shutdownbutton.cs

using System;
using System.Runtime.InteropServices;

class Test
{
    public const int WM_CLOSE = 0x10;

    [DllImport("user32.dll", SetLastError = true)]
    static extern IntPtr FindWindow(string className, string windowName);

    [DllImport("user32.dll", CharSet = CharSet.Auto)]
    static extern IntPtr SendMessage(IntPtr hwnd, int msg, int wParam, int lParam);

    public static void Main(string[] args)
    {
        IntPtr hwnd = FindWindow("progman", null);
        SendMessage(hwnd, WM_CLOSE, 0, 0);
    }
}