使用Disposables

Question

最近经过一堆代码整理和“改进”的事情，我注意到这段代码用于发送电子邮件：

Dim SmtpServer As New SmtpClient() With {
    .Port = 25,
    .Host = Email.Hostname,
    .Credentials = New NetworkCredential() With {
        .UserName = Email.UserName,
        .Password = Email.Password      
    }
}

Dim mail As New MailMessage() With {
    .From = Email.Sender,
    .Subject = Email.Subject,
    .IsBodyHtml = True,
    .Body = Email.WrappedHtml
}
If Not Email.Attachments Is Nothing Then
    For Each a In Email.Attachments
        mail.Attachments.Add(a)
    Next
End If    
mail.To.Add(New MailAddress(Email.Recipient.Email, Email.Recipient.Name))
Call SmtpServer.Send(mail)

SmtpServer.Dispose()
mail.Dispose()
SmtpServer = Nothing
mail = Nothing

在该示例中，Email对象是包含基本电子邮件信息，收件人，发件人，附件等的类。

上面的代码完美无缺......

但是，我注意到SmtpServer和MailMessage都是Disposable个对象，最好将它们包装在Using语句中，因此：

Using SmtpServer As New SmtpClient() With {
    .Port = 25,
    .Host = Email.Hostname,
    .Credentials = New NetworkCredential() With {
        .UserName = Email.UserName,
        .Password = Email.Password      
    }
    }

    Using mail As New MailMessage() With {
            .From = Email.Sender,
            .Subject = Email.Subject,
            .IsBodyHtml = True,
            .Body = Email.WrappedHtml
        }
        If Not Email.Attachments Is Nothing Then
            For Each a In Email.Attachments
                mail.Attachments.Add(a)
            Next
        End If    
        mail.To.Add(New MailAddress(Email.Recipient.Email, Email.Recipient.Name))
        Call SmtpServer.Send(mail)
    End Using
End Using

然而......当我这样做的时候，我会快速连续发送几封电子邮件：

  

System.Net.Mail.SmtpException：发送邮件失败。 ---＆GT; System.IO.IOException：无法从传输连接读取数据：net_io_connectionclosed。在System.Net.Mail.SmtpReplyReaderFactory.ProcessRead（字节[]缓冲液，的Int32偏移的Int32读，布尔read430）在System.Net.Mail.SmtpReplyReaderFactory.Read430s（SmtpReplyReader呼叫者，布尔one430）在System.Net.Mail.SmtpReplyReaderFactory .Read430（SmtpReplyReader调用者）在System.Net.Mail.CheckCommand.Send（SmtpConnection conn，String＆amp; response）处于System.Net.Mail.MailCommand.Send（SmtpConnection conn，Byte []命令，MailAddress from，Boolean allowUnicode）at at System.Net.Mail.SmtpClient.Send（MailMessage消息）中的System.Net.Mail.SmtpTransport.SendMail（MailAddress sender，MailAddressCollection recipients，String deliveryNotify，Boolean allowUnicode，SmtpFailedRecipientException＆amp; exception）

Using语句似乎导致连接过早关闭....

其他人对解决方案有类似的经验/提示吗？

我已经恢复了旧代码，但更好的做法是用using语句换行，对吗？