How to shut down the computer from C#

Works starting with windows XP, not available in win 2000 or lower:

This is the quickest way to do it:

Process.Start("shutdown","/s /t 0");

Otherwise use P/Invoke or WMI like others have said.

Edit: how to avoid creating a window

var psi = new ProcessStartInfo("shutdown","/s /t 0");
psi.CreateNoWindow = true;
psi.UseShellExecute = false;
Process.Start(psi);

Taken from: a Geekpedia post

This method uses WMI to shutdown windows.

You'll need to add a reference to System.Management to your project to use this.

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);
    }
}

This thread provides the code necessary: http://bytes.com/forum/thread251367.html

but here's the relevant code:

using System.Runtime.InteropServices;

[StructLayout(LayoutKind.Sequential, Pack=1)]
internal struct TokPriv1Luid
{
    public int Count;
    public long Luid;
    public int Attr;
}

[DllImport("kernel32.dll", ExactSpelling=true) ]
internal static extern IntPtr GetCurrentProcess();

[DllImport("advapi32.dll", ExactSpelling=true, SetLastError=true) ]
internal static extern bool OpenProcessToken( IntPtr h, int acc, ref IntPtr
phtok );

[DllImport("advapi32.dll", SetLastError=true) ]
internal static extern bool LookupPrivilegeValue( string host, string name,
ref long pluid );

[DllImport("advapi32.dll", ExactSpelling=true, SetLastError=true) ]
internal static extern bool AdjustTokenPrivileges( IntPtr htok, bool disall,
ref TokPriv1Luid newst, int len, IntPtr prev, IntPtr relen );

[DllImport("user32.dll", ExactSpelling=true, SetLastError=true) ]
internal static extern bool ExitWindowsEx( int flg, int rea );

internal const int SE_PRIVILEGE_ENABLED = 0x00000002;
internal const int TOKEN_QUERY = 0x00000008;
internal const int TOKEN_ADJUST_PRIVILEGES = 0x00000020;
internal const string SE_SHUTDOWN_NAME = "SeShutdownPrivilege";
internal const int EWX_LOGOFF = 0x00000000;
internal const int EWX_SHUTDOWN = 0x00000001;
internal const int EWX_REBOOT = 0x00000002;
internal const int EWX_FORCE = 0x00000004;
internal const int EWX_POWEROFF = 0x00000008;
internal const int EWX_FORCEIFHUNG = 0x00000010;

private void DoExitWin( int flg )
{
    bool ok;
    TokPriv1Luid tp;
    IntPtr hproc = GetCurrentProcess();
    IntPtr htok = IntPtr.Zero;
    ok = OpenProcessToken( hproc, TOKEN_ADJUST_PRIVILEGES | TOKEN_QUERY, ref htok );
    tp.Count = 1;
    tp.Luid = 0;
    tp.Attr = SE_PRIVILEGE_ENABLED;
    ok = LookupPrivilegeValue( null, SE_SHUTDOWN_NAME, ref tp.Luid );
    ok = AdjustTokenPrivileges( htok, false, ref tp, 0, IntPtr.Zero, IntPtr.Zero );
    ok = ExitWindowsEx( flg, 0 );
    }

Usage:

DoExitWin( EWX_SHUTDOWN );

or

DoExitWin( EWX_REBOOT );