Program.cs

using System.Drawing;
using System.Reflection;
using Microsoft.Web.WebView2.Core;
using Windows.Win32;
using Windows.Win32.Foundation;
using Windows.Win32.Graphics.Gdi;
using Windows.Win32.UI.WindowsAndMessaging;

namespace MinimalWebView;

class Program
{
    internal const uint WM_SYNCHRONIZATIONCONTEXT_WORK_AVAILABLE = PInvoke.WM_USER + 1;
    private const string StaticFileDirectory = "wwwroot";
    private static CoreWebView2Controller _controller;
    private static UiThreadSynchronizationContext _uiThreadSyncCtx;

    [STAThread]
    static int Main(string[] args)
    {
#if DEBUG // By default GUI apps have no console. Open one to enable Console.WriteLine debugging 🤠
        PInvoke.AllocConsole();
#endif
        HWND hwnd;

        unsafe
        {
            HINSTANCE hInstance = PInvoke.GetModuleHandle((char*)null);
            ushort classId;

            HBRUSH backgroundBrush = PInvoke.CreateSolidBrush(0x271811); // this is actually #111827, Windows uses BBGGRR
            if (backgroundBrush.IsNull)
            {
                // fallback to the system background color in case it fails
                backgroundBrush = (HBRUSH)(IntPtr)(SYS_COLOR_INDEX.COLOR_BACKGROUND + 1);
            }

            fixed (char* classNamePtr = "MinimalWebView")
            {
                WNDCLASSW wc = new()
                {
                    lpfnWndProc = WndProc,
                    lpszClassName = classNamePtr,
                    hInstance = hInstance,
                    hbrBackground = backgroundBrush,
                    style = WNDCLASS_STYLES.CS_VREDRAW | WNDCLASS_STYLES.CS_HREDRAW
                };
                classId = PInvoke.RegisterClass(wc);

                if (classId == 0)
                    throw new Exception("class not registered");
            }

            fixed (char* windowNamePtr = $"MinimalWebView {Assembly.GetExecutingAssembly().GetName().Version}")
            {
                hwnd = PInvoke.CreateWindowEx(
                    0,
                    (char*)classId,
                    windowNamePtr,
                    WINDOW_STYLE.WS_OVERLAPPEDWINDOW,
                    PInvoke.CW_USEDEFAULT, PInvoke.CW_USEDEFAULT, 600, 500,
                    new HWND(),
                    new HMENU(),
                    hInstance,
                    null);
            }
        }

        if (hwnd.Value == 0)
            throw new Exception("hwnd not created");

        PInvoke.ShowWindow(hwnd, SHOW_WINDOW_CMD.SW_NORMAL);

        _uiThreadSyncCtx = new UiThreadSynchronizationContext(hwnd);
        SynchronizationContext.SetSynchronizationContext(_uiThreadSyncCtx);

        // Start initializing WebView2 in a fire-and-forget manner. Errors will be handled in the initialization function
        _ = CreateCoreWebView2Async(hwnd);

        Console.WriteLine("Starting message pump...");
        MSG msg;
        while (PInvoke.GetMessage(out msg, new HWND(), 0, 0))
        {
            PInvoke.TranslateMessage(msg);
            PInvoke.DispatchMessage(msg);
        }

        return (int)msg.wParam.Value;
    }

    private static LRESULT WndProc(HWND hwnd, uint msg, WPARAM wParam, LPARAM lParam)
    {
        switch (msg)
        {
            case PInvoke.WM_SIZE:
                OnSize(hwnd, wParam, GetLowWord(lParam.Value), GetHighWord(lParam.Value));
                break;
            case WM_SYNCHRONIZATIONCONTEXT_WORK_AVAILABLE:
                _uiThreadSyncCtx.RunAvailableWorkOnCurrentThread();
                break;
            case PInvoke.WM_CLOSE:
                PInvoke.PostQuitMessage(0);
                break;
        }

        return PInvoke.DefWindowProc(hwnd, msg, wParam, lParam);
    }

    private static void OnSize(HWND hwnd, WPARAM wParam, int width, int height)
    {
        if (_controller != null)
            _controller.Bounds = new Rectangle(0, 0, width, height);
    }

    private static async Task CreateCoreWebView2Async(HWND hwnd)
    {
        try
        {
            Console.WriteLine("Initializing WebView2...");
            var environment = await CoreWebView2Environment.CreateAsync(null, null, null);
            _controller = await environment.CreateCoreWebView2ControllerAsync(hwnd);

            _controller.DefaultBackgroundColor = Color.Transparent; // avoids flash of white when page first renders
            _controller.CoreWebView2.WebMessageReceived += CoreWebView2_WebMessageReceived;
            _controller.CoreWebView2.SetVirtualHostNameToFolderMapping("minimalwebview.example", StaticFileDirectory, CoreWebView2HostResourceAccessKind.Allow);
            PInvoke.GetClientRect(hwnd, out var hwndRect);
            _controller.Bounds = new Rectangle(0, 0, hwndRect.right, hwndRect.bottom);
            _controller.IsVisible = true;
            _controller.CoreWebView2.Navigate("https://minimalwebview.example/index.html");

            Console.WriteLine("WebView2 initialization succeeded.");
        }
        catch (WebView2RuntimeNotFoundException)
        {
            var result = PInvoke.MessageBox(hwnd, "WebView2 runtime not installed.", "Error", MESSAGEBOX_STYLE.MB_OK | MESSAGEBOX_STYLE.MB_ICONERROR);

            if (result == MESSAGEBOX_RESULT.IDYES)
            {
                //TODO: download WV2 bootstrapper from https://go.microsoft.com/fwlink/p/?LinkId=2124703 and run it
            }

            Environment.Exit(1);
        }
        catch (Exception ex)
        {
            PInvoke.MessageBox(hwnd, $"Failed to initialize WebView2:{Environment.NewLine}{ex}", "Error", MESSAGEBOX_STYLE.MB_OK | MESSAGEBOX_STYLE.MB_ICONERROR);
            Environment.Exit(1);
        }
    }

    private static async void CoreWebView2_WebMessageReceived(object sender, CoreWebView2WebMessageReceivedEventArgs e)
    {
        var webMessage = e.TryGetWebMessageAsString();
        if (string.IsNullOrEmpty(webMessage))
            return;

        // simulate moving some slow operation to a background thread
        await Task.Run(() => Thread.Sleep(200));

        // this will blow up if not run on the UI thread, so the SynchronizationContext needs to have been wired up correctly
        await _controller.CoreWebView2.ExecuteScriptAsync($"alert('Hi from the UI thread! I got a message from the browser: {webMessage}')");
    }

    private static int GetLowWord(nint value)
    {
        uint xy = (uint)value;
        int x = unchecked((short)xy);
        return x;
    }

    private static int GetHighWord(nint value)
    {
        uint xy = (uint)value;
        int y = unchecked((short)(xy >> 16));
        return y;
    }
}