C# window message loop generated by ChatGPT

cswinloop.cs

using System;
using System.Runtime.InteropServices;

public static class Program
{
    [DllImport("user32.dll")]
    private static extern bool RegisterClassEx(ref WNDCLASSEX wcex);

    [DllImport("user32.dll")]
    private static extern IntPtr CreateWindowEx(
        uint dwExStyle,
        string lpClassName,
        string lpWindowName,
        uint dwStyle,
        int x,
        int y,
        int nWidth,
        int nHeight,
        IntPtr hWndParent,
        IntPtr hMenu,
        IntPtr hInstance,
        IntPtr lpParam);

    [DllImport("user32.dll")]
    private static extern bool ShowWindow(IntPtr hWnd, int nCmdShow);

    [DllImport("user32.dll")]
    private static extern bool UpdateWindow(IntPtr hWnd);

    [DllImport("user32.dll")]
    private static extern int GetMessage(out MSG lpMsg, IntPtr hWnd, uint wMsgFilterMin, uint wMsgFilterMax);

    [DllImport("user32.dll")]
    private static extern bool TranslateMessage(ref MSG lpMsg);

    [DllImport("user32.dll")]
    private static extern IntPtr DispatchMessage(ref MSG lpMsg);

    [DllImport("user32.dll")]
    private static extern bool DestroyWindow(IntPtr hWnd);

    private const string WindowClassName = "MyWindowClass";
    private static IntPtr _hWnd;

    [StructLayout(LayoutKind.Sequential)]
    private struct WNDCLASSEX
    {
        public int cbSize;
        public uint style;
        public IntPtr lpfnWndProc;
        public int cbClsExtra;
        public int cbWndExtra;
        public IntPtr hInstance;
        public IntPtr hIcon;
        public IntPtr hCursor;
        public IntPtr hbrBackground;
        public string lpszMenuName;
        public string lpszClassName;
        public IntPtr hIconSm;
    }

    [StructLayout(LayoutKind.Sequential)]
    private struct MSG
    {
        public IntPtr hWnd;
        public uint message;
        public IntPtr wParam;
        public IntPtr lParam;
        public uint time;
        public POINT pt;
    }

    [StructLayout(LayoutKind.Sequential)]
    private struct POINT
    {
        public int x;
        public int y;
    }

    private static IntPtr WindowProc(IntPtr hWnd, uint msg, IntPtr wParam, IntPtr lParam)
    {
        switch (msg)
        {
            case WM_DESTROY:
                PostQuitMessage(0);
                return IntPtr.Zero;
            default:
                return DefWindowProc(hWnd, msg, wParam, lParam);
        }
    }

    private const uint CS_HREDRAW = 0x0002;
    private const uint CS_VREDRAW = 0x0001;
    private const uint CW_USEDEFAULT = 0x80000000;
    private const uint WS_OVERLAPPEDWINDOW = 0xcf0000;
    private const uint WM_DESTROY = 0x0002;

    [DllImport("user32.dll")]
    private static extern IntPtr DefWindowProc(IntPtr hWnd, uint uMsg, IntPtr wParam, IntPtr lParam);

    [DllImport("user32.dll")]
    private static extern void PostQuitMessage(int nExitCode);

    public static void Main()
    {
        var hInstance = Marshal.GetHINSTANCE(typeof(Program).Module);
        var wcex = new WNDCLASSEX
        {
            cbSize = Marshal.SizeOf<WNDCLASSEX>(),
            style = CS_HREDRAW | CS_VREDRAW,
            lpfnWndProc = Marshal.GetFunctionPointerForDelegate<WndProc>(WindowProc),
            hInstance = hInstance,
            hCursor = LoadCursor(IntPtr.Zero, IDC_ARROW),
            hbrBackground = (IntPtr)(COLOR_WINDOW),
            lpszClassName = WindowClassName
        };
        RegisterClassEx(ref wcex);
        unchecked
        {
            _hWnd = CreateWindowEx(
                0,
                WindowClassName,
                "My Window",
                WS_OVERLAPPEDWINDOW,
                (int)CW_USEDEFAULT,
                (int)CW_USEDEFAULT,
                (int)CW_USEDEFAULT,
                (int)CW_USEDEFAULT,
                IntPtr.Zero,
                IntPtr.Zero,
                hInstance,
                IntPtr.Zero);
        }
        if (_hWnd == IntPtr.Zero)
        {
            throw new InvalidOperationException("Failed to create window.");
        }

        ShowWindow(_hWnd, 1);
        UpdateWindow(_hWnd);

        var msg = new MSG();
        while (GetMessage(out msg, IntPtr.Zero, 0, 0) != 0)
        {
            TranslateMessage(ref msg);
            DispatchMessage(ref msg);
        }

        DestroyWindow(_hWnd);
    }

    private delegate IntPtr WndProc(IntPtr hWnd, uint msg, IntPtr wParam, IntPtr lParam);

    private const uint IDC_ARROW = 32512;
    private const uint COLOR_WINDOW = 5;
    private const int SW_SHOWNORMAL = 1;

    [DllImport("user32.dll")]
    private static extern IntPtr LoadCursor(IntPtr hInstance, uint lpCursorName);
}