Multipart Class

public class Multipart : MimeEntity, IMultipart, 
	IMimeEntity, IDisposable, ICollection<MimeEntity>, IEnumerable<MimeEntity>, 
	IEnumerable, IList<MimeEntity>

                            public static MimeMessage CreateMessageWithAttachment (string path)
        {
            var message = new MimeMessage ();
            message.From.Add (new MailboxAddress ("Joey", "joey@friends.com"));
            message.To.Add (new MailboxAddress ("Alice", "alice@wonderland.com"));
            message.Subject = "How you doin?";

            var body = new TextPart (TextFormat.Plain) {
                Text = @"Hey Alice,

What are you up to this weekend? Monica is throwing one of her parties on
Saturday and I was hoping you could make it.

Will you be my +1?

-- Joey
"
            };

            var attachment = new MimePart (MimeTypes.GetMimeType (path)) {
                Content = new MimeContent (File.OpenRead (path)),
                ContentDisposition = new ContentDisposition (ContentDisposition.Attachment),
                ContentTransferEncoding = ContentEncoding.Base64,
                FileName = Path.GetFileName (path)
            };

            // A multipart/mixed is a container for a list of independent MIME parts. The
            // message body is conventionally the first child, followed by the attachments.
            // A boundary string is generated automatically.
            var multipart = new Multipart ("mixed");
            multipart.Add (body);
            multipart.Add (attachment);

            message.Body = multipart;

            return message;
        }
                  

                    public static void RemoveAttachments (MimeMessage message)
{
    if (message.Body is Multipart multipart)
        RemoveAttachments (multipart);
}

static void RemoveAttachments (Multipart multipart)
{
    // A Multipart is an IList<MimeEntity>, so its children can be added, removed or
    // replaced just like any other list. Iterate in reverse so that removing an item
    // does not affect the indexes of the children not yet visited.
    for (int i = multipart.Count - 1; i >= 0; i--) {
        var child = multipart[i];

        if (child is Multipart nested) {
            // Recurse into nested multiparts such as multipart/alternative.
            RemoveAttachments (nested);
        } else if (child.IsAttachment) {
            // Replace the attachment with a short note so that the recipient knows that
            // something was removed.
            var fileName = (child as MimePart)?.FileName ?? "attachment";

            multipart[i] = new TextPart (TextFormat.Plain) {
                Text = string.Format ("The attachment \"{0}\" has been removed.", fileName)
            };
        }
    }
}