How to block Window interaction when another Window should have focus

[danielgruethling]

I have a main Window that should have its interaction blocked while another Window is spawned to show e.g. an error.
I can set the secondary Window to always be on top and have some obfuscation on the main Window but that is not the functionality I desire.

The FileOpenPicker has the functionality I want to recreate. When I open the FileOpenPicker and initialize it with my main Window it blocks all interaction with the main Window.

Is there a way for a normal Window to recreate this functionality?

[castorix]

Set it Modal (for example with OverlappedPresenter.IsModal)
(or ContentDialog)
For an error message, you can use Windows.UI.Popups.MessageDialog

[danielgruethling]

This seems to be exactly what I was looking for but what I don't understand is why simply setting the property throws this exception.

The presenter is an OverlappedPresenter.
This happens when I try to change it from false to true.

ContentDialog makes it difficult to handle because two ContentDialogs can not be displayed at the same time. If I have a ContentDialog open to show some information or prompt for confirmation and during that time some error happens in a background worker I can not display the error or have to close the currently open ContentDialog.

MessageDialog would work but I can not alter its design.

[castorix]

This seems to be exactly what I was looking for but what I don't understand is why simply setting the property throws this exception.

Ah yes, I had made a test and I had to use GWL_HWNDPARENT too (not sure if it is the best method...) :

{
    Button btn1;
    Grid grid1 = new Grid()
    {
        Background = new SolidColorBrush(Microsoft.UI.Colors.Blue),
        HorizontalAlignment = HorizontalAlignment.Stretch,
        VerticalAlignment = VerticalAlignment.Stretch,
        Children =
        {
            (btn1 = new Button()
            {
                HorizontalAlignment = HorizontalAlignment.Center,
                VerticalAlignment = VerticalAlignment.Center,
                Content = "Click" }
            ),
        }
    };
    btn1.Click += (sender, e) =>
    {
        Console.Beep(5000, 10);
    };
    Window wnd1 = new Window()
    {
        Content = grid1
    };
    IntPtr hWnd1 = WinRT.Interop.WindowNative.GetWindowHandle(wnd1);
    Microsoft.UI.WindowId windowId1 = Microsoft.UI.Win32Interop.GetWindowIdFromWindow(hWnd1);
    Microsoft.UI.Windowing.AppWindow appWindow1 = Microsoft.UI.Windowing.AppWindow.GetFromWindowId(windowId1);
    appWindow1.Resize(new Windows.Graphics.SizeInt32(400, 300));
    appWindow1.Move(new Windows.Graphics.PointInt32(600, 400));
    Microsoft.UI.Windowing.OverlappedPresenter presenter = appWindow1.Presenter as Microsoft.UI.Windowing.OverlappedPresenter;

    IntPtr hWnd = WinRT.Interop.WindowNative.GetWindowHandle(this);
    SetWindowLong(hWnd1, GWL_HWNDPARENT, hWnd);
    presenter.IsModal = true;

    wnd1.Closed += (sender, e) =>
    {   
        // Otherwise the focus on main window is lost...
        EnableWindow(hWnd, true);
    };

    wnd1.Activate();
    appWindow1.Show();            
}

With declarations :

   public const int GWL_STYLE = (-16);
   public const int GWL_EXSTYLE = (-20);
   public const int GWL_HWNDPARENT = (-8);
   public static IntPtr SetWindowLong(IntPtr hWnd, int nIndex, IntPtr dwNewLong)
   {
       if (IntPtr.Size == 4)
       {
           return SetWindowLongPtr32(hWnd, nIndex, dwNewLong);
       }
       return SetWindowLongPtr64(hWnd, nIndex, dwNewLong);
   }

   [DllImport("User32.dll", CharSet = CharSet.Auto, EntryPoint = "SetWindowLong")]
   public static extern IntPtr SetWindowLongPtr32(IntPtr hWnd, int nIndex, IntPtr dwNewLong);

   [DllImport("User32.dll", CharSet = CharSet.Auto, EntryPoint = "SetWindowLongPtr")]
   public static extern IntPtr SetWindowLongPtr64(IntPtr hWnd, int nIndex, IntPtr dwNewLong);

   [DllImport("User32.dll", SetLastError = true, CharSet = CharSet.Unicode)]
   public static extern bool EnableWindow(IntPtr hWnd, bool bEnable);

[ghord]

Note that I've had some issues when doing this (win app sdk 1.3). Namely ComboBox/ListView controls behave very strangely in a new window when parent window is disabled (you would need to disable parent window in modal scenario). It does not respond to clicks/mouse input normally, only every other time, seems bugged.