(The strangely-phrased subject line is for search engine optimization.)

A customer reported that when they called ShellExecute, the function sometimes fails with SE_ERR_ACCESSDENIED, depending on what they are trying to execute. (If they had tried ShellExecuteEx they would have gotten the error ERROR_ACCESS_DENIED.)

After a good amount of back-and-forth examing file type registrations, a member of the development team had psychic insight to ask, "Are you calling it from an MTA?"

"Yes," the customer replied. "ShellExecute is being called from a dedicated MTA thread. Would that cause the failure?"

Why yes, as a matter of fact, and it's called out in the documentation for ShellExecute.

Because ShellExecute can delegate execution to Shell extensions (data sources, context menu handlers, verb implementations) that are activated using Component Object Model (COM), COM should be initialized before ShellExecute is called. Some Shell extensions require the COM single-threaded apartment (STA) type.

As a general rule, shell functions require STA. Recall that MTA implies no user interface. If you try to use an apartment-threaded object from your MTA thread, a marshaller is required, and if no such marshaller exists, the call fails.

This also explains why the failure occurs only for certain file types: If handling the file type happens not to involve creating a COM object, then the MTA/STA mismatch situation never occurs.