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