ASP.NET 身份发送电子邮件：TaskCanceledException

Question

我正在尝试在 asp.net 身份 2.0 应用程序中发送验证电子邮件地址电子邮件，但收到 TaskCanceledException 错误。

我尝试了这个帖子中的内容：Asp.Net Identity 2.0 - How to Implement IIdentityMessageService to do Async SMTP using SmtpClient?

如果我使用await，应用程序将永远运行：没有错误消息。但第二个选项返回错误。

错误抛出于：return smtpClient.SendMailAsync(msg);

IdentityConfig.cs

    public async Task SendAsync(IdentityMessage message)
    {
        // Plug in your email service here to send an email.

        try
        {
            #region formatter
            string text = string.Format("Please click on this link to {0}: {1}", message.Subject, message.Body);
            string html = "Please confirm your account by clicking this link: <a href=\"" + message.Body + "\">link</a><br/>";

            html += HttpUtility.HtmlEncode(@"Or click on the copy the following link on the browser:" + message.Body);
            #endregion

            MailMessage msg = new MailMessage();
            msg.From = new MailAddress("");
            msg.To.Add(new MailAddress(message.Destination));
            msg.Subject = message.Subject;
            msg.AlternateViews.Add(AlternateView.CreateAlternateViewFromString(text, null, MediaTypeNames.Text.Plain));
            msg.AlternateViews.Add(AlternateView.CreateAlternateViewFromString(html, null, MediaTypeNames.Text.Html));

            using (var smtpClient = new SmtpClient())
            {
                smtpClient.EnableSsl = true;
                smtpClient.SendCompleted += (s, e) => { smtpClient.Dispose(); };
                await smtpClient.SendMailAsync(msg);
            }


        }
        catch (Exception ex)
        {

            throw ex;
        }


    }

注册.aspx.cs

        protected void CreateUser_Click(object sender, EventArgs e)
    {
        var manager = Context.GetOwinContext().GetUserManager<ApplicationUserManager>();
        var user = new ApplicationUser()
        {
            UserName = Email.Text,
            Email = Email.Text,
            UserProfileInfo = new UserProfileInfo
            {
                FirstName = Firstname.Text,
                LastName = Lastname.Text,
                Adress = Adress.Text,
                Zip = Zip.Text,
                City = City.Text,
                Mobile = Mobile.Text

            }

        };
        IdentityResult result = manager.Create(user, Password.Text);
        if (result.Succeeded)
        {
            // For more information on how to enable account confirmation and password reset please visit http://go.microsoft.com/fwlink/?LinkID=320771
            string code = manager.GenerateEmailConfirmationToken(user.Id);
            string callbackUrl = IdentityHelper.GetUserConfirmationRedirectUrl(code, user.Id, Request);
            manager.SendEmail(user.Id, "Confirm your account", "Please confirm your account by clicking <a href=\"" + callbackUrl + "\">here</a>.");

            manager.AddToRole(user.Id, "Konsument");
            IdentityHelper.SignIn(manager, user, isPersistent: false);
            IdentityHelper.RedirectToReturnUrl(Request.QueryString["ReturnUrl"], Response);
        }
        else
        {
            ErrorMessage.Text = result.Errors.FirstOrDefault();
        }
    }

Answer 1

@ntl 正确识别了问题：SmtpClient 实例在请求完成之前被释放。不过，我更喜欢使用async 和await 作为解决方案：

public async Task SendAsync(IdentityMessage message)
{
  ...
  using (var smtpClient = new SmtpClient())
  {
    smtpClient.EnableSsl = true;
    smtpClient.SendCompleted += (s, e) => { smtpClient.Dispose(); };
    await smtpClient.SendMailAsync(msg);
  }
}

那么，让我们解决原来的问题：

如果我使用 await 应用程序将永远运行：没有错误消息。

我感觉你的代码在调用SendAsync 之后可能会调用Task.Wait 或Task<T>.Result。这才是真正的问题：调用代码应该改为使用await（这使得调用方法async 并返回Task/Task<T>，这使得its调用代码必须使用await 等）。让async 通过代码库自然地“成长”。这就是为什么我的MSDN article on the subject 中的async 最佳实践之一是“一路异步”。

造成死锁是因为当您await 一个任务时，await 将默认捕获“当前上下文”（在本例中为 ASP.NET SynchronizationContext），并使用该上下文来恢复async 方法。但是，ASP.NET SynchronizationContext 一次只允许每个请求一个线程，因此当调用代码阻塞（通过调用Wait 或Result）时，它阻塞了一个线程在这种情况下，async 方法无法恢复。我更详细地描述了这种死锁情况on my blog。

最好的办法是用await，一路用async。

Answer 2

问题可能是在执行 SendMailAsync 时您的 smpt 客户端已经被释放。这就是引发异常的原因，但由于此操作在单独的线程（异步）中运行，您会得到 TaskCanceledException。

您可以尝试删除您的 using 语句。

public Task SendAsync(IdentityMessage message)
{
    // Plug in your email service here to send an email.
    try
    {
        #region formatter
        string text = string.Format("Please click on this link to {0}: {1}", message.Subject, message.Body);
        string html = "Please confirm your account by clicking this link: <a href=\"" + message.Body + "\">link</a><br/>";
        html += HttpUtility.HtmlEncode(@"Or click on the copy the following link on the browser:" + message.Body);
        #endregion

        MailMessage msg = new MailMessage();
        msg.From = new MailAddress("info@emailadress.com");
        msg.To.Add(new MailAddress(message.Destination));
        msg.Subject = message.Subject;
        msg.AlternateViews.Add(AlternateView.CreateAlternateViewFromString(text, null, MediaTypeNames.Text.Plain));
        msg.AlternateViews.Add(AlternateView.CreateAlternateViewFromString(html, null, MediaTypeNames.Text.Html));

        var smtpClient = new SmtpClient();            
        smtpClient.EnableSsl = true;
        smtpClient.SendCompleted += (s, e) => { smtpClient.Dispose(); };
        return smtpClient.SendMailAsync(msg);
    }
    catch (Exception)
    {
        return Task.FromResult(0);
    }
}

Smtp 客户端将在 SendCompleted 处理程序的帮助下进行处理。