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I did a notifier app in C# that sits on the taskbar and display a balloon when I get something from it, similar to Google Talk when you receive a new email but at the momment it ignores if I am running an application on fullscreen or not and just do it is job.
How can I detect and make sure an application is in fullscreen mode? I don't mean maximized (as an app could be maximized on your screen but still not in fullscreen) mode but true fullscreen.
For example when you run a game most will run in fullscreen mode.
Initially GetForegroundWindow() and GetWindowRect() came to my mind but even so I could make a window go larger then my screen if I am not on fullscreen which wouldn't help me so I am wondering if there are other flags or ways to approach this?
For PowerPoint, see this MS KB article here: http://support.microsoft.com/kb/913045
Screen Saver: Windows API: FindWindow("WindowsScreenSaverClass"). (For more details see this web article: http://bobmoore.mvps.org/Win32/w32tip22.htm - it has a more detailsd way to do it, but I think FindWindow will work ok for you)
For detecting if a full screen game is in use: IDirect3DDevice9::TestCooperativeLevel http://msdn.microsoft.com/en-us/library/bb174472(VS.85).aspx
Otherwise, I think you are on the right path with GetForegroundWindow, GetWindowRect, and other window positioning api.
Things get complicated with you have more than one monitor - so don't forget that scenario as it relates to your app.
来源：https://stackoverflow.com/questions/3536373/detect-if-user-has-any-application-running-in-fullscreen