You probably want to continue using MailBee... but in case you are open to alternatives, let me answer this question as if you were using my open source MimeKit/MailKit libraries instead:
When MimeKit's MIME parser encounters attachments with a Content-Type header that matches application/vnd.ms-tnef or application/ms-tnef, it will automatically use the special-purpose TnefPart class to represent that attachment.
To extract the encapsulated attachments from that, you can simply use the
ExtractAttachments() method which probably works a lot like MailBee's GetAttachmentsFromTnef() method.
The translation of your code from MailBee to MimeKit would look like this:
var attachments = message.Attachments.ToList();
var encapsulated = attachments.OfType<TnefPart>().SelectMany(a => a.ExtractAttachments ());
// Add encapsulated attachments
attachments.AddRange(encapsulated);
Most likely the reason that MailBee's IsTnef property is returning false for you, however, is probably because the Content-Type header doesn't match the tnef mime-types I mentioned earlier. If that is indeed the case, then the translated code I posted above will also fail under MimeKit.
However...
MimeKit also provides lower-level TNEF support classes that you can use if you decide to use an alternative method for determining of a MIME part contains TNEF content.
static void ExtractAttachments (MimePart attachment, IList<MimeEntity> attachments)
{
using (var reader = new TnefReader (attachment.ContentObject.Open (), 0, TnefComplianceMode.Loose)) {
// skip past the non-attachment tnef content...
while (reader.ReadNextAttribute ()) {
if (reader.AttributeLevel == TnefAttributeLevel.Attachment)
break;
}
if (reader.AttributeLevel == TnefAttributeLevel.Attachment)
ExtractAttachments (reader, attachments);
}
}
static void ExtractAttachments (TnefReader reader, IList<MimeEntity> attachments)
{
var attachMethod = TnefAttachMethod.ByValue;
var filter = new BestEncodingFilter ();
var prop = reader.TnefPropertyReader;
MimePart attachment = null;
int outIndex, outLength;
TnefAttachFlags flags;
string[] mimeType;
byte[] attachData;
string text;
do {
if (reader.AttributeLevel != TnefAttributeLevel.Attachment)
break;
switch (reader.AttributeTag) {
case TnefAttributeTag.AttachRenderData:
attachMethod = TnefAttachMethod.ByValue;
attachment = new MimePart ();
break;
case TnefAttributeTag.Attachment:
if (attachment == null)
break;
while (prop.ReadNextProperty ()) {
switch (prop.PropertyTag.Id) {
case TnefPropertyId.AttachLongFilename:
attachment.FileName = prop.ReadValueAsString ();
break;
case TnefPropertyId.AttachFilename:
if (attachment.FileName == null)
attachment.FileName = prop.ReadValueAsString ();
break;
case TnefPropertyId.AttachContentLocation:
text = prop.ReadValueAsString ();
if (Uri.IsWellFormedUriString (text, UriKind.Absolute))
attachment.ContentLocation = new Uri (text, UriKind.Absolute);
else if (Uri.IsWellFormedUriString (text, UriKind.Relative))
attachment.ContentLocation = new Uri (text, UriKind.Relative);
break;
case TnefPropertyId.AttachContentBase:
text = prop.ReadValueAsString ();
attachment.ContentBase = new Uri (text, UriKind.Absolute);
break;
case TnefPropertyId.AttachContentId:
attachment.ContentId = prop.ReadValueAsString ();
break;
case TnefPropertyId.AttachDisposition:
text = prop.ReadValueAsString ();
if (attachment.ContentDisposition == null)
attachment.ContentDisposition = new ContentDisposition (text);
else
attachment.ContentDisposition.Disposition = text;
break;
case TnefPropertyId.AttachData:
var stream = prop.GetRawValueReadStream ();
var content = new MemoryStream ();
var guid = new byte[16];
if (attachMethod == TnefAttachMethod.EmbeddedMessage) {
var tnef = new TnefPart ();
foreach (var param in attachment.ContentType.Parameters)
tnef.ContentType.Parameters[param.Name] = param.Value;
if (attachment.ContentDisposition != null)
tnef.ContentDisposition = attachment.ContentDisposition;
attachment = tnef;
}
// read the GUID
stream.Read (guid, 0, 16);
// the rest is content
using (var filtered = new FilteredStream (content)) {
filtered.Add (filter);
stream.CopyTo (filtered, 4096);
filtered.Flush ();
}
content.Position = 0;
attachment.ContentTransferEncoding = filter.GetBestEncoding (EncodingConstraint.SevenBit);
attachment.ContentObject = new ContentObject (content);
filter.Reset ();
attachments.Add (attachment);
break;
case TnefPropertyId.AttachMethod:
attachMethod = (TnefAttachMethod) prop.ReadValueAsInt32 ();
break;
case TnefPropertyId.AttachMimeTag:
mimeType = prop.ReadValueAsString ().Split ('/');
if (mimeType.Length == 2) {
attachment.ContentType.MediaType = mimeType[0].Trim ();
attachment.ContentType.MediaSubtype = mimeType[1].Trim ();
}
break;
case TnefPropertyId.AttachFlags:
flags = (TnefAttachFlags) prop.ReadValueAsInt32 ();
if ((flags & TnefAttachFlags.RenderedInBody) != 0) {
if (attachment.ContentDisposition == null)
attachment.ContentDisposition = new ContentDisposition (ContentDisposition.Inline);
else
attachment.ContentDisposition.Disposition = ContentDisposition.Inline;
}
break;
case TnefPropertyId.AttachSize:
if (attachment.ContentDisposition == null)
attachment.ContentDisposition = new ContentDisposition ();
attachment.ContentDisposition.Size = prop.ReadValueAsInt64 ();
break;
case TnefPropertyId.DisplayName:
attachment.ContentType.Name = prop.ReadValueAsString ();
break;
}
}
break;
case TnefAttributeTag.AttachCreateDate:
if (attachment != null) {
if (attachment.ContentDisposition == null)
attachment.ContentDisposition = new ContentDisposition ();
attachment.ContentDisposition.CreationDate = prop.ReadValueAsDateTime ();
}
break;
case TnefAttributeTag.AttachModifyDate:
if (attachment != null) {
if (attachment.ContentDisposition == null)
attachment.ContentDisposition = new ContentDisposition ();
attachment.ContentDisposition.ModificationDate = prop.ReadValueAsDateTime ();
}
break;
case TnefAttributeTag.AttachTitle:
if (attachment != null && string.IsNullOrEmpty (attachment.FileName))
attachment.FileName = prop.ReadValueAsString ();
break;
case TnefAttributeTag.AttachMetaFile:
if (attachment == null)
break;
// TODO: what to do with the meta data?
break;
case TnefAttributeTag.AttachData:
if (attachment == null || attachMethod != TnefAttachMethod.ByValue)
break;
attachData = prop.ReadValueAsBytes ();
filter.Flush (attachData, 0, attachData.Length, out outIndex, out outLength);
attachment.ContentTransferEncoding = filter.GetBestEncoding (EncodingConstraint.EightBit);
attachment.ContentObject = new ContentObject (new MemoryStream (attachData, false));
filter.Reset ();
attachments.Add (attachment);
break;
}
} while (reader.ReadNextAttribute ());
}
Of course... you could also just cheat by doing this:
var tnef = new TnefPart { ContentObject = attachment.ContentObject };
attachments.AddRange (tnef.ExtractAttachments ());
So the end result might look like this (if you decide to match against the FileName instead):
var attachments = message.Attachments.ToList();
var encapsulated = attachments.OfType<MimePart>()
.Where(x => (x is TnefPart) || x.FileName == "winmail.dat")
.SelectMany(x => {
var tnef = (x as TnefPart) ?? new TnefPart { ContentObject = x.ContentObject };
return tnef.ExtractAttachments ();
});
// Add encapsulated attachments
attachments.AddRange(encapsulated);
