EWS - FileAttachment Content is empty / byte[0] - exchangewebservices

Question

I have written a WebAPI controller method that finds a mail by its unique ID from ExchangeOnline. I wrote a small model class in order to store some attributes of a mail like the subject, the sender, the date received and so on.
Now I also want to access file attachments if the mail has such attachments. Therefore, I wrote this code (just the relevant part):
List<AttachmentItem> attDataContainer = new List<AttachmentItem>();
EmailMessage originalMail = EmailMessage.Bind(service, new ItemId(uniqueID), new PropertySet(ItemSchema.Attachments));
foreach (Attachment att in originalMail.Attachments)
{
if (att is FileAttachment && !att.IsInline)
{
FileAttachment fa = att as FileAttachment;
fa.Load();
attDataContainer.Add(
new AttachmentItem
{
ID = fa.Id,
Name = fa.Name,
ContentID = fa.ContentId,
ContentType = fa.ContentType,
ContentLocation = fa.ContentLocation,
Content = Convert.ToBase64String(fa.Content),
Size = fa.Size
});
}
}
The method indeed finds the attachments and displays all of the attributes you can see in the "AttachmentItem" object - BUT NOT the fa.Content attribute.
I have crwaled almost any document I could find on this (especially the *.Load() part as well as much examples. But in my case I get "byte[0]" when debugging the output.
Do you have any idea for me what could be the reason for this?
PS: By the way, I have version v2.0.50727 of Microsoft.Exchange.WebServices referenced.
Best regards and thanks in advance,
Marco

Answer 1

When you call the load method on the Attachment that should make a GetAttachment request to the server which will return the data for that Attachment. If you enable tracing https://msdn.microsoft.com/en-us/library/office/dn495632(v=exchg.150).aspx that should allow you to follow the underlying SOAP requests which should help with troubleshooting (eg you can see what the server is returning which is important).
The other thing to check is that is this is a real attachment vs a One Drive attachment which could be the case on ExchangeOnline.