'WM_ACTIVATEAPP' tag RSSI recently had a requirement where a user was able to perform an action externally to my application, and my application then had to detect this for processing.
I could of course just had a poller running away in the background to check, but as the requirement also needed user input, why not just wait until the user switched back to my application, then check and deal with accordingly?
This article describes how to intercept the WM_ACTIVATEAPP message from your C# application and put it to good use.