When an instance method declaration includes an override modifier, the
method is said to be an override
method. An override method overrides an inherited virtual method with the
same signature. Whereas a virtual
method declaration introduces a new method, an override method declaration
specializes an existing inherited
virtual method by providing a new implementation of that method.
The method overridden by an override declaration is known as the overridden
base method. For an override
method M declared in a class C, the overridden base method is determined by
examining each base class of C,
starting with the direct base class of C and continuing with each
successive direct base class, until an accessible
method with the same signature as M is located. For the purposes of
locating the overridden base method, a
method is considered accessible if it is public, if it is protected, if it
is protected internal, or if it is
internal and declared in the same program as C.
A compile-time error occurs unless all of the following are true for an
override declaration:
? An overridden base method can be located as described above.
? The overridden base method is a virtual, abstract, or override method. In
other words, the overridden base
method cannot be static or non-virtual.
? The overridden base method is not a sealed method.
? The override declaration and the overridden base method have the same
return type.
? The override declaration and the overridden base method have the same
declared accessibility. In other
words, an override declaration cannot change the accessibility of the
virtual method.
An override declaration can access the overridden base method using a
base-access (§14.5.8). [Example: In the
example
class A
{
int x;
public virtual void PrintFields() {
Console.WriteLine("x = {0}", x);
}
}
C# LANGUAGE SPECIFICATION
236
class B: A
{
int y;
public override void PrintFields() {
base.PrintFields();
Console.WriteLine("y = {0}", y);
}
}
the base.PrintFields() invocation in B invokes the PrintFields method
declared in A. A base-access
disables the virtual invocation mechanism and simply treats the base method
as a non-virtual method. Had the
invocation in B been written ((A)this).PrintFields(), it would recursively
invoke the PrintFields
method declared in B, not the one declared in A, since PrintFields is
virtual and the run-time type of
((A)this) is B. end example]
Only by including an override modifier can a method override another
method. In all other cases, a method
with the same signature as an inherited method simply hides the inherited
method. [Example: In the example
class A
{
public virtual void F() {}
}
class B: A
{
public virtual void F() {} // Warning, hiding inherited F()
}
the F method in B does not include an override modifier and therefore does
not override the F method in A.
Rather, the F method in B hides the method in A, and a warning is reported
because the declaration does not
include a new modifier. end example]
[Example: In the example
class A
{
public virtual void F() {}
}
class B: A
{
new private void F() {} // Hides A.F within B
}
class C: B
{
public override void F() {} // Ok, overrides A.F
}
the F method in B hides the virtual F method inherited from A. Since the
new F in B has private access, its scope
only includes the class body of B and does not extend to C. Therefore, the
declaration of F in C is permitted to
override the F inherited from A. end example]