VoiDeD

using System;
using System.Runtime.InteropServices;

unsafe class Program {
    [DllImport("Win.dll")]
    public static extern void CreateWndClass(
        string name // wndclass name
    );
    [DllImport("user32.dll")]
    public static extern IntPtr CreateWindowEx(
        uint dwExStyle,	// extended window style
        string lpClassName,	// pointer to registered class name
        string lpWindowName,	// pointer to window name
        uint dwStyle,	// window style
        uint x,	// horizontal position of window
        uint y,	// vertical position of window
        uint nWidth,	// window width
        uint nHeight,	// window height
        IntPtr hWndParent,	// handle to parent or owner window
        IntPtr hMenu,	// handle to menu, or child-window identifier
        IntPtr hInstance,	// handle to application instance
        void* lpParam 	// pointer to window-creation data
    );
    [DllImport("user32.dll")]
    public static extern uint DefWindowProc(
        IntPtr hWnd,	// handle to window
        uint Msg,	// message identifier
        uint wParam,	// first message parameter
        uint lParam 	// second message parameter
    );
    [DllImport("user32.dll")]
    public static extern bool ShowWindow(
        IntPtr hWnd,	// handle of window
        uint nCmdShow 	// show state of window
    );
    [DllImport("user32.dll")]
    public static extern int GetMessage(
        IntPtr lpMsg,	// address of structure with message
        IntPtr hWnd,	// handle of window
        uint wMsgFilterMin,	// first message
        uint wMsgFilterMax 	// last message
    );
    [DllImport("user32.dll")]
    public static extern long DispatchMessage(
        IntPtr lpmsg 	// pointer to structure with message
    );
    [DllImport("user32.dll")]
    public static extern void MessageBox(
        IntPtr hWnd,	// handle of owner window
        string lpText,	// address of text in message box
        string lpCaption,	// address of title of message box  
        uint uType 	// style of message box
    );
    [DllImport("user32.dll")]
    public static extern bool SetWindowText(
        IntPtr hWnd,	// handle of window or control
        string lpString 	// address of string
    );
    [DllImport("user32.dll")]
    public static extern void PostQuitMessage(
        int nExitCode 	// exit code
    );
    [StructLayout(LayoutKind.Sequential)]
    public struct MSG { //WinAPI MSG struct
        public IntPtr hwnd;
        public uint message;
        public uint wParam;
        public uint lParam;
        public uint time;
        public long x;
        public long y;
    }
    public class WinAPI { //WinAPI defines
        public const uint WS_SYSMENU = 0x80000;
        public const uint WS_BORDER = 0x800000;
        public const uint WS_CAPTION = 0xc00000;
        public const uint WS_CHILD = 0x40000000;
        public const uint SW_SHOW = 5;
        public const uint CW_USEDEFAULT = 0x80000000;
        public const uint WM_LBUTTONUP = 514;
        public const uint SS_CENTER = 1;
    }
    public static void Main() {
        //the only non WinAPI call
        //this function calls a C library function
        //which creates and registers a WNDCLASSEX
        //it also contains a WndProc function which only calls DefWindowProc
        CreateWndClass("wndclass");

        //create a window, a label, and a button
        IntPtr wind = CreateWindowEx(0, "wndclass", "Window! >:D", WinAPI.WS_SYSMENU | WinAPI.WS_CAPTION | WinAPI.WS_BORDER, WinAPI.CW_USEDEFAULT, WinAPI.CW_USEDEFAULT, 200, 200, IntPtr.Zero, IntPtr.Zero, IntPtr.Zero, null);
        IntPtr label = CreateWindowEx(0, "STATIC", "This window was created using the WinAPI inside a C# application...", WinAPI.WS_CHILD | WinAPI.SS_CENTER, 10, 10, 175, 60, wind, IntPtr.Zero, IntPtr.Zero, null);
        IntPtr button = CreateWindowEx(0, "BUTTON", "Close", WinAPI.WS_CHILD, 55, 80, 80, 30, wind, IntPtr.Zero, IntPtr.Zero, null);

        //show all 3 windows
        ShowWindow(wind, WinAPI.SW_SHOW);
        ShowWindow(label, WinAPI.SW_SHOW);
        ShowWindow(button, WinAPI.SW_SHOW);

        //create a new MSG struct
        MSG msg = new MSG();

        //allocate memory for the MSG struct
        IntPtr mem = Marshal.AllocHGlobal(Marshal.SizeOf(msg));
        //marshals the MSG struct into unmanaged memory
        Marshal.StructureToPtr(msg, mem, false);

        //run message loop
        while (GetMessage(mem, wind, 0, 0) > 0) { //GetMessage fills the unmanaged memory with a message
            //marshals the memory into a MSG struct
            MSG t_msg = (MSG)Marshal.PtrToStructure(mem, typeof(MSG));

            if (t_msg.message == WinAPI.WM_LBUTTONUP) { //check for a mouse button up message
                if (t_msg.hwnd == button) { //check if the message window is the button
                    MessageBox(wind, "Processing close message.", "C# WinAPI", 0);
                    //exit message loop
                    break;
                }
            }
            //dispatches message to a C library WndProc which calls DefWindowProc
            DispatchMessage(mem);
        }
        //sends a WM_QUIT message
        //this is handled by the C library WndProc
        PostQuitMessage(0);
    }
}