How to make outlook mail attachment from memory?

Question

Adding new System.Net.Mail.Attachment to Outlook.MailItem.Attachments via Attachments.Add() results in an System.ArgumentException: 'Sorry, something went wrong. You may want to try again.'

Trying to add a JPEG image encoded in Base64 as an attachment to a mail item in Outlook. I am storing encoded image as a variable, converting it to a memory stream, then to an attachment.

public void CreateMessageWithAttachment() {
    Outlook.MailItem mailIttem = thisAddIn.Application.CreateItem(Outlook.OlItemType.olMailItem);
    string base64Attachment = "/...base64 gibberish";
    MemoryStream ms = new MemoryStream(Convert.FromBase64String(base64Attachment));
    ContentType ct = new ContentType(MediaTypeNames.Image.Jpeg);
    Attachment attachment = new Attachment(ms, ct);

    attachment.ContentDisposition.FileName = "look_at_dis.jpg";
    mailIttem.Subject = "Test e-mail message with attachment";
    mailIttem.To = "[email protected]";
    mailIttem.Body = "This message has been generated programmatically";
    mailIttem.Attachments.Add(attachment); // This raises "Sorry..." expression
    mailIttem.Display(true);
}

Raises System.ArgumentException: 'Sorry, something went wrong. You may want to try again.', which tells me nothing :-/

Answer 1

MailItem.Attachments.Add only allows to pass a string (a fully qualified path to a file) or another Outlook item (e.g. MailItem) as the parameter.

On the Extended MAPI level (C++ or Delphi only), it is only taking an IStream (you are supposed to open PR_ATTACH_DATA_BIN as IStream using IAttach::OpenProperty). If using Redemption (I am its author) is an option, it allows to pass a url, a file name, another Outlook item, IStream or IStorage COM interface, another attachment (Outlook.Attachment or Redemption.RDOAttachment or IAttach MAPI interface) or an array (of Variant or of byte) to RDOMail.Attachments.Add

Answer 2

Official docs gave me an impression that Attachments.Add is only supposed to really work with file paths, so saving MemoryStream to a temporary file and attaching it solved the problem.

string tempFilePath = Path.GetTempPath() + "look_at_dis.jpg";
FileStream fs = new FileStream(tempFilePath, FileMode.Create);

ms.CopyTo(fs);
fs.Close();

mailIttem.Attachments.Add(tempFilePath, Outlook.OlAttachmentType.olByValue, 1, "look_at_dis.jpg");