Calling Shell32.dll from .NET Windows Service

Question

I have a .NET 4.0 library that uses Shell32 and Folder.GetDetailsOf() to get metadata from WTV files. I have used it successfully with Console and Windows Forms apps without issue. But for some reason, when calling the component from a .NET 4.0 Windows Service, the call to initiate the Shell class causes a COM error.

The code that fails inside the library:

Shell32.Shell shell = new Shell();

The error:

Unable to cast COM object of type 'System.__ComObject' to interface type 'Shell32.Shell'. This operation failed because the QueryInterface call on the COM component for the interface with IID '{286E6F1B-7113-4355-9562-96B7E9D64C54}' failed due to the following error: No such interface supported (Exception from HRESULT: 0x80004002 (E_NOINTERFACE)).

I read my fill of Apartment Threading, COM Interops, Dynamic, PIA's, etc, etc, etc :) But no combination of solutions I've found has solved the problem. It must be a calling from another thread that can't see the Interop. Help, please :)

Answer 1

I had the same problem just recently with a command line application (console). Turns out, it was required to annotate the program's Main() method with the [STAThread] attribute. It has also been noted that it fails miserably in the exact same way if the entry point is annotated with [MTAThread] instead. I hope it helps.

Answer 2

I suspect this may be related to the fact that, by default, a Windows Service does not have permission to interact with the desktop.

To test that theory, reconfigure (at least on a temporary basis) your service permissions to allow for desktop interaction. The following link walks you through doing that

https://superuser.com/questions/415204/how-do-i-allow-interactive-services-in-windows-7

UPDATE

The Shell32 functionality works just fine as LocalSystem, even when the "Allow service to interact with desktop" checkbox is unchecked, but doesn't seem to work at all under a specific user account (whether limited or admin)

Using SHFileOperation within a Windows service

If you succeed in getting this to work, make sure that you suppress any UI interaction. Information on how to do that is available in this answer:

https://stackoverflow.com/a/202519/141172