using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace Tom
{
class TOM
{
public void Display()
{
System.Console.WriteLine("TOM::Display");
}
}
class JOY : TOM
{
public new void Display()
{
System.Console.WriteLine("JOY::Display");
Console.ReadLine();
}
}
class Show
{
public static void Main()
{
JOY obj = new JOY();
obj.Display();
}
}
}
If a method Test() is declared in the base class A and classes B or C has no methods as shown below.
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(); // output --> "A::Test()"
B b = new B();
b.Test(); // output --> "A::Test()"
C c = new C();
c.Test(); // output --> "A::Test()"
Console.ReadKey();
}
}
}
Suppose you have Test() method in all the classes A, B, C as shown below:
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(); // output --> "A::Test()"
b.Test(); // output --> "B::Test()"
c.Test(); // output --> "C::Test()"
a = new B();
a.Test(); // output --> "A::Test()"
b = new C();
b.Test(); // output --> "B::Test()"
Console.ReadKey();
}
}
}
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:
'Polymorphism.B.Test()' hides inherited member 'Polymorphism.A.Test()'. Use the new keyword if hiding was intended.
'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 :
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(); // output --> "A::Test()"
b.Test(); // output --> "B::Test()"
c.Test(); // output --> "C::Test()"
a = new B();
a.Test(); // output --> "A::Test()"
b = new C();
b.Test(); // output --> "B::Test()"
Console.ReadKey();
}
}
}
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:
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(); // output --> "A::Test()"
b.Test(); // output --> "B::Test()"
c.Test(); // output --> "C::Test()"
a = new B();
a.Test(); // output --> "B::Test()"
b = new C();
b.Test(); // output --> "C::Test()"
Console.ReadKey();
}
}
}
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:
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(); // output --> "A::Test()"
b.Test(); // output --> "B::Test()"
c.Test(); // output --> "C::Test()"
a = new B();
a.Test(); // output --> "A::Test()"
b = new C();
b.Test(); // output --> "C::Test()"
Console.ReadKey();
}
}
}
Note
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.
The override keyword is used to extend or modify a virtual/abstract method, property, indexer, or event of base class into derived class.
The new keyword is used to hide a method, property, indexer, or event of base class into derived class.
Can we restrict a method not to be overridden under the child class?
If you want to do restrict a method not to be overridden by a child classes it should not be declared as sealed but every method by default sealed unless it was declared as virtual.
If a method is declared as virtual under parent can any child class overwrite it
Yes, if a method is declared as virtual in parent any child class as a write to overwrite it.
Parent class 1
Public virtual void Test ()
Child class 2
Public override void Test ()
Child class 3
Public override void Test ()
Can we restrict further overriding of a method at a child class which was declared as virtual in its parent
Yes, this can be performed by using sealed modifier on the method while overriding.
Public virtual void Test()
Public sealed override void Test()
Public override void Test()
Dynamic Polymorphism
When you have a function defined in a class that you want to be implemented in an inherited class(es), you use virtual functions. The virtual functions could be implemented differently in different inherited class and the call to these functions will be decided at runtime.
Dynamic polymorphism is implemented by abstract classes and virtual functions.
classShape
{
protectedintwidth, height;
publicShape(int a = 0, int b = 0)
{
width = a;
height = b;
}
publicvirtualintarea()
{
Console.WriteLine("Parent class area :");
return0;
}
}
classRectangle : Shape
{
publicRectangle(int a = 0, int b = 0)
: base(a, b)
{
}
publicoverrideintarea()
{
Console.WriteLine("Rectangle class area :");
return(width * height);
}
}
classTriangle : Shape
{
publicTriangle(int a = 0, int b = 0)
: base(a, b)
{
}
publicoverrideintarea()
{
Console.WriteLine("Triangle class area :");
return(width * height / 2);
}
}
classCaller
{
publicvoidCallArea(Shapesh)
{
int a;
a = sh.area();
Console.WriteLine("Area: {0}", a);
}
}
classTester
{
staticvoidMain(string[] args)
{
Callerc = newCaller();
Rectangler = newRectangle(10, 7);
Trianglet = newTriangle(10, 5);
c.CallArea(r);
c.CallArea(t);
Console.ReadKey();
}
}
Output:
