Microsoft.Office.Interop.Word.Document.set_AttachedTemplate() locks template file

Question

Our c# UI application is using Microsoft.Office.Interop heavily to integrate Microsoft Office products visually.
We switched from the legacy document formats to the new OOXML document formats.

Now the behaviour for Microsoft.Office.Interop.Word.Document.set_AttachedTemplate() seems to have changed:
it locks the template file

Previously this wasn't the case. In our application, multiple documents get opened, the second document gets the locked file warning now.
How to prevent this?

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I traced the filesystem operations using Process Monitor and I can see a difference at the end: dotx has no CloseFile operation at the end, while dot has

dot template gets closed:

dotx template doesn't get closed:

Answer 1

We solved this using a workaround:

1. Open the file with Aspose.Words
2. Set AttachedTemplate
3. Save and close the file
4. Open the file with Microsoft.Office.Interop.Word

It isn't pretty, but it works.