There is an ongoing problem with app-to-app communications between UWP apps and Outlook Desktop versions.
The problem arises when creating attachments in UWP and then trying to display and use them in desktop version email clients such as Outlook 2013 or 2016.
It's relatively trivial to create an email and attachment in UWP app and display it to the user with the following code.
EmailMessage emailMessage = new EmailMessage();
emailMessage.Body = "Text for mail body here";
var attachment = new EmailAttachment("myattachment.jpg", attachmentContentsStreamRef);
emailMessage.Attachments.Add(attachment);
await EmailManager.ShowComposeNewEmailAsync(emailMessage);
This works if the Windows 10 Mail app is the user's default email client. If however, the user has a desktop email client set as default, it fails to generate the mail attachment correctly and displays a blank email.
This has been queried many times on various forums, but the general reply from Microsoft representatives seems to be that "it may need some time before this issue can be fixed."
See following links for representative sample.
Add attachment to desktop mail app
How to attach file programatically as Email attachment using C# for Windows App store
Is it possible to attach Attachements to mailto app
Sending email attachments via UWP EmailManager not working
There seems to be no work-around unless you use Brokered Windows Runtime Components but this requires users to sideload your UWP app and install a component via Powerscript. Not practical in the real world of non-tech users.
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My thoughts so far
If the user is using a Microsoft web-based message store (such as Office 365/Hotmail.com/Outlook.com etc), it looks like it's going to be possible to generate the mail and attachment using the Outlook Mail REST API. This might be a solution for these cases, but it would seem much more reasonable to communicate app-to-app locally rather than via a remote server, and the user experience will depend largely on how quickly the client syncs with the server. On the other hand, if the user's email is POP3-based we're basically out of luck.
Alternatively, I'm speculating that it might be possible to write an add-in for the Outlook Desktop version to enable it to expose a UWP compatible share target interface and transfer attachments from the UWP app that way? But it's not clear to me how you might go about trying to implement that.
Has anyone found a workable solution to the problem?
