Properly maximizing WPF window with WindowStyle=None

Question

There are two problems with WPF windows when the WindowStyle=None option is used.

1. The window covers the Taskbar when maximized.
2. Once maximized, the wi

Answer 1

leebickmtu's answer is basically correct, but has some extraneous code and picks the monitor based one where the cursor is, rather than where the window is. That would have the wrong behavior if the mouse is on a different monitor and the user presses Win+Up to maximize. We should be using MonitorFromWindow to identify the monitor to maximize to.

Put the following in your window codebehind:

protected override void OnSourceInitialized(EventArgs e)
{
    base.OnSourceInitialized(e);
    ((HwndSource)PresentationSource.FromVisual(this)).AddHook(HookProc);
}

public static IntPtr HookProc(IntPtr hwnd, int msg, IntPtr wParam, IntPtr lParam, ref bool handled)
{
    if (msg == WM_GETMINMAXINFO)
    {
        // We need to tell the system what our size should be when maximized. Otherwise it will
        // cover the whole screen, including the task bar.
        MINMAXINFO mmi = (MINMAXINFO)Marshal.PtrToStructure(lParam, typeof(MINMAXINFO));

        // Adjust the maximized size and position to fit the work area of the correct monitor
        IntPtr monitor = MonitorFromWindow(hwnd, MONITOR_DEFAULTTONEAREST);

        if (monitor != IntPtr.Zero)
        {
            MONITORINFO monitorInfo = new MONITORINFO();
            monitorInfo.cbSize = Marshal.SizeOf(typeof(MONITORINFO));
            GetMonitorInfo(monitor, ref monitorInfo);
            RECT rcWorkArea = monitorInfo.rcWork;
            RECT rcMonitorArea = monitorInfo.rcMonitor;
            mmi.ptMaxPosition.X = Math.Abs(rcWorkArea.Left - rcMonitorArea.Left);
            mmi.ptMaxPosition.Y = Math.Abs(rcWorkArea.Top - rcMonitorArea.Top);
            mmi.ptMaxSize.X = Math.Abs(rcWorkArea.Right - rcWorkArea.Left);
            mmi.ptMaxSize.Y = Math.Abs(rcWorkArea.Bottom - rcWorkArea.Top);
        }

        Marshal.StructureToPtr(mmi, lParam, true);
    }

    return IntPtr.Zero;
}

private const int WM_GETMINMAXINFO = 0x0024;

private const uint MONITOR_DEFAULTTONEAREST = 0x00000002;

[DllImport("user32.dll")]
private static extern IntPtr MonitorFromWindow(IntPtr handle, uint flags);

[DllImport("user32.dll")]
private static extern bool GetMonitorInfo(IntPtr hMonitor, ref MONITORINFO lpmi);

[Serializable]
[StructLayout(LayoutKind.Sequential)]
public struct RECT
{
    public int Left;
    public int Top;
    public int Right;
    public int Bottom;

    public RECT(int left, int top, int right, int bottom)
    {
        this.Left = left;
        this.Top = top;
        this.Right = right;
        this.Bottom = bottom;
    }
}

[StructLayout(LayoutKind.Sequential)]
public struct MONITORINFO
{
    public int cbSize;
    public RECT rcMonitor;
    public RECT rcWork;
    public uint dwFlags;
}

[Serializable]
[StructLayout(LayoutKind.Sequential)]
public struct POINT
{
    public int X;
    public int Y;

    public POINT(int x, int y)
    {
        this.X = x;
        this.Y = y;
    }
}

[StructLayout(LayoutKind.Sequential)]
public struct MINMAXINFO
{
    public POINT ptReserved;
    public POINT ptMaxSize;
    public POINT ptMaxPosition;
    public POINT ptMinTrackSize;
    public POINT ptMaxTrackSize;
}