Abstract
Abstract classes are closely related to interfaces. They are classes that cannot be instantiated, and are frequently either partially implemented, or not at all implemented. One key difference between abstract classes and interfaces is that a class may implement an unlimited number of interfaces, but may inherit from only one abstract (or any other kind of) class. A class that is derived from an abstract class may still implement interfaces. Abstract classes are useful when creating components because they allow you specify an invariant level of functionality in some methods, but leave the implementation of other methods until a specific implementation of that class is needed. They also version well, because if additional functionality is needed in derived classes, it can be added to the base class without breaking code.
The abstract modifier can be used with classes, methods, properties, indexers, and events.
Use the abstract modifier in a class declaration to indicate that a class is intended only to be a base class of other classes.
Abstract classes have the following features:
- An abstract class cannot be instantiated.
- An abstract class may contain abstract methods and accessors.
- It is not possible to modify an abstract class with the sealed modifier, which means that the class cannot be inherited.
- A non-abstract class derived from an abstract class must include actual implementations of all inherited abstract methods and accessors.
Use the abstract modifier in a method or property declaration to indicate that the method or property does not contain implementation.
Abstract methods have the following features:
- An abstract method is implicitly a virtual method.
- Abstract method declarations are only permitted in abstract classes.
- Because an abstract method declaration provides no actual implementation, there is no method body; the method declaration simply ends with a semicolon and there are no braces ({ }) following the signature. For example:
public abstract void MyMethod();
- The implementation is provided by an overriding method, which is a member of a non-abstract class.
- It is an error to use the static or virtual modifiers in an abstract method declaration.
Abstract properties behave like abstract methods, except for the differences in declaration and invocation syntax.
- It is an error to use the abstract modifier on a static property.
- An abstract inherited property can be overridden in a derived class by including a property declaration that uses the override modifier.
An abstract class must provide implementation for all interface members.
An abstract class that implements an interface might map the interface methods onto abstract methods. For example:
interface I
{
void M();
}
abstract class C: I
{
public abstract void M();
}
Example
In this example, the classMyDerivedCis derived from an abstract classMyBaseC. The abstract class contains an abstract method,MyMethod(), and two abstract properties,GetX()andGetY().
// abstract_keyword.cs
// Abstract Classes
using System;
abstract class MyBaseC // Abstract class
{
protected int x = 100;
protected int y = 150;
public abstract void MyMethod(); // Abstract method
public abstract int GetX // Abstract property
{
get;
}
public abstract int GetY // Abstract property
{
get;
}
}
class MyDerivedC: MyBaseC
{
public override void MyMethod()
{
x++;
y++;
}
public override int GetX // overriding property
{
get
{
return x+10;
}
}
public override int GetY // overriding property
{
get
{
return y+10;
}
}
public static void Main()
{
MyDerivedC mC = new MyDerivedC();
mC.MyMethod();
Console.WriteLine("x = {0}, y = {1}", mC.GetX, mC.GetY);
}
}Abstract classes are closely related to interfaces. They are classes that cannot be instantiated, and are frequently either partially implemented, or not at all implemented. One key difference between abstract classes and interfaces is that a class may implement an unlimited number of interfaces, but may inherit from only one abstract (or any other kind of) class. A class that is derived from an abstract class may still implement interfaces. Abstract classes are useful when creating components because they allow you specify an invariant level of functionality in some methods, but leave the implementation of other methods until a specific implementation of that class is needed. They also version well, because if additional functionality is needed in derived classes, it can be added to the base class without breaking code.
// C#
abstract class WashingMachine
{
public WashingMachine()
{
// Code to initialize the class goes here.
}
abstract public void Wash();
abstract public void Rinse(int loadSize);
abstract public long Spin(int speed);
}
// C#
class MyWashingMachine : WashingMachine
{
public MyWashingMachine()
{
// Initialization code goes here.
}
override public void Wash()
{
// Wash code goes here.
}
override public void Rinse(int loadSize)
{
// Rinse code goes here.
}
override public long Spin(int speed)
{
// Spin code goes here.
}
}
Ref:http://msdn.microsoft.com/en-us/library/sf985hc5(v=vs.71).aspx
