Standby, Hibernate or Shut Down In .NET

See Standby or Hibernate Windows Programmatically
Using Windows forms static methods is the fastes way
Suspend is:

System.Windows.Forms.Application.SetSuspendState(PowerState.Suspend, false, false);

Hibernate is:

System.Windows.Forms.Application.SetSuspendState(PowerState.Hibernate, true, true);

Alternatively invoke the underlying API SetSuspendState directly.
To shut windows down

[Flags]
public enum ExitWindows : uint
{
    // ONE of the following five:
    LogOff = 0x00,
    ShutDown = 0x01,
    Reboot = 0x02,
    PowerOff = 0x08,
    RestartApps = 0x40,
    // plus AT MOST ONE of the following two:
    Force = 0x04,
    ForceIfHung = 0x10,
}

[Flags]
enum ShutdownReason : uint
{
    MajorApplication = 0x00040000,
    MajorHardware = 0x00010000,
    MajorLegacyApi = 0x00070000,
    MajorOperatingSystem = 0x00020000,
    MajorOther = 0x00000000,
    MajorPower = 0x00060000,
    MajorSoftware = 0x00030000,
    MajorSystem = 0x00050000,

    MinorBlueScreen = 0x0000000F,
    MinorCordUnplugged = 0x0000000b,
    MinorDisk = 0x00000007,
    MinorEnvironment = 0x0000000c,
    MinorHardwareDriver = 0x0000000d,
    MinorHotfix = 0x00000011,
    MinorHung = 0x00000005,
    MinorInstallation = 0x00000002,
    MinorMaintenance = 0x00000001,
    MinorMMC = 0x00000019,
    MinorNetworkConnectivity = 0x00000014,
    MinorNetworkCard = 0x00000009,
    MinorOther = 0x00000000,
    MinorOtherDriver = 0x0000000e,
    MinorPowerSupply = 0x0000000a,
    MinorProcessor = 0x00000008,
    MinorReconfig = 0x00000004,
    MinorSecurity = 0x00000013,
    MinorSecurityFix = 0x00000012,
    MinorSecurityFixUninstall = 0x00000018,
    MinorServicePack = 0x00000010,
    MinorServicePackUninstall = 0x00000016,
    MinorTermSrv = 0x00000020,
    MinorUnstable = 0x00000006,
    MinorUpgrade = 0x00000003,
    MinorWMI = 0x00000015,

    FlagUserDefined = 0x40000000,
    FlagPlanned = 0x80000000
}

[DllImport("user32.dll", SetLastError = true)]
[return: MarshalAs(UnmanagedType.Bool)]
static extern bool ExitWindowsEx(ExitWindows uFlags, ShutdownReason dwReason);

internal void ShutDownWindows()
{
    AddShutDownPrivilegeToApp();
    bool res = ExitWindowsEx(
        ExitWindows.ShutDown | ExitWindows.Force, 
        ShutdownReason.MajorOther | ShutdownReason.MinorOther);
}

taken from PInvoke
Trouble is the application must have the correct privileges:

#region Adjust Priveleges

//This snippet is tested on WinXP and Vista, only needed in Vista when using SetTimeZoneInformation
[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("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);

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

internal const int SE_PRIVILEGE_ENABLED = 0x00000002;
internal const int TOKEN_QUERY = 0x00000008;
internal const int TOKEN_ADJUST_PRIVILEGES = 0x00000020;
internal const string SE_TIME_ZONE_NAMETEXT = "SeTimeZonePrivilege"; //http://msdn.microsoft.com/en-us/library/bb530716(VS.85).aspx
internal const string SE_SHUTDOWN_NAME = "SeShutdownPrivilege";

private bool AddShutDownPrivilegeToApp()
{
    try
    {
        bool retVal;
        TokPriv1Luid tp;
        IntPtr hproc = GetCurrentProcess();
        IntPtr htok = IntPtr.Zero;
        retVal = OpenProcessToken(hproc, TOKEN_ADJUST_PRIVILEGES | TOKEN_QUERY, ref htok);
        tp.Count = 1;
        tp.Luid = 0;
        tp.Attr = SE_PRIVILEGE_ENABLED;
        retVal = LookupPrivilegeValue(null, SE_SHUTDOWN_NAME, ref tp.Luid);
        retVal = AdjustTokenPrivileges(htok, false, ref tp, 0, IntPtr.Zero, IntPtr.Zero);
        return retVal;
    }
    catch (Exception ex)
    {
        //throw;
        return false;
    }

}
#endregion

Again taken from PInvoke a great web-site