>I created a small executable to run a process in the background, so the application does not hang while the process is executed. A timer then checks for the file created and shows the result. I did not use ParallelFox, because I did not want to register any additional libraries at the clients, but not sure if I can succeed.
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>Because there is one small problem: When starting the process (using api_Apprun with "HID") the main application loses its focus for a short time. That can be annoying when you are typing or doing anything, and then for a split second the focus goes away, and that every few minutes. So I wonder if there is a way to run the process without losing focus of the calling application, or if there is a way to use ParallelFox without registering, or perhaps another idea?

You can run the process as a service. Or like I used to do, run it on a server or a separate machine. All "communication" with the process is done by sending a file with "instructions", and the process responds by creating a file with the "answers".