Application stuck in full screen?

Question

To reproduce my problem please do the following:

1. Create a new Windows Form Application in C#.
2. In the Properties window of Form1 set FormBorderSt

Answer 1

Windows does this by calling SetWindowPos() to change the position and size of the window. A window can be notified about this by listening for the WM_WINDOWPOSCHANGING message and override the settings. Lots of things you can do, like still giving the operation a meaning by adjusting the size and position to your liking. You completely prevent it by turning on the NOSIZE and NOMOVE flags.

Paste this code into your form:

    private bool AllowWindowChange;

    private struct WINDOWPOS {
        public IntPtr hwnd, hwndInsertAfter;
        public int x, y, cx, cy;
        public int flags;
    }

    protected override void WndProc(ref Message m) {
        // Trap WM_WINDOWPOSCHANGING
        if (m.Msg == 0x46 && !AllowWindowChange) {
            var wpos = (WINDOWPOS)System.Runtime.InteropServices.Marshal.PtrToStructure(m.LParam, typeof(WINDOWPOS));
            wpos.flags |= 0x03; // Turn on SWP_NOSIZE | SWP_NOMOVE
            System.Runtime.InteropServices.Marshal.StructureToPtr(wpos, m.LParam, false);
        }
        base.WndProc(ref m);
    }

When you want to change the window yourself, simply set the AllowWindowChange field temporarily to true.