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 == )
{
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。