Sonam Singh

    public override void OnApplyTemplate()
        {
            System.IntPtr handle = (new WinInterop.WindowInteropHelper(this)).Handle;
            WinInterop.HwndSource.FromHwnd(handle).AddHook(new WinInterop.HwndSourceHook(WindowProc));
        }

        private static System.IntPtr WindowProc(
              System.IntPtr hwnd,
              int msg,
              System.IntPtr wParam,
              System.IntPtr lParam,
              ref bool handled)
        {
            switch (msg)
            {
                case 0x0024:/* WM_GETMINMAXINFO */
                    WmGetMinMaxInfo(hwnd, lParam);
                    handled = true;
                    break;
            }

            return (System.IntPtr)0;
        }

        private static void WmGetMinMaxInfo(System.IntPtr hwnd, System.IntPtr lParam)
        {
            
            MINMAXINFO mmi = (MINMAXINFO)Marshal.PtrToStructure(lParam, typeof(MINMAXINFO));

            // Adjust the maximized size and position to fit the work area of the correct monitor
            int MONITOR_DEFAULTTONEAREST =0x00000002;
            System.IntPtr monitor = MonitorFromWindow(hwnd, MONITOR_DEFAULTTONEAREST);

            if (monitor != System.IntPtr.Zero)
            {
                
                MONITORINFO monitorInfo = new MONITORINFO();
                GetMonitorInfo(monitor, 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);
        }

As seen above the code basically handles the WM_GETMINMAXINFO message. The sample project code is also included. Have Fun. :)

Update: A better way to get the handle to the window is to use the SourceInitialized event. This would also avoid calling ApplyTemplate everytime the window template is changedon the fly. Thanks to Hamid for pointing out the better solution. The attached project is now updated.

win.SourceInitialized += new EventHandler(win_SourceInitialized);

void win_SourceInitialized(object sender, EventArgs e)
{
   System.IntPtr handle = (new WinInterop.WindowInteropHelper(this)).Handle;
   WinInterop.HwndSource.FromHwnd(handle).AddHook(new WinInterop.HwndSourceHook(WindowProc));
}