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17.5.6 Abstract methods

王朝other·作者佚名  2006-05-09
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When an instance method declaration includes an abstract modifier, that

method is said to be an abstract

method. Although an abstract method is implicitly also a virtual method, it

cannot have the modifier virtual.

An abstract method declaration introduces a new virtual method but does not

provide an implementation of that

method. Instead, non-abstract derived classes are required to provide their

own implementation by overriding that

method. Because an abstract method provides no actual implementation, the

method-body of an abstract method

simply consists of a semicolon.

Abstract method declarations are only permitted in abstract classes (§17.1.1

.1).

[Example: In the example

public abstract class Shape

{

public abstract void Paint(Graphics g, Rectangle r);

}

public class Ellipse: Shape

{

public override void Paint(Graphics g, Rectangle r) {

g.DrawEllipse(r);

}

}

public class Box: Shape

{

public override void Paint(Graphics g, Rectangle r) {

g.DrawRect(r);

}

}

the Shape class defines the abstract notion of a geometrical shape object

that can paint itself. The Paint method

is abstract because there is no meaningful default implementation. The

Ellipse and Box classes are concrete

Shape implementations. Because these classes are non-abstract, they are

required to override the Paint method

and provide an actual implementation. end example]

It is a compile-time error for a base-access (§14.5.8) to reference an

abstract method. [Example: In the example

abstract class A

{

public abstract void F();

}

C# LANGUAGE SPECIFICATION

238

class B: A

{

public override void F() {

base.F(); // Error, base.F is abstract

}

}

a compile-time error is reported for the base.F() invocation because it

references an abstract method. end

example]

An abstract method declaration is permitted to override a virtual method.

This allows an abstract class to force reimplementation

of the method in derived classes, and makes the original implementation of

the method

unavailable. [Example: In the example

using System;

class A

{

public virtual void F() {

Console.WriteLine("A.F");

}

}

abstract class B: A

{

public abstract override void F();

}

class C: B

{

public override void F() {

Console.WriteLine("C.F");

}

}

class A declares a virtual method, class B overrides this method with an

abstract method, and class C overrides

that abstract method to provide its own implementation. end example]

17.5.7 External methods

When a method declaration includes an extern modifier, the method is said

to be an external method. External

methods are implemented externally, typically using a language other than

C#. Because an external method

declaration provides no actual implementation, the method-body of an

external method simply consists of a

semicolon.

The mechanism by which linkage to an external method is achieved, is

implementation-defined.

[Example: The following example demonstrates the use of the extern modifier

in combination with a

DllImport attribute that specifies the name of the external library in

which the method is implemented:

using System.Text;

using System.Security.Permissions;

using System.Runtime.InteropServices;

class Path

{

[DllImport("kernel32", SetLastError=true)]

static extern bool CreateDirectory(string name, SecurityAttribute sa);

[DllImport("kernel32", SetLastError=true)]

static extern bool RemoveDirectory(string name);

[DllImport("kernel32", SetLastError=true)]

static extern int GetCurrentDirectory(int bufSize, StringBuilder buf);

[DllImport("kernel32", SetLastError=true)]

static extern bool SetCurrentDirectory(string name);

}

end example]

 
 
 
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