一个简单的依赖倒置／依赖注入（Dependency Injection）的例子

一个简单的依赖倒置／依赖注入（Dependency Injection）的例子

1.

一个邮件发送程序

class MailService
   {
     public void SendEmail(string address, string subject, string
       body)
     {
       var mail = new MailMessage();
       mail.To.Add(address);
       mail.Subject = subject;
       mail.Body = body;
       var client = new SmtpClient();
       // Setup client with smtp server address and port here
       client.Send(mail);
} }

2.

增加一个日志功能

class MailService
{
  public void SendMail(string address, string subject, string
    body)
  {
    Console.WriteLine("Creating mail message...");
    var mail = new MailMessage();
    mail.To.Add(address);
    mail.Subject = subject;
    mail.Body = body;
    var client = new SmtpClient();
    // Setup client with smtp server address and port here
    Console.WriteLine("Sending message...");
    client.Send(mail);
    Console.WriteLine("Message sent successfully.");
} }

现在MailService具有2个职责，发送邮件和记录日志。

3.

把日志的实现细节分离出去。

class MailService
   {
     private ConsoleLogger logger;
     public MailService()
     {
       logger = new ConsoleLogger();
     }
     public void SendMail(string address, string subject, string
       body)
     {
       logger.Log("Creating mail message...");
       var mail = new MailMessage();
       mail.To.Add(address);
       mail.Subject = subject;
       mail.Body = body;
       var client = new SmtpClient();
       // Setup client with smtp server address and port here
       logger.Log("Sending message...");
       client.Send(mail);
       logger.Log("Message sent successfully.");
} }

class ConsoleLogger
   {
     public void Log(string message)
     {
       Console.WriteLine("{0}: {1}", DateTime.Now, message);
     }
}

现在可以日志功能的变更就分离到ConsoleLogger里面了。比如换成Windows Event Log方法记录日志。

4.

现在假设要根据开发环境和生产环境，分别用不同的Logger。比如开发环境用ConsoleLogger，生产环境用EventLogger。

class EventLogger
   {
     public void Log(string message)
     {
       EventLog.WriteEntry("MailService", message);
     }
}

接口

interface ILogger
   {
     void Log(string message);
   }

class ConsoleLogger:ILogger
   {
     public void Log(string message)
     {
       Console.WriteLine("{0}: {1}", DateTime.Now, message);
     }
}

class EventLogger:ILogger
{
  public void Log(string message)
  {
    EventLog.WriteEntry("MailService", message);
  }
｝

通过构造函数参数，MailService不依赖任何一个具体的Logger类，无论怎样对日志功能的修改，都不影响MailService。

public MailService(ILogger logger)
   {
     this.logger = logger;
   }

MailService不再关心Logger类的创建，只负责使用日志类提供的功能。

internal class Program
   {
     private static void Main(string[] args)
     {
       var mailService = new MailService(new EventLogger());
       mailService.SendMail("[email protected]", "My first DI
         App", "Hello World!");
     }
}