Open default mail client along with a attachment

We can make use of the fact that most email clients support the .EML file format to be loaded.

So if we Extend the System.Net.Mail.MailMessage Class in a way that it can be saved to the filesystem as an .EML file.
The resulting file can be opened with the default mail client using Process.Start(filename)

For this to work properly we have to add a line containing “X-Unsent: 1” to the .EML file. This line tells the email client loading the .EML file the message must be presented in “New message” mode.

Use the “addUnsentHeader” bool parameter of the extension method to add this line to the .EML file

The extension method looks like this:

using System;
using System.IO;
using System.Net.Mail;
using System.Reflection;

namespace Fsolutions.Fbase.Common.Mail
{
    public static class MailUtility
    {
        //Extension method for MailMessage to save to a file on disk
        public static void Save(this MailMessage message, string filename, bool addUnsentHeader = true)
        {
            using (var filestream = File.Open(filename, FileMode.Create))
            {
                if (addUnsentHeader)
                {
                    var binaryWriter = new BinaryWriter(filestream);
                    //Write the Unsent header to the file so the mail client knows this mail must be presented in "New message" mode
                    binaryWriter.Write(System.Text.Encoding.UTF8.GetBytes("X-Unsent: 1" + Environment.NewLine));
                }

                var assembly = typeof(SmtpClient).Assembly;
                var mailWriterType = assembly.GetType("System.Net.Mail.MailWriter");

                // Get reflection info for MailWriter contructor
                var mailWriterContructor = mailWriterType.GetConstructor(BindingFlags.Instance | BindingFlags.NonPublic, null, new[] { typeof(Stream) }, null);

                // Construct MailWriter object with our FileStream
                var mailWriter = mailWriterContructor.Invoke(new object[] { filestream });

                // Get reflection info for Send() method on MailMessage
                var sendMethod = typeof(MailMessage).GetMethod("Send", BindingFlags.Instance | BindingFlags.NonPublic);

                sendMethod.Invoke(message, BindingFlags.Instance | BindingFlags.NonPublic, null, new object[] { mailWriter, true, true }, null);

                // Finally get reflection info for Close() method on our MailWriter
                var closeMethod = mailWriter.GetType().GetMethod("Close", BindingFlags.Instance | BindingFlags.NonPublic);

                // Call close method
                closeMethod.Invoke(mailWriter, BindingFlags.Instance | BindingFlags.NonPublic, null, new object[] { }, null);
            }
        }
    }
}

Use the extension method like this:

        var mailMessage = new MailMessage();
        mailMessage.From = new MailAddress("[email protected]");
        mailMessage.Subject = "Your subject here";
        mailMessage.IsBodyHtml = true;
        mailMessage.Body = "<span style="font-size: 12pt; color: red;">My HTML formatted body</span>";

        mailMessage.Attachments.Add(new Attachment("C://Myfile.pdf"));

        var filename = "C://Temp/mymessage.eml";

        //save the MailMessage to the filesystem
        mailMessage.Save(filename);

        //Open the file with the default associated application registered on the local machine
        Process.Start(filename);