Question

Is there any easy way to convert a System.Net.Mail.MailMessage object to the raw mail message text, like when you open a eml file in notepad.

Answer 1

Here’s the same solution, but as an extension method to MailMessage.

Some of the reflection overhead is minimized by grabbing the ConstructorInfo and MethodInfo members once in the static context.

/// <summary>
/// Uses reflection to get the raw content out of a MailMessage.
/// </summary>
public static class MailMessageExtensions
{
    private static readonly BindingFlags Flags = BindingFlags.Instance | BindingFlags.NonPublic;
    private static readonly Type MailWriter = typeof(SmtpClient).Assembly.GetType("System.Net.Mail.MailWriter");
    private static readonly ConstructorInfo MailWriterConstructor = MailWriter.GetConstructor(Flags, null, new[] { typeof(Stream) }, null);
    private static readonly MethodInfo CloseMethod = MailWriter.GetMethod("Close", Flags);
    private static readonly MethodInfo SendMethod = typeof(MailMessage).GetMethod("Send", Flags);

    /// <summary>
    /// A little hack to determine the number of parameters that we
    /// need to pass to the SaveMethod.
    /// </summary>
    private static readonly bool IsRunningInDotNetFourPointFive = SendMethod.GetParameters().Length == 3;

    /// <summary>
    /// The raw contents of this MailMessage as a MemoryStream.
    /// </summary>
    /// <param name="self">The caller.</param>
    /// <returns>A MemoryStream with the raw contents of this MailMessage.</returns>
    public static MemoryStream RawMessage(this MailMessage self)
    {
        var result = new MemoryStream();
        var mailWriter = MailWriterConstructor.Invoke(new object[] { result });
        SendMethod.Invoke(self, Flags, null, IsRunningInDotNetFourPointFive ? new[] { mailWriter, true, true } : new[] { mailWriter, true }, null);
        result = new MemoryStream(result.ToArray());
        CloseMethod.Invoke(mailWriter, Flags, null, new object[] { }, null);
        return result;
    }
}

To grab the underlying MemoryStream:

var email = new MailMessage();
using (var m = email.RawMessage()) {
    // do something with the raw message
}