
17 Classes
17.5 Methods
17.5.3 Virtual methods
Paragraph 11 When an instance method declaration includes a virtual modifier, that method is said to be a virtual method. 2 When no virtual modifier is present, the method is said to be a non-virtual method.
Paragraph 21 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in which it is declared or an instance of a derived class. 2 In contrast, the implementation of a virtual method can be superseded by derived classes. 3 The process of superseding the implementation of an inherited virtual method is known as overriding that method (§17.5.4).
Paragraph 31 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. 2 In a non-virtual method invocation, the compile-time type of the instance is the determining factor. 3 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:
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.GNotice 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] Paragraph 51 Because methods are allowed to hide inherited methods, it is possible for a class to contain several virtual methods with the same signature. 2 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.FNote 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]
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