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偶然兴起，想做一个后台监控PLC状态的服务。功能如下：监控到PLC状态值异常后触发邮件推送，状态改变后只推送一次。开始使用的是.net6.0开发框架开发，一切都很顺利，邮件也能正常推送。但由于现场工控机系统不是WIN10 20H2的最新版本，导致系统未安装.Net6.0 Runtime。而我也没有再去安装的打算。我重新使用了.net FrameWork4.7 框架进行开发。开发完成后，我以为能正常运行。但出现了不可预知的错误——服务器响应：5.7.1 Client was not authenticated。下面分别是2个框架下发送邮件的代码：
.Net 6.0框架：
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public bool Send() { try { SmtpClient smtp = new SmtpClient(this.Host!, (int)this.Port!) { Credentials = new NetworkCredential(this.User!, this.Password!), EnableSsl = true, UseDefaultCredentials = false }; MailMessage message = new MailMessage(this.SenderAddress!, this.ReciverAddress!, this.Subject, this.Body) { From = new MailAddress(this.SenderAddress!, this.SenderName) }; ServicePointManager.ServerCertificateValidationCallback = delegate (object obj, X509Certificate certificate, X509Chain chain, SslPolicyErrors errors) { return true; }; smtp.Send(message); return true; } catch (Exception) { return false; } } .Net FrameWork 4.7 框架：
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public bool Send() { try { SmtpClient smtp = new SmtpClient(this.Host!, (int)this.Port!) { Credentials = new NetworkCredential(this.User!, this.Password!), EnableSsl = true, UseDefaultCredentials = false }; MailMessage message = new MailMessage(this.SenderAddress!, this.ReciverAddress!, this.Subject, this.Body) { From = new MailAddress(this.SenderAddress!, this.SenderName) }; ServicePointManager.ServerCertificateValidationCallback = delegate (object obj, X509Certificate certificate, X509Chain chain, SslPolicyErrors errors) { return true; }; smtp.Send(message); return true; } catch (Exception) { return false; } } 在不同的开发框架下，使用的代码完全一致。但是在.Net FrameWork 4.7 框架下发送邮件才会出现异常：**5.7.1 Client was not authenticated**，而.Net 6.0 环境下不会。
我不得不怀疑是不是微软的封装类System.Net.Mail存在问题。经过断点调试，终于发现了2个环境发送邮件时存在的差异。
.Net 6.0框架下用户传入的凭证（账号密码）SMTP服务器可正常获取到
.Net FrameWork 框架下竟然获取的凭证为空
经过短暂思考，我决定修改下.Net FrameWork框架下的开发代码。如下：
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public bool Send() { try { smtp = new SmtpClient(this.Host, (int)this.Port); smtp.Credentials = new NetworkCredential(this.User,this.Password); smtp.EnableSsl = true; //smtp.UseDefaultCredentials = false; MailMessage message = new MailMessage(this.SenderAddress, this.ReciverAddress, this.Subject, this.Body) { From = new MailAddress(this.SenderAddress, this.SenderName) }; ServicePointManager.ServerCertificateValidationCallback = delegate (object obj, X509Certificate certificate, X509Chain chain, SslPolicyErrors errors) { return true; }; smtp.Send(message); return true; } catch (Exception) { return false; } } 上述代码中，取消了UseDefaultCredentials属性的使用。运行后，邮件发送正常。 那么问题来了，为什么使用了UseDefaultCredentials属性就会导致Credentials 凭证为空呢？看下.Net FrameWork框架System.Net.Mail源码：
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public bool UseDefaultCredentials { get { if (!(transport.Credentials is SystemNetworkCredential)) { return false; } return true; } set { if (InCall) { throw new InvalidOperationException(SR.GetString("SmtpInvalidOperationDuringSend")); } transport.Credentials = (value ? CredentialCache.DefaultNetworkCredentials : null); } } 再对比下.net6.0 框架System.Net.Mail源码：
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// // 摘要: // Gets or sets a System.Boolean value that controls whether the System.Net.CredentialCache.DefaultCredentials // are sent with requests. // // 返回结果: // true if the default credentials are used; otherwise false. The default value // is false. // // 异常: // T:System.InvalidOperationException: // You cannot change the value of this property when an email is being sent. public bool UseDefaultCredentials { get; set; } 水落石出，微软误我啊！?
