WinUI - MessageDialog in App.xaml.cs does not work

Question

Before the MainWindow is created and called, the programme should establish a connection to the database and read data. In case of an error, a message should be displayed and the programme closed without the main window appearing. I have tried various attempts and tried everything, but to no avail. Maybe someone knows how it could work.Enclosed are two of my examples

Accepted Answer

MessageDialog needs a window

You can use MessageBox API, which does not need a window :

        protected override void OnLaunched(Microsoft.UI.Xaml.LaunchActivatedEventArgs args)
        {
            MessageBox(IntPtr.Zero, "Test", "Information", MB_OK | MB_ICONINFORMATION);
            m_window = new MainWindow();
            m_window.Activate();
        }

Declarations :

    [DllImport("User32.dll", SetLastError = true, CharSet = CharSet.Unicode)]
    public static extern int MessageBox(IntPtr HWND, string lpText, string lpCaption, uint Type);

    public const int MB_OK = 0x00000000;
    public const int MB_OKCANCEL = 0x00000001;
    public const int MB_ABORTRETRYIGNORE = 0x00000002;
    public const int MB_YESNOCANCEL = 0x00000003;
    public const int MB_YESNO = 0x00000004;
    public const int MB_RETRYCANCEL = 0x00000005;
    public const int MB_CANCELTRYCONTINUE = 0x00000006;

    public const int MB_ICONHAND = 0x00000010;
    public const int MB_ICONQUESTION = 0x00000020;
    public const int MB_ICONEXCLAMATION = 0x00000030;
    public const int MB_ICONASTERISK = 0x00000040;
    public const int MB_USERICON = 0x00000080;
    public const int MB_ICONWARNING = MB_ICONEXCLAMATION;
    public const int MB_ICONERROR = MB_ICONHAND;
    public const int MB_ICONINFORMATION = MB_ICONASTERISK;
    public const int MB_ICONSTOP = MB_ICONHAND;

    public const int MB_DEFBUTTON1 = 0x00000000;
    public const int MB_DEFBUTTON2 = 0x00000100;
    public const int MB_DEFBUTTON3 = 0x00000200;
    public const int MB_DEFBUTTON4 = 0x00000300;

    public const int MB_APPLMODAL = 0x00000000;
    public const int MB_SYSTEMMODAL = 0x00001000;
    public const int MB_TASKMODAL = 0x00002000;
    public const int MB_HELP = 0x00004000; // Help Button
    public const int MB_NOFOCUS = 0x00008000;
    public const int MB_SETFOREGROUND = 0x00010000;
    public const int MB_DEFAULT_DESKTOP_ONLY = 0x00020000;
    public const int MB_TOPMOST = 0x00040000;
    public const int MB_RIGHT = 0x00080000;
    public const int MB_RTLREADING = 0x00100000;
    public const int MB_SERVICE_NOTIFICATION = 0x00200000;
    public const int MB_SERVICE_NOTIFICATION_NT3X = 0x00040000;

    public const int MB_TYPEMASK = 0x0000000F;
    public const int MB_ICONMASK = 0x000000F0;
    public const int MB_DEFMASK = 0x00000F00;
    public const int MB_MODEMASK = 0x00003000;
    public const int MB_MISCMASK = 0x0000C000;

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WinUI - MessageDialog in App.xaml.cs does not work

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