
10.
{
11.
public override void Test() { Console.WriteLine("B::Test()"); }
12.
}
13.
14.
class C : B
15.
{
16.
public override void Test() { Console.WriteLine("C::Test()"); }
17.
}
18.
19.
class Program
20.
{
21.
static void Main(string[] args)
22.
{
23.
24.
A a = new A();
25.
B b = new B();
26.
C c = new C();
27.
a.Test(); // output
--> "A::Test()"
28.
b.Test(); // output
--> "B::Test()"
29.
c.Test(); // output
--> "C::Test()"
30.
31.
a = new B();
32.
a.Test(); // output
--> "B::Test()"
33.
34.
b = new C();
35.
b.Test(); // output
--> "C::Test()"
36.
37.
Console.ReadKey();
38.
}
39.
}
40.
}
Mixing Method Overriding
and Method Hiding
You can also mix the method hiding and method
overriding by using virtual and new keyword since the method of a derived class
can be virtual and new at the same time. This is required when you want to
further override the derived class method into next level as I am overriding
Class B, Test() method in Class C as shown below:
1.
using System;
2.
namespace Polymorphism
3.
{
4.
class A
5.
{
6.
public void Test() { Console.WriteLine("A::Test()"); }
7.
}
8.
9.
class B : A
10.
{
11.
public new virtual void Test() { Console.WriteLine("B::Test()"); }
12.
}
13.
14.
class C : B
15.
{
16.
public override void Test() { Console.WriteLine("C::Test()"); }
17.
}
18.
19.
class Program
20.
{
21.
static void Main(string[] args)
22.
{
23.
24.
A a = new A();
25.
B b = new B();
26.
C c = new C();
27.
28.
a.Test(); // output -->
"A::Test()"
29.
b.Test(); // output
--> "B::Test()"
30.
c.Test(); // output
--> "C::Test()"
31.
32.
a = new B();
33.
a.Test(); // output
--> "A::Test()"
34.
35.
b = new C();
36.
b.Test(); // output
--> "C::Test()"
37.
38.
Console.ReadKey();
39.
}
40.
}
41.
}
Note
1. The virtual keyword is used to modify a
method, property, indexer, or event declared in the base class and allow it to
be overridden in the derived class.
2. The override keyword is used to extend or
modify a virtual/abstract method, property, indexer, or event of base class
into derived class.
3. The new keyword is used to hide a method,
property, indexer, or event of base class into derived class.
Virtual Keyword
Virtual keyword is used for generating a virtual path
for its derived classes on implementing method overriding. Virtual keyword
is used within a set with override keyword. It is used as:
Hide Copy Code
// Base Class
class A
{
public virtual void show()
{
Console.WriteLine("Hello: Base Class!");
Console.ReadLine();
}
}
Override Keyword
Override keyword is used in the derived class of the
base class in order to override the base class method.Override keyword
is used with virtual keyword, as:
Hide Copy Code
// Base Class
class A
{
public virtual void show()
{
Console.WriteLine("Hello: Base Class!");
Console.ReadLine();
}
}
// Derived Class
class B : A
{
public override void show()
{
Console.WriteLine("Hello: Derived Class!");
Console.ReadLine();
}
}
New Keyword
New keyword is also used in polymorphism concept,
but in the case of method overloading So what does overloading means, in simple
words we can say procedure of hiding your base class through your derived
class.
It
is implemented as:
Hide Copy Code
class A
{
public void show()
{
Console.WriteLine("Hello: Base Class!");
Console.ReadLine();
}
}
class B : A
{
public new void show()
{
Console.WriteLine("Hello: Derived Class!");
Console.ReadLine();
}
}
Demo Example
Sample Implementation
Here’s
a simple implementation in C# without using any keyword. Do you think it will
run fine or show some RUN or COMPILE time errors?
Let’s
see:
Hide Shrink 
Copy Code
Copy Code
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace Generics
{
class A
{
public void show()
{
Console.WriteLine("Hello: Base Class!");
Console.ReadLine();
}
}
class B : A
{
public void show()
{
Console.WriteLine("Hello: Derived Class!");
Console.ReadLine();
}
}
class Polymorphism
{
public static void Main()
{
A a1 = new A();
a1.show();
B b1 = new B();
b1.show();
A a2 = new B();
a2.show();
}
}
}
Output Window
It
is showing some sort of output which means there is neither run time nor
compile time error. But it will definitely show a Warning to you in your Visual
Studio. So what is it and how to remove it. Do you want to know?
Keep
reading and you will go through that.
Warning Message
Here’s
a warning message that you will get:
Solution
The
solution of this problem is already in the warning. Just read it carefully and
you will get that. Yes, you got that right, for removing that warning we need
to use NEW keyword.
In
the next sample, the demo example shows you a simple demo snippet and
implementation of new keyword.
(I
hope now you get why we are using new keyword in here.)
New Keyword | Method Overloading
Here’s
a simple snippet of method overloading mechanism. So just go through it and
guess the output:
Hide Shrink Copy Code
Copy Code
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace Generics
{
class A
{
public void show()
{
Console.WriteLine("Hello: Base Class!");
Console.ReadLine();
}
}
class B : A
{
public new void show()
{
Console.WriteLine("Hello: Derived Class!");
Console.ReadLine();
}
}
class Polymorphism
{
public static void Main()
{
A a1 = new A();
a1.show();
B b1 = new B();
b1.show();
A a2 = new B();
a2.show();
}
}
}
Output Window
Explanation
The
procedure goes something like this:
(using overhiding as a ref for overloading)
(using overhiding as a ref for overloading)
Virtual & Override Keywords | Method
Overriding
This
is a simple example of method overriding. Just go through it and guess the
output again and also try to differentiate between the previous snippet and
this snippet.
Hide Shrink Copy Code
Copy Code
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace Generics
{
class A
{
public virtual void show()
{
Console.WriteLine("Hello: Base Class!");
Console.ReadLine();
}
}
class B : A
{
public override void show()
{
Console.WriteLine("Hello: Derived
Class!");
Console.ReadLine();
}
}
class Polymorphism
{
public static void Main()
{
A a1 = new A();
a1.show();
B b1 = new B();
b1.show();
A a2 = new B();
a2.show();
}
}
}
Output Window
Explanation
The
flow goes something like this:
Overriding + Hiding | Together
In
this snippet, I show how both these methods can work together in the same snippet.
So just go through this and guess the output.
Hide Shrink Copy Code
Copy Code
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace Generics
{
class A
{
public virtual void show()
{
Console.WriteLine("Hello: Base Class!");
Console.ReadLine();
}
}
class B : A
{
public override void show()
{
Console.WriteLine("Hello: Derived Class!");
Console.ReadLine();
}
}
class C : B
{
public new void show()
{
Console.WriteLine("Am Here!");
Console.ReadLine();
}
}
class Polymorphism
{
public static void Main()
{
A a1 = new A();
a1.show();
B b1 = new B();
b1.show();
C c1 = new C();
c1.show();
A a2 = new B();
a2.show();
A a3 = new C();
a3.show();
B b3 = new C();
b3.show();
}
}
}
Output Window
Explanation
The
flow goes something like this:
(using
overhiding as a ref for overloading)
Key Points
I
am giving some key points about these keywords by taking reference of method
overloading and overriding concepts, as these keywords are used in these
mechanism.
Virtual & Override
·
Used in polymorphism
implementation
·
Includes same method
name and same params’
·
Used in method
overriding concept
·
It is also called run
time polymorphism
·
Causes late binding
New
·
It is also used in
polymorphism concept
·
Includes same method
name and different params’
·
Used in method
overloading concept
·
It is compile time
polymorphism
·
Cause early binding
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