using System.Management;

    void Shutdown()
    {
        ManagementBaseObject mboShutdown = null;
        ManagementClass mcWin32 = new ManagementClass("Win32_OperatingSystem");
        mcWin32.Get();

        // You can't shutdown without security privileges
        mcWin32.Scope.Options.EnablePrivileges = true;
        ManagementBaseObject mboShutdownParams =
                 mcWin32.GetMethodParameters("Win32Shutdown");

        // Flag 1 means we want to shut down the system. Use "2" to reboot.
        mboShutdownParams["Flags"] = "1";
        mboShutdownParams["Reserved"] = "0";
        foreach (ManagementObject manObj in mcWin32.GetInstances())
        {
            mboShutdown = manObj.InvokeMethod("Win32Shutdown", 
                                           mboShutdownParams, null);
        }
    }

System.Diagnostics.Process p=new System.Diagnostics.Process();
p.StartInfo.FileName="cmd.exe";
p.StartInfo.Arguments="/c shutdown -s";
p.Start();