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Editorial Team
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Asked: 2026-05-29T23:03:47+00:00

In my C# .NET 4 application, I use WndProc to process some messages mostly

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In my C# .NET 4 application, I use WndProc to process some messages mostly dealing with resizing the application to and from full screen.

Right now I am just handling SC_MAXIMIZE and WM_NCLBUTTONDBLCLK to determine if the window is being resized to or from a maximized state (I know I don’t need WndProc to handle SC_MAXIMIZE, but Form_Resize didn’t seem to fire for a WM_NCLBUTTONDBLCLK message when I double-click on the application’s title bar.

Now I noticed that if I Aero Snap the window to the top of the screen to maximize it, neither of the above messages are posted so certain logic is not applied when the window is maximized via Aero Snap. I only want to handle the message if the window is snapped to the top of the screen rather than the right or left, or if a window is unsnappped from maximized position.

I couldn’t find any of the window messages related to Aero Snap. Does anyone know of any references for those messages?

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1 Answer

  1. Editorial Team

    I’m guessing there aren’t any special messages here; Aero is likely just using the plain Win32 APIs – ShowWindow(SW_MAXIMIZE) and similar.

    The thing to uderstand with the SC_ messages are that those are requests from a menu asking the window to resize/restore/etc itself, but that is not the only mechanism for changing the windows’s size. What’s probably happening is that when a window gets SC_MAXIMIZE, the DefWndProc implements this by calling ShowWindow(SW_MAXIMIZE).

    Your best best is to listen to the WM_SIZE message, which the window receives, regardless of what triggered the size change: system menu, API, or other means. In particular, the lParam will let you know if the window was maximized (SIZE_MAXIMIZED) or restored (SIZE_RESTORED).

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