.Net FrameWork 框架下使用System.Net.Mail封装类 发送邮件失败:服务器响应：5.7.1 Client was not authenticated 解决方案

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：

点击查看代码

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：

点击查看代码

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：

点击查看代码

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：

点击查看代码

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：

点击查看代码

 //
 // 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！😂