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 In contrast, the implementation of a virtual method can be superseded by derived classes.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: First, overload resolution is applied to C, N, and A, to select a specific method M from the set of methods declared in and inherited by C. This is described in 14.5.5.1. Then, if M is a non-virtual method, M is invoked. Otherwise, M is a virtual method, and the most derived implementation of M with respect to R is invoked.
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: If R contains the introducing virtual declaration of M, then this is the most derived implementation of M. Otherwise, if R contains an override of M, then this is the most derived implementation of M. Otherwise, the most derived implementation of M is the same as that of the direct base class of R.
[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]