发送批量电子邮件"An asynchronous call is already in progress. It must be completed or canceled before you ca

Question

我创建了一个控制台应用程序，用于将数据从一个数据库表迁移到另一个数据库表迁移的记录，因此我必须通知它们以更改密码。

public static async Task<bool> SendRegisterEmail(List<MailMessage> mailMessage)
{
    bool flag = true;
    try
    {
        var smtp = new SmtpClient();
        var taskEmails = mailMessage.Select(x => smtp.SendMailAsync(x));
        await Task.WhenAll(taskEmails); // **Error : An asynchronous call is already in progress. It must be completed or canceled before you can call this method**
    }
    catch (Exception ex)
    {
        throw ex;
    }
    return flag;
}

如果我删除该等待任务，电子邮件将成功发送。whenall(taskemails(，但是当迁移操作完成时，它不会发送所有电子邮件。他们中的许多人在完成操作后仍未关闭Console应用程序，我有超过1,00,000个记录，因此我如何继续在运行的后台或应用程序中发送电子邮件发送流程，直到成功发送所有电子邮件？

这是数据迁移的守则：

foreach (DataRow SourceReader in DS.Tables[0].Rows)
{
    insertCounter++;
    using (SqlCommand DestinationCommand = DestinationConnection.CreateCommand())
    {
        Console.WriteLine("inserting row...");
        var date = (SourceReader["LockedUntil"] == DBNull.Value ? Convert.ToDateTime("01/01/1753") : Convert.ToDateTime(SourceReader["LockedUntil"]));
        DestinationCommand.CommandText = string.Format(insertQuery1, (SourceReader["CustomerGUID"] == DBNull.Value ? null : SourceReader["CustomerGUID"].ToString()), (SourceReader["FirstName"] == DBNull.Value ? null : SourceReader["FirstName"].ToString()), (SourceReader["LastName"] == DBNull.Value ? null : SourceReader["LastName"].ToString()), (SourceReader["Email"] == DBNull.Value ? null : SourceReader["Email"].ToString()), 1, PasswordManager.Encrypt(SourceReader["FirstName"].ToString() + "1!"), (SourceReader["Phone"] == DBNull.Value ? null : SourceReader["Phone"].ToString()), 1, 0, 0, date.ToString("yyyy-MM-ddTHH:mm:ss"), Convert.ToInt16(SourceReader["BadLoginCount"] == DBNull.Value ? 0 : SourceReader["BadLoginCount"]), Convert.ToInt16(SourceReader["OkToEmail"] == DBNull.Value ? 0 : SourceReader["OkToEmail"]), (SourceReader["CustomerGUID"] == DBNull.Value ? null : SourceReader["CustomerGUID"].ToString()), 2);
        DestinationCommand.ExecuteNonQuery();
        Console.WriteLine("AspNetUser Row inserted...!!! ");
    }
    if ((insertCounter % 100) == emailCounter)
    {
        var message = Email.AddUserForEmail(new User() { Email = (SourceReader["Email"] == DBNull.Value ? null : SourceReader["Email"].ToString()), Password = (SourceReader["FirstName"].ToString() + "1!"), FirstName = (SourceReader["FirstName"] == DBNull.Value ? null : SourceReader["FirstName"].ToString()), LastName = (SourceReader["LastName"] == DBNull.Value ? null : SourceReader["LastName"].ToString()) });
        mailList.Add(message);
        var flag = Email.SendRegisterEmail(mailList);
        emailCounter++;
    }
    else if (insertCounter == totalCount)
    {
        var message = Email.AddUserForEmail(new User() { Email = (SourceReader["Email"] == DBNull.Value ? null : SourceReader["Email"].ToString()), Password = (SourceReader["FirstName"].ToString() + "1!"), FirstName = (SourceReader["FirstName"] == DBNull.Value ? null : SourceReader["FirstName"].ToString()), LastName = (SourceReader["LastName"] == DBNull.Value ? null : SourceReader["LastName"].ToString()) });
        mailList.Add(message);
        var flag = Email.SendRegisterEmail(mailList);
    }
    else
    {
        var message = Email.AddUserForEmail(new User() { Email = (SourceReader["Email"] == DBNull.Value ? null : SourceReader["Email"].ToString()), Password = (SourceReader["FirstName"].ToString() + "1!"), FirstName = (SourceReader["FirstName"] == DBNull.Value ? null : SourceReader["FirstName"].ToString()), LastName = (SourceReader["LastName"] == DBNull.Value ? null : SourceReader["LastName"].ToString()) });
        mailList.Add(message);
    }
    // var i = Email.SendRegisterEmail((SourceReader["Email"] == DBNull.Value ? null : SourceReader["Email"].ToString()), (SourceReader["FirstName"].ToString() + "1!"), (SourceReader["FirstName"] == DBNull.Value ? null : SourceReader["FirstName"].ToString()), (SourceReader["LastName"] == DBNull.Value ? null : SourceReader["LastName"].ToString()));
}

Answer 1

SmtpClient不允许您同时执行多个异步操作，这就是错误消息告诉您的。您正在这样做：

var smtp = new SmtpClient();
var taskEmails = mailMessage.Select(x => smtp.SendMailAsync(x));
await Task.WhenAll(taskEmails);

您也没有处理SmtpClient，这也无济于事。

相反，要么一个人发送：

using (var smtp = new SmtpClient()){
    foreach (var email in mailMessage) {
        await smtp.SendMailAsync(email);
    }
}

或每次发送单独的SmtpClient：

Func<MailMessage, Task> sendFunc = async (x) => {
    using (var smtp = new SmtpClient()) {
        await smtp.SendMailAsync(x);
    }
};
var taskEmails = mailMessage.Select(sendFunc);
await Task.WhenAll(taskEmails);

看来您也不在等待从迁移功能发送的发送：

// flag is Task<bool> here
var flag = Email.SendRegisterEmail(mailList);

如果这不是错字 - 您需要等待它们到位，或者在某些列表中收集任务并在循环全部循环后等待它们(使用await Task.WhenAll(。

请注意，如果您发送了很多电子邮件，尤其是向同一域发送，尤其是在并行的 - 您使用的SMTP服务器(或收件人的SMTP服务器(可能对此不太满意，并且可能会暂时将您列入一段时间。

最好使用单独的背景过程，这将使您的电子邮件完全与您的迁移过程无关。只需让迁移过程插入有关挂起电子邮件的信息到某些持久存储(数据库表(，然后让另一个应用程序探索该表并发送电子邮件。这样，迁移过程就不会被电子邮件失败中断，而发送电子邮件的背景过程可以根据需要重试。