If a base class have a virtual method it means that method must be override by the derived class using override keyword.

A virtual function is a member function of a class, whose functionality can be over-ridden in its derived classes
or its called do nothing function .

virtual keyword is used in the base class method
when derived class use virtual method we use override key with derived class method and the implementation of method can be changed but not affecting the type signature of base class.

eg: base class
public virtual method1()
{
int a,b,c;
c=a+b;
Console.WriteLine(c);
}

derived class

public override method1()
{
int a,b,c;
c=a*b;
Console.WriteLine(c);
}

virtual meathod is define in base class and which is over-ridden in child class..
base:class
public virtual method1()
{
string c="virtual method1"
Console.WriteLine(c);
}

derived class

public override method1()
{
string c="override meathod"
Console.WriteLine(c);
}

If a Method is declared as Virutval in Base class then you can override the Method in derived Class by using Override keyword.

class Program
{
static void Main(string[] args)
{
CA a;
a= new CB();
a.Foo();
}

}
class CA
{
public virtual void Foo()
{
Console.WriteLine("Foo in parent Class");
}
}
class CB : CA
{

public override void Foo()
{
Console.WriteLine("Foo in Child Class");
}
}

Basically virtual method has drastical affect on performance of application. No Doubt we can do all the things without virtual function but if we use Virtual then we have following benefits.

1. Same of name of function can be used in derived class after overriding and can change its body there but with same signature

2. Overrided function will point to same memory location that is of the virtual function. means same memory location can be shared.

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