App.xaml.cs

using Microsoft.UI.Xaml;
using Microsoft.UI.Xaml.Controls;
using Microsoft.UI.Xaml.Controls.Primitives;
using Microsoft.UI.Xaml.Data;
using Microsoft.UI.Xaml.Input;
using Microsoft.UI.Xaml.Media;
using Microsoft.UI.Xaml.Navigation;
using Microsoft.UI.Xaml.Shapes;
using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Runtime.InteropServices.WindowsRuntime;
using Windows.ApplicationModel;
using Windows.ApplicationModel.Activation;
using Windows.Foundation;
using Windows.Foundation.Collections;
using System.Runtime.InteropServices;
using WinRT;
using System.Xml.Linq;

// To learn more about WinUI, the WinUI project structure,
// and more about our project templates, see: http://aka.ms/winui-project-info.

namespace ITATKWinUI
{
    /// <summary>
    /// Provides application-specific behavior to supplement the default Application class.
    /// </summary>
    ///

    public partial class App : Application
    {
        /// <summary>
        /// Initializes the singleton application object.  This is the first line of authored code
        /// executed, and as such is the logical equivalent of main() or WinMain().
        /// </summary>
        public App()
        {
            this.InitializeComponent();

            //Set the theme and app icon from settings at startup
            XDocument settingsXML = XDocument.Load(@"Settings.xml");
            foreach (XElement item in from y in settingsXML.Descendants("Item") select y)
            {
                if (item.Attribute("Name").Value == "SettingTheme")
                {
                    switch (item.Attribute("Setting").Value)
                    {
                        case "Dark Mode":
                            App.Current.RequestedTheme = ApplicationTheme.Dark;
                            break;

                        case "Light Mode":
                            App.Current.RequestedTheme = ApplicationTheme.Light;
                            break;

                        case "System Theme":
                            //We don't need to do anything, this is already the default
                            break;

                        default:
                            break;
                    }
                }

                if (item.Attribute("Name").Value == "SettingApplicationIconImage")
                {
                    AppIcon = item.Attribute("Setting").Value;
                }
            }
        }

        public static string AppIcon;

        /// <summary>
        /// Invoked when the application is launched normally by the end user.  Other entry points
        /// will be used such as when the application is launched to open a specific file.
        /// </summary>
        /// <param name="args">Details about the launch request and process.</param>
        /*protected override void OnLaunched(Microsoft.UI.Xaml.LaunchActivatedEventArgs args)
        {
            m_window = new MainWindow();
            m_window.Activate();
        }

        private Window m_window;*/

        protected override void OnLaunched(Microsoft.UI.Xaml.LaunchActivatedEventArgs args)
        {
            m_window = new MainWindow();
            //Get the Window's HWND
            var windowNative = m_window.As<IWindowNative>();
            m_windowHandle = windowNative.WindowHandle;

            void LoadIcon(string iconName)
            {
                string i = iconName.Replace("ms-appx:///","");
                //Get the Window's HWND
                var hwnd = WinRT.Interop.WindowNative.GetWindowHandle(m_window);
                var hIcon = PInvoke.User32.LoadImage(System.IntPtr.Zero, i,
                          PInvoke.User32.ImageType.IMAGE_ICON, 16, 16, PInvoke.User32.LoadImageFlags.LR_LOADFROMFILE);

                PInvoke.User32.SendMessage(hwnd, PInvoke.User32.WindowMessage.WM_SETICON, (System.IntPtr)0, hIcon);
            }

            LoadIcon(AppIcon);

            m_window.Activate();

            // The Window object doesn't have Width and Height properties in WInUI 3 Desktop yet.
            // To set the Width and Height, you can use the Win32 API SetWindowPos.
            // Note, you should apply the DPI scale factor if you are thinking of dpi instead of pixels.
            SetWindowSize(m_windowHandle, 1450, 820);
        }

        public static string Title;

        private void SetWindowSize(IntPtr hwnd, int width, int height)
        {
            var dpi = PInvoke.User32.GetDpiForWindow(hwnd);
            float scalingFactor = (float)dpi / 96;
            width = (int)(width * scalingFactor);
            height = (int)(height * scalingFactor);

            PInvoke.User32.SetWindowPos(hwnd, PInvoke.User32.SpecialWindowHandles.HWND_TOP,
                                        0, 0, width, height,
                                        PInvoke.User32.SetWindowPosFlags.SWP_NOMOVE);
        }

        private Window m_window;
        private IntPtr m_windowHandle;
        public Window Window => m_window;
        public IntPtr WindowHandle { get { return m_windowHandle; } }

        [ComImport]
        [InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
        [Guid("EECDBF0E-BAE9-4CB6-A68E-9598E1CB57BB")]
        internal interface IWindowNative
        {
            IntPtr WindowHandle { get; }
        }
    }
}