最近因故需要用.Net（C#）将邮件信息保存为本地eml文件，看起来是个费劲的活：首先得读懂MIME协议（RFC 2045~2049），然后写代码。还好，google后发现以前有朋友也有过这样的需求，并有现成解决方案，呵呵：System.Net.Mail命名空间里已经实现了MailWriter，只不知道为啥没有公开而已。

用Xenocode Fox 2007（Community Edition）加载System.dll，找到System.Net.Mail.SmtpClient类，可看到其Send(MailMessage)方法中有这么两句代码：

MailWriter writer;

writer = GetFileMailWriter(PickupDirectoryLocation);

我们来看看GetFileMailWriter方法：

internal MailWriter GetFileMailWriter (string pickupDirectory)
{
  string path;
  ...

  do
  {
    Guid guid1 = Guid.NewGuid();
    string path2 = guid1.ToString() + ".eml";
    path = Path.Combine(pickupDirectory, path2);
  }
  while (File.Exists(path));
  return new MailWriter(new FileStream(path, FileMode.CreateNew));
}

再往深里看便可知：MailWriter直接将MailMessage内容写成了“.eml”文件，而且是按MIME协议写入的，Yeah！

那接下来要做的就是用上MailWriter这个内部（internal）类，当然这是很简单的事：

public static byte[] MailMessageToArray(MailMessage msg)
{
    const BindingFlags flags = BindingFlags.Instance | BindingFlags.NonPublic | BindingFlags.FlattenHierarchy;
    using (var ms = new MemoryStream())
    {
        var assembly = typeof (System.Net.Mail.SmtpClient).Assembly;
        var writerType = assembly.GetType("System.Net.Mail.MailWriter");
        var writer = Activator.CreateInstance(writerType, flags, null, new object[] {ms},
                                              CultureInfo.InvariantCulture);
        msg.GetType().GetMethod("Send", flags).Invoke(msg, new[] {writer, true});
        return ms.ToArray();
    }
}

如果我们使用的是C#3.0以后的版本，还可以为MailMessage添加扩展方法，这样使用起来更为自然。代码如下：

//定义扩展方法

public static class MailMessageExtend
{
    public static byte[] ToArray(this MailMessage msg)
    {
        const BindingFlags flags = BindingFlags.Instance | BindingFlags.NonPublic | BindingFlags.FlattenHierarchy;
        using (var ms = new MemoryStream())
        {
            var assembly = typeof(System.Net.Mail.SmtpClient).Assembly;
            var writerType = assembly.GetType("System.Net.Mail.MailWriter");
            var writer = Activator.CreateInstance(writerType, flags, null, new object[] { ms },
                                                  CultureInfo.InvariantCulture);
            msg.GetType().GetMethod("Send", flags).Invoke(msg, new[] { writer, true });
            return ms.ToArray();
        }
    }

}

//调用

private void button1_Click(object sender, EventArgs e)
{
    var msg = new MailMessage();
    ...
    var content = msg.ToArray();
    ...
}