SmtpClient发邮件时为什么用MailMessage.From而不用MailMessage.Sender

今天在看C#高级编程（第9版）的时候，在768页看到这样的一段代码

SmtpClient sc = new SmtpClient();
sc.Host = "邮箱服务器地址";
MailMessage mm = new MailMessage();
mm.Sender = new MailAddress("公司邮箱", "发件人");
mm.To.Add(new MailAddress("我的163邮箱", "接收人"));
mm.CC.Add(new MailAddress("抄送的邮箱", "抄送人"));
mm.Subject = "测试程序发送邮件";
mm.Body = "<b>我发送了邮件---我是程序发出的</b>";
mm.IsBodyHtml = true;
mm.Priority = MailPriority.High;
sc.Send(mm);

以前没有做过Email项目，所以，直接就敲到了VS下，编译，但是出问题，报了个错

我心思是没有指定发件人的用户名和密码，因此又增加了一行代码

sc.Credentials = new System.Net.NetworkCredential("发件邮箱", "密码");

但是依然报这个错误，继续找办法吧，看了看网上其他人发邮件的代码，发现他们用的是From，因此，又增加了一行代码

mm.From = new MailAddress("公司邮箱", "发件人");

发现顿时就好使了，而且两个可以同时存在，或单独From存在，都好使。

这……我就郁闷了，因为F12定位到定义，From和Sender的注解里面只差了一个字

一个是发信人，一个是发件人，而且From是发信人，Sender是发件人，一字之差不应该Sender不好使，From好使啊，疑惑增加ing……

那就用ILSpy反编译看一下吧

我擦，更加疑惑，不可能只是因为From比Sender多了一个判断是否为null就用From吧。

那就继续看反编译的代码吧。在SmtpClient里找到Send方法，代码如下

public void Send(MailMessage message)
        {
            if (Logging.On)
            {
                Logging.Enter(Logging.Web, this, "Send", message);
            }
            if (this.disposed)
            {
                throw new ObjectDisposedException(base.GetType().FullName);
            }
            try
            {
                if (Logging.On)
                {
                    Logging.PrintInfo(Logging.Web, this, "Send", "DeliveryMethod=" + this.DeliveryMethod.ToString());
                }
                if (Logging.On)
                {
                    Logging.Associate(Logging.Web, this, message);
                }
                SmtpFailedRecipientException ex = null;
                if (this.InCall)
                {
                    throw new InvalidOperationException(SR.GetString("net_inasync"));
                }
                if (message == null)
                {
                    throw new ArgumentNullException("message");
                }
                if (this.DeliveryMethod == SmtpDeliveryMethod.Network)
                {
                    this.CheckHostAndPort();
                }
                MailAddressCollection mailAddressCollection = new MailAddressCollection();
                if (message.From == null)
                {
                    throw new InvalidOperationException(SR.GetString("SmtpFromRequired"));
                }
                if (message.To != null)
                {
                    foreach (MailAddress current in message.To)
                    {
                        mailAddressCollection.Add(current);
                    }
                }
                if (message.Bcc != null)
                {
                    foreach (MailAddress current2 in message.Bcc)
                    {
                        mailAddressCollection.Add(current2);
                    }
                }
                if (message.CC != null)
                {
                    foreach (MailAddress current3 in message.CC)
                    {
                        mailAddressCollection.Add(current3);
                    }
                }
                if (mailAddressCollection.Count == 0)
                {
                    throw new InvalidOperationException(SR.GetString("SmtpRecipientRequired"));
                }
                this.transport.IdentityRequired = false;
                try
                {
                    this.InCall = true;
                    this.timedOut = false;
                    this.timer = new Timer(new TimerCallback(this.TimeOutCallback), null, this.Timeout, this.Timeout);
                    string pickupDirectory = this.PickupDirectoryLocation;
                    switch (this.DeliveryMethod)
                    {
                    case SmtpDeliveryMethod.Network:
                        goto IL_235;
                    case SmtpDeliveryMethod.SpecifiedPickupDirectory:
                        break;
                    case SmtpDeliveryMethod.PickupDirectoryFromIis:
                        pickupDirectory = IisPickupDirectory.GetPickupDirectory();
                        break;
                    default:
                        goto IL_235;
                    }
                    if (this.EnableSsl)
                    {
                        throw new SmtpException(SR.GetString("SmtpPickupDirectoryDoesnotSupportSsl"));
                    }
                    bool allowUnicode = this.IsUnicodeSupported();
                    this.ValidateUnicodeRequirement(message, mailAddressCollection, allowUnicode);
                    MailWriter mailWriter = this.GetFileMailWriter(pickupDirectory);
                    goto IL_275;
                    IL_235:
                    this.GetConnection();
                    allowUnicode = this.IsUnicodeSupported();
                    this.ValidateUnicodeRequirement(message, mailAddressCollection, allowUnicode);
                    mailWriter = this.transport.SendMail(message.Sender ?? message.From, mailAddressCollection, message.BuildDeliveryStatusNotificationString(), allowUnicode, out ex);
                    IL_275:
                    this.message = message;
                    message.Send(mailWriter, this.DeliveryMethod > SmtpDeliveryMethod.Network, allowUnicode);
                    mailWriter.Close();
                    this.transport.ReleaseConnection();
                    if (this.DeliveryMethod == SmtpDeliveryMethod.Network && ex != null)
                    {
                        throw ex;
                    }
                }
                catch (Exception ex2)
                {
                    if (Logging.On)
                    {
                        Logging.Exception(Logging.Web, this, "Send", ex2);
                    }
                    if (ex2 is SmtpFailedRecipientException && !((SmtpFailedRecipientException)ex2).fatal)
                    {
                        throw;
                    }
                    this.Abort();
                    if (this.timedOut)
                    {
                        throw new SmtpException(SR.GetString("net_timeout"));
                    }
                    if (ex2 is SecurityException || ex2 is AuthenticationException || ex2 is SmtpException)
                    {
                        throw;
                    }
                    throw new SmtpException(SR.GetString("SmtpSendMailFailure"), ex2);
                }
                finally
                {
                    this.InCall = false;
                    if (this.timer != null)
                    {
                        this.timer.Dispose();
                    }
                }
            }
            finally
            {
                if (Logging.On)
                {
                    Logging.Exit(Logging.Web, this, "Send", null);
                }
            }
        }

看代码里我注红色的部分，可以发现，Send方法，必须要求From存在，如果From不存在的话，就会抛出异常，但是并没有强制要求Sender存在，Sender可以存在，如果Sender存在就用Sender，不存在就用From，但是必须保证From存在。