brief what virtual void means

Question

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

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

it is print "B.G" twice .why and virtual what does

Answer 1

Virtual allows for a derived function to overwrite the functionality of a base members function.

it prints b.g twice because a=b;

here: http://www.cplusplus.com/doc/tutorial/polymorphism/