Windows Service starts multiple instances of out-of-process local COM Server

04 July 2018

Here is a problem I can’t solve. I have a 32-bit client application which calls in to a third party out-of-process COM server (also 32-bit). My client application is a C# app built on Topshelf, so it can run either as a command line app or as a Windows Service.

• When I run my client application as an exe, as the same user as the COM Server, it connects fine.
• When I run my client as a Windows Service, even if I specify RunAs options to make it use the same user as the COM Server, a new instance of the COM Server exe is started.

Because of the implementation details of the COM Server, this means that I can’t use it with my Windows Service, because it detects a licensing violation (single instance only) and refuses to run.

A number of other people have spotted this behaviour where Windows Services in particular cause COM-serving apps to start additional instances:

• A comment on this Larry Osterman blog post about COM activiation
• This StackOverflow question (and plenty of others)

Things I’ve tried:

• Running as same user
• Ensuring same architecture (both server and client are x86)
• Trying different CLSCTX options, like CLSCTX_ENABLE_AAA and CLSCTX_DISABLE_AAA (combined as flags with CLSCTX_LOCAL_SERVER)

I have no solution, so if you have any ideas, please tweet me @SteGriff or email ste at this domain :)