ECMA-334 C# Language Specification

17.5.3: Virtual methods

When an instance method declaration includes a virtual modifier, that method is said to be a virtual method. When no virtual modifier is present, the method is said to be a non-virtual method.

The implementation of a non-virtual method is invariant: The implementation is the same whether the method is invoked on an instance of the class in which it is declared or an instance of a derived class. In contrast, the implementation of a virtual method can be superseded by derived classes. The process of superseding the implementation of an inherited virtual method is known as overriding that method (17.5.4).

In a virtual method invocation, the run-time type of the instance for which that invocation takes place determines the actual method implementation to invoke. In a non-virtual method invocation, the compile-time type of the instance is the determining factor. In precise terms, when a method named N is invoked with an argument list A on an instance with a compile-time type C and a run-time type R (where R is either C or a class derived from C), the invocation is processed as follows:

For every virtual method declared in or inherited by a class, there exists a most derived implementation of the method with respect to that class. The most derived implementation of a virtual method M with respect to a class R is determined as follows:

[Example: The following example illustrates the differences between virtual and non-virtual methods: 
using System;  
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 Test  
{  
   static void Main() {  
      B b = new B();  
      A a = b;  
      a.F();  
      b.F();  
      a.G();  
      b.G();  
   }  
}

In the example, A introduces a non-virtual method F and a virtual method G. The class B introduces a new non-virtual method F, thus hiding the inherited F, and also overrides the inherited method G. The example produces the output: 
A.F  
B.F  
B.G  
B.G

Notice that the statement a.G() invokes B.G, not A.G. This is because the run-time type of the instance (which is B), not the compile-time type of the instance (which is A), determines the actual method implementation to invoke. end example]

Because methods are allowed to hide inherited methods, it is possible for a class to contain several virtual methods with the same signature. This does not present an ambiguity problem, since all but the most derived method are hidden. [Example: In the example 
using System;  
class A  
{  
   public virtual void F() { Console.WriteLine("A.F"); }  
}  
class B: A  
{  
   public override void F() { Console.WriteLine("B.F"); }  
}  
class C: B  
{  
   new public virtual void F() { Console.WriteLine("C.F"); }  
}  
class D: C  
{  
   public override void F() { Console.WriteLine("D.F"); }  
}  
class Test  
{  
   static void Main() {  
      D d = new D();  
      A a = d;  
      B b = d;  
      C c = d;  
      a.F();  
      b.F();  
      c.F();  
      d.F();  
   }  
}
the C and D classes contain two virtual methods with the same signature: The one introduced by A and the one introduced by C. The method introduced by C hides the method inherited from A. Thus, the override declaration in D overrides the method introduced by C, and it is not possible for D to override the method introduced by A. The example produces the output: 
B.F  
B.F  
D.F  
D.F

Note that it is possible to invoke the hidden virtual method by accessing an instance of D through a less derived type in which the method is not hidden. end example]