一、控制反转：

从简单的代码示例入手：

    /// <summary>/// 邮件服务类/// </summary>public class EmailService{public string SendMessage(){return "发送通知邮件";}}/// <summary>/// 邮件通知类/// </summary>public class NotifycationSystem{private EmailService service;public NotifycationSystem(){service = new EmailService(); //邮件通知类必须精确的知道创建和使用了哪种类型的服务，此处高耦合了。}public string InterestingEventHappened(){return service.SendMessage();}}

共两个类，一个邮件服务类，一个邮件通知类，邮件通知类依赖于邮件服务类。邮件通知类必须精确的知道创建和使用了哪种类型的服务，此处高耦合了。

改进一：在两代码块中引入抽象层，提取接口。

    /// <summary>/// 邮件服务接口/// </summary>public interface IMessageService{string SendMessage();}/// <summary>/// 邮件服务类/// </summary>public class EmailService : IMessageService{public string SendMessage(){return "发送通知邮件";}}/// <summary>/// 邮件通知类/// </summary>public class NotifycationSystem{private IMessageService service;//邮件通知类保存服务实现的接口public NotifycationSystem(){service = new EmailService(); }public string InterestingEventHappened(){return service.SendMessage();}}

上面将依赖具体实现改为了依赖接口，减少了部分耦合。但是邮件服务类还是在邮件通知类内实例化的，也就是说邮件通知类还是要完全知道邮件服务类的具体细节。

改进二：将选择抽象实现的责任移到服务消费者类的外部。

    /// <summary>/// 第二层抽象： 服务定位器/// </summary>public interface IServiceLocator{IMessageService GetMessageService();}/// <summary>/// 第一层抽象：邮件服务接口/// </summary>public interface IMessageService{string SendMessage();}/// <summary>/// 邮件服务类/// </summary>public class EmailService : IMessageService{public string SendMessage(){return "发送通知邮件";}}/// <summary>/// 邮件通知类/// </summary>public class NotifycationSystem{private IMessageService service;//邮件通知类保存服务实现的接口。public NotifycationSystem(IServiceLocator locator){service = locator.GetMessageService();//实现依赖关系被转移到类外。}public string InterestingEventHappened(){return service.SendMessage();}}

扩展一：弱类型服务定位器。

/// <summary>/// 第二层抽象： 服务定位器/// </summary>public interface IServiceLocator{object GetService(Type serviceType);}/// <summary>/// 第一层抽象：邮件服务接口/// </summary>public interface IMessageService{string SendMessage();}/// <summary>/// 邮件服务类/// </summary>public class EmailService : IMessageService{public string SendMessage(){return "发送通知邮件";}}/// <summary>/// 邮件通知类/// </summary>public class NotifycationSystem{private IMessageService service;//邮件通知类保存服务实现的接口。public NotifycationSystem(IServiceLocator locator){service = (IMessageService)locator.GetService(typeof(IMessageService));//实现依赖关系被转移到类外。}public string InterestingEventHappened(){return service.SendMessage();}}

弱类型服务定位器使得这种模式更加灵活，因为他允许请求任意类型的服务类型。采用Type类型的参数，并返回一个非类型化的示例，也就是一个object类型对象。

扩展二：泛型方法。

    /// <summary>/// 第二层抽象： 服务定位器/// </summary>public interface IServiceLocator{T GetService<T>();//泛型接口object GetService(Type serviceType);}/// <summary>/// 第一层抽象：邮件服务接口/// </summary>public interface IMessageService{string SendMessage();}/// <summary>/// 邮件服务类/// </summary>public class EmailService : IMessageService{public string SendMessage(){return "发送通知邮件";}}/// <summary>/// 邮件通知类/// </summary>public class NotifycationSystem{private IMessageService service;//邮件通知类保存服务实现的接口。public NotifycationSystem(IServiceLocator locator){service = locator.GetService<IMessageService>();//实现依赖关系被转移到类外。}public string InterestingEventHappened(){return service.SendMessage();}}

泛型方法，让依赖反转代码看上去更加高效优雅。

二、依赖注入：

1.构造函数注入：

    /// <summary>/// 邮件通知类/// </summary>public class NotifycationSystem{private IMessageService _service;public NotifycationSystem(IMessageService service)//构造函数注入{_service = service;}public string InterestingEventHappened(){return _service.SendMessage();}}

2.属性注入：

    /// <summary>/// 邮件通知类/// </summary>public class NotifycationSystem{private IMessageService MessageService { get; set; }public string InterestingEventHappened(){if (MessageService == null){throw new InvalidOperationException("服务类型为赋值！");}return MessageService.SendMessage();}}