如果接口中有事件，如何实现事件，以及如何在依赖注入中使用它

Question

我创建了一个单独的类来处理邮件发送。为了让它免于未来的变化，我使用了依赖注入来设计它。现在，当我尝试使用SendMessageAsync() 方法时，我想检查 SendMailCompleted 事件以查看发送失败/成功的消息状态。我很困惑我应该如何在实现它的类中实现事件。如果不在界面中提及此事件，我将无法在注入类中捕获它。任何人都可以建议如何解决这个问题？我的界面如下所示

public interface IMailing
{
    string Host { get; set; }
    int Port { get; set; }
    Task SendMailAsync(string toAddress, string subject, string body);

    event SendCompletedEventHandler OnEmailSendComplete;
}

实现接口的类如下-

public class Mailing : IMailing
{
    private SmtpClient client = new SmtpClient();
    MailMessage mm = null;

    public string Host{ get;  set; }
    public int Port { get; set; }

// do i need this? without event being in the interface I would have had this //two following lines to manually raise the event 
    public event SendCompletedEventHandler OnEmailSendComplete;

    public delegate void SendCompletedEventHandler(object source, AsyncCompletedEventArgs e);



// following lines were generated from the vs2017 IDE
// how do I use it when actual Send mail completed event fires?
    event System.Net.Mail.SendCompletedEventHandler IMailing.OnEmailSendComplete
    {
        add
        {
            throw new NotImplementedException();
        }

        remove
        {
            throw new NotImplementedException();
        }
    }

    public async Task SendMailAsync(string toAddress, string subject, string body)
    {            
        mm = new MailMessage(User, toAddress, subject, body);
        client.SendCompleted += Client_SendCompleted;
        await client.SendMailAsync(mm).ConfigureAwait(false);
    }

    private void Client_SendCompleted(object sender, AsyncCompletedEventArgs e)
    {
        OnEmailSendComplete?.Invoke(sender, e);
    }
}

现在，注入类使用构造函数注入，如下所示-

public class MailingInjection
{
    IMailing mailing = null;

    private MailingInjection()
    { }
    public MailingInjection(IMailing imail)
    {
        mailing = imail;
    }              

    public async Task SenMailAsync(string to, string subject, string body)
    {
        mailing.OnEmailSendComplete += EmailSendCompleted;
        await mailing.SendMailAsync(to, subject,body).ConfigureAwait(false);   
    }

    private void EmailSendCompleted(object sender, System.ComponentModel.AsyncCompletedEventArgs e)
    {
        mailing.OnEmailSendComplete -= EmailSendCompleted;
    }
}

我尝试使用尽可能少的代码来解释我的困惑，因此这段代码在实际场景中不起作用，但具有我相信的结构。如果我夸大了，请告诉我。我很感激任何帮助。

Answer 1

您的代码将两种不同的异步模式混为一谈：旧的event-based asynchronous pattern 和新的task-based asynchronous pattern。特别是，SmtpClient.SendCompleted 仅与SmtpClient.SendAsync 一起使用。你的代码没有使用SendAsync，所以它不需要SendCompleted。它可以只使用SmtpClient.SendMailAsync：

public interface IMailing
{
  string Host { get; set; }
  int Port { get; set; }
  Task SendMailAsync(string toAddress, string subject, string body);
  // No event necessary.
}

public class Mailing : IMailing
{
  private SmtpClient client = new SmtpClient();
  MailMessage mm = null;

  public string Host{ get;  set; }
  public int Port { get; set; }

  public async Task SendMailAsync(string toAddress, string subject, string body)
  {            
    mm = new MailMessage(User, toAddress, subject, body);
    await client.SendMailAsync(mm).ConfigureAwait(false);
  }
}

用法：

public class MailingInjection
{
  ...

  public async Task SenMailAsync(string to, string subject, string body)
  {
    await mailing.SendMailAsync(to, subject,body).ConfigureAwait(false);   
  }
}

最后提醒一下，如果您的抽象只有一个实现，那么它就不是抽象。 ;)