using System;
using System.Runtime.InteropServices;

unsafe class Program
{
    public const int CS_VREDRAW = 0x0001;
    public const int CS_HREDRAW = 0x0002;

    public const int CW_USEDEFAULT = unchecked((int)0x80000000);

    public const int SW_SHOWDEFAULT = 10;

    public const uint WS_OVERLAPPED = 0x00000000;
    public const uint WS_CAPTION = 0x00C00000;
    public const uint WS_SYSMENU = 0x00080000;
    public const uint WS_THICKFRAME = 0x00040000;
    public const uint WS_MINIMIZEBOX = 0x00020000;
    public const uint WS_MAXIMIZEBOX = 0x00010000;

    public const uint WS_OVERLAPPEDWINDOW = WS_OVERLAPPED | WS_CAPTION | WS_SYSMENU | WS_THICKFRAME | WS_MINIMIZEBOX | WS_MAXIMIZEBOX;

    public unsafe partial struct WNDCLASSEXW
    {
        public uint cbSize;
        public uint style;
        public delegate* stdcall<IntPtr, uint, nuint, nint, nint> lpfnWndProc;
        public int cbClsExtra;
        public int cbWndExtra;
        public IntPtr hInstance;
        public IntPtr hIcon;
        public IntPtr hCursor;
        public IntPtr hbrBackground;
        public ushort* lpszMenuName;
        public ushort* lpszClassName;
        public IntPtr hIconSm;
    }

    [DllImport("user32", EntryPoint = "CreateWindowExW", ExactSpelling = true)]
    public static extern IntPtr CreateWindowExW(uint dwExStyle, ushort* lpClassName, ushort* lpWindowName, uint dwStyle, int X, int Y, int nWidth, int nHeight, IntPtr hWndParent, IntPtr hMenu, IntPtr hInstance, void* lpParam);

    [DllImport("user32", EntryPoint = "DefWindowProcW", ExactSpelling = true)]
    public static extern nint DefWindowProcW(IntPtr hWnd, uint Msg, nuint wParam, nint lParam);

    [DllImport("user32", EntryPoint = "RegisterClassExW", ExactSpelling = true)]
    public static extern ushort RegisterClassExW(WNDCLASSEXW* param0);

    [UnmanagedCallersOnly]
    private static nint WindowProc(IntPtr hWnd, uint Msg, nuint wParam, nint lParam)
    {
        return DefWindowProcW(hWnd, Msg, wParam, lParam);
    }

    static void Main(string[] args)
    {
        fixed (char* lpszClassName = "SampleClass")
        fixed (char* lpWindowName = "SampleTitle")
        {
            var hInstance = Marshal.GetHINSTANCE(typeof(Program).Module);

            // Requires an explicit cast until C# handles UnmanagedCallersOnly
            var wndProc = (delegate* stdcall<IntPtr, uint, nuint, nint, nint>)(delegate* managed<IntPtr, uint, nuint, nint, nint>)&WindowProc;

            // Initialize the window class.
            var windowClass = new WNDCLASSEXW
            {
                cbSize = (uint)sizeof(WNDCLASSEXW),
                style = CS_HREDRAW | CS_VREDRAW,
                lpfnWndProc = wndProc,
                hInstance = hInstance,
                lpszClassName = (ushort*)lpszClassName
            };
            _ = RegisterClassExW(&windowClass);

            var hwnd = CreateWindowExW(
                0,
                windowClass.lpszClassName,
                (ushort*)lpWindowName,
                WS_OVERLAPPEDWINDOW,
                X: CW_USEDEFAULT,
                Y: CW_USEDEFAULT,
                nWidth: CW_USEDEFAULT,
                nHeight: CW_USEDEFAULT,
                hWndParent: IntPtr.Zero,
                hMenu: IntPtr.Zero,
                hInstance,
                lpParam: null
            );
        }
    }
}