BodyPartBasic Class

public class BodyPartBasic : BodyPart

public static void DownloadBodyAndAttachments (string baseDirectory)
{
    using (var client = new ImapClient ()) {
        client.Connect ("imap.gmail.com", 993, SecureSocketOptions.SslOnConnect);

        client.Authenticate ("username", "password");

        client.Inbox.Open (FolderAccess.ReadOnly);

        // search for messages where the Subject header contains either "MimeKit" or "MailKit"
        var query = SearchQuery.SubjectContains ("MimeKit").Or (SearchQuery.SubjectContains ("MailKit"));
        var uids = client.Inbox.Search (query);

        // fetch summary information for the search results (we will want the UID and the BODYSTRUCTURE
        // of each message so that we can extract the text body and the attachments)
        var items = client.Inbox.Fetch (uids, MessageSummaryItems.UniqueId | MessageSummaryItems.BodyStructure);

        foreach (var item in items) {
            // determine a directory to save stuff in
            var directory = Path.Combine (baseDirectory, item.UniqueId.ToString ());

            // create the directory
            Directory.CreateDirectory (directory);

            // IMessageSummary.TextBody is a convenience property that finds the 'text/plain' body part for us
            var bodyPart = item.TextBody;

            if (bodyPart != null) {
                // download the 'text/plain' body part
                var plain = (TextPart) client.Inbox.GetBodyPart (item.UniqueId, bodyPart);

                // TextPart.Text is a convenience property that decodes the content and converts the result to
                // a string for us
                var text = plain.Text;

                File.WriteAllText (Path.Combine (directory, "body.txt"), text);
            }

            // IMessageSummary.HtmlBody is a convenience property that finds the 'text/html' body part for us
            bodyPart = item.HtmlBody;

            if (bodyPart != null) {
                // download the 'text/html' body part
                var html = (TextPart) client.Inbox.GetBodyPart (item.UniqueId, bodyPart);

                // TextPart.Text is a convenience property that decodes the content and converts the result to
                // a string for us
                var text = html.Text;

                File.WriteAllText (Path.Combine (directory, "body.html"), text);
            }

            // now iterate over all of the attachments and save them to disk
            foreach (var attachment in item.Attachments) {
                // download the attachment just like we did with the body
                var entity = client.Inbox.GetBodyPart (item.UniqueId, attachment);

                // attachments can be either message/rfc822 parts or regular MIME parts
                if (entity is MessagePart) {
                    var rfc822 = (MessagePart) entity;

                    var path = Path.Combine (directory, attachment.PartSpecifier + ".eml");

                    rfc822.Message.WriteTo (path);
                } else {
                    var part = (MimePart) entity;

                    // default to using the sending client's suggested fileName value
                    var fileName = attachment.FileName;

                    if (string.IsNullOrEmpty (fileName)) {
                        // the FileName wasn't defined, so generate one...
                        if (!MimeTypes.TryGetExtension (attachment.ContentType.MimeType, out string extension))
                            extension = ".dat";

                        fileName = Guid.NewGuid ().ToString () + extension;
                    }

                    var path = Path.Combine (directory, fileName);

                    // decode and save the content to a file
                    using (var stream = File.Create (path))
                        part.Content.DecodeTo (stream);
                }
            }
        }

        client.Disconnect (true);
    }
}