继承（加上封装和多态性）是面向对象的编程的三个主要特性（也称为[支柱]）之一。 继承用于创建可重用、扩展和修改在其他类中定义的行为的新类。其成员被继承的类称为[基类]，继承这些成员的类称为[派生类]。派生类只能有一个直接基类。但是，继承是可传递的。如果 ClassB 派生出 ClassC，ClassA 派生出 ClassB，则 ClassC 会继承 ClassB 和 ClassA 中声明的成员。
注意
结构不支持继承，但可以实现接口。
从概念上来说，派生类是基类的特例。 例如，如果您有一个基类 Animal，则可以有一个名为 Mammal 的派生类和一个名为 Reptile 的派生类。 Mammal 是一个 Animal，Reptile 也是一个 Animal，但每个派生类均表示基类的不同专用化。
定义一个类从其他类派生时，派生类隐式获得基类的除构造函数和析构函数以外的所有成员。因此，派生类可以重用基类中的代码而无需重新实现这些代码。可以在派生类中添加更多成员。派生类以这种方式扩展基类的功能。
下图演示一个 WorkItem 类，该类表示某业务流程中的一个工作项。和所有的类一样，该类派生自 System.Object 并继承其所有方法。 WorkItem 添加了自己的五个成员。其中包括一个构造函数，因为构造函数不能继承。类ChangeRequest 继承自 WorkItem 并表示特定种类的工作项。 ChangeRequest 在它从 WorkItem 和 Object 继承的成员中另外添加了两个成员。它必须添加其自己的构造函数，还添加 originalItemID。利用属性 originalItemID，可将 ChangeRequest 实例与更改请求将应用到的原始 WorkItem 相关联。
类继承
下面的示例演示如何以 C# 表示上图所示的类关系。该示例还演示 WorkItem 如何重写虚方法 Object.ToString，以及 ChangeRequest 类如何继承该方法的 WorkItem 实现。
// WorkItem implicitly inherits from the Object class.
public class WorkItem
{
// Static field currentID stores the job ID of the last WorkItem that
// has been created.
private static int currentID;
//Properties.
protected int ID { get ; set ; }
protected string Title { get ; set ; }
protected string Description { get ; set ; }
protected TimeSpan jobLength { get ; set ; }
// Default constructor. If a derived class does not invoke a base-
// class constructor explicitly, the default constructor is called
// implicitly.
public WorkItem()
{
ID = 0;
Title = "Default title" ;
Description = "Default description." ;
jobLength = new TimeSpan();
}
// Instance constructor that has three parameters.
public WorkItem( string title, string desc, TimeSpan joblen)
{
this .ID = GetNextID();
this .Title = title;
this .Description = desc;
this .jobLength = joblen;
}
// Static constructor to initialize the static member, currentID. This
// constructor is called one time, automatically, before any instance
// of WorkItem or ChangeRequest is created, or currentID is referenced.
static WorkItem()
{
currentID = 0;
}
protected int GetNextID()
{
// currentID is a static field. It is incremented each time a new
// instance of WorkItem is created.
return ++currentID;
}
// Method Update enables you to update the title and job length of an
// existing WorkItem object.
public void Update( string title, TimeSpan joblen)
{
this .Title = title;
this .jobLength = joblen;
}
// Virtual method override of the ToString method that is inherited
// from System.Object.
public override string ToString()
{
return String.Format( "{0} - {1}" , this .ID, this .Title);
}
}
// ChangeRequest derives from WorkItem and adds a property (originalItemID)
// and two constructors.
public class ChangeRequest : WorkItem
{
protected int originalItemID { get ; set ; }
// Constructors. Because neither constructor calls a base-class
// constructor explicitly, the default constructor in the base class
// is called implicitly. The base class must contain a default
// constructor.
// Default constructor for the derived class.
public ChangeRequest() { }
// Instance constructor that has four parameters.
public ChangeRequest( string title, string desc, TimeSpan jobLen,
int originalID)
{
// The following properties and the GetNexID method are inherited
// from WorkItem.
this .ID = GetNextID();
this .Title = title;
this .Description = desc;
this .jobLength = jobLen;
// Property originalItemId is a member of ChangeRequest, but not
// of WorkItem.
this .originalItemID = originalID;
}
}
class Program
{
static void Main()
{
// Create an instance of WorkItem by using the constructor in the
// base class that takes three arguments.
WorkItem item = new WorkItem( "Fix Bugs" ,
"Fix all bugs in my code branch" ,
new TimeSpan(3, 4, 0, 0));
// Create an instance of ChangeRequest by using the constructor in
// the derived class that takes four arguments.
ChangeRequest change = new ChangeRequest( "Change Base Class Design" ,
"Add members to the class" ,
new TimeSpan(4, 0, 0),
1);
// Use the ToString method defined in WorkItem.
Console.WriteLine(item.ToString());
// Use the inherited Update method to change the title of the
// ChangeRequest object.
change.Update( "Change the Design of the Base Class" ,
new TimeSpan(4, 0, 0));
// ChangeRequest inherits WorkItem's override of ToString.
Console.WriteLine(change.ToString());
// Keep the console open in debug mode.
Console.WriteLine( "Press any key to exit." );
Console.ReadKey();
}
}
输出：
1 - Fix Bugs
2 - Change the Design of the Base Class
dy("nrwz");
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