HOWTO:Turn Off PDA Display while your application is running.(VB.Net/c#+.Net CF+PPC2003) from CodeProject

Core code for VB.NET:

Namespace PDANamespace PDA
    Public Class VideoClass Video
        Private Const SETPOWERMANAGEMENT As Int32 = 6147

        Declare Function ExtEscapeSet()Function ExtEscapeSet Lib "coredll" Alias "ExtEscape" (ByVal hdc As IntPtr, _
                                                  ByVal nEscape As Int32, _
                                                  ByVal cbInput As Int32, _
                                                  ByVal plszInData As Byte(), _
                                                  ByVal cbOutput As Int32, _
                                                  ByVal lpszOutData As IntPtr) As Int32

        Declare Function GetDC()Function GetDC Lib "coredll" (ByVal hwnd As IntPtr) As IntPtr

        Public Enum VideoPowerStateEnum VideoPowerState As Integer
            VideoPowerOn = 1
            VideoPowerStandBy
            VideoPowerSuspend
            VideoPowerOff
        End Enum

        Public Shared Sub PowerOff()Sub PowerOff()
            Dim hdc As IntPtr = GetDC(IntPtr.Zero)
            Dim vpm() As Byte = {12, 0, 0, 0, 1, 0, 0, 0, VideoPowerState.VideoPowerOff, 0, 0, 0, 0}
            ExtEscapeSet(hdc, SETPOWERMANAGEMENT, 12, vpm, 0, IntPtr.Zero)
        End Sub

        Public Shared Sub PowerOn()Sub PowerOn()
            Dim hdc As IntPtr = GetDC(IntPtr.Zero)
            Dim vpm() As Byte = {12, 0, 0, 0, 1, 0, 0, 0, VideoPowerState.VideoPowerOn, 0, 0, 0, 0}
            ExtEscapeSet(hdc, SETPOWERMANAGEMENT, 12, vpm, 0, IntPtr.Zero)
        End Sub
    End Class
End Namespace

Core code for C#

using System;
using System.Runtime.InteropServices;

Namespace OpenNETCFnamespace OpenNETCF
{
/// <summary>
/// Summary description for Video.
/// </summary>
public Class Videoclass Video
{
         // GDI Escapes for ExtEscape()
         private const uint QUERYESCSUPPORT   = 8;

         // The following are unique to CE
         private const uint GETVFRAMEPHYSICAL =  6144;
         private const uint GETVFRAMELEN   = 6145;
         private const uint DBGDRIVERSTAT   = 6146;
         private const uint SETPOWERMANAGEMENT =  6147;
         private const uint GETPOWERMANAGEMENT =  6148;

         public static void PowerOff()
         {
                 IntPtr hdc = GetDC(IntPtr.Zero);
                 uint func = SETPOWERMANAGEMENT;

                 uint size = 12;
                 byte[] vpm = new byte[size];
                                   
                 //Structure sizestructure size
                 BitConverter.GetBytes(size).CopyTo(vpm, 0);
                 //dpms version
                 BitConverter.GetBytes(0x0001).CopyTo(vpm, 4);
                 //power state
                 BitConverter.GetBytes((uint)VideoPowerState.VideoPowerOff).CopyTo(vpm, 8);

                 ExtEscapeSet(hdc, SETPOWERMANAGEMENT, size, vpm, 0, IntPtr.Zero);
         }

         public static void PowerOn()
         {
         IntPtr hdc = GetDC(IntPtr.Zero);

                 uint size = 12;
                 byte[] vpm = new byte[size];
                          
                 //Structure sizestructure size
                 BitConverter.GetBytes(size).CopyTo(vpm, 0);
                 //dpms version
                 BitConverter.GetBytes(0x0001).CopyTo(vpm, 4);
                 //power state
                 BitConverter.GetBytes((uint)VideoPowerState.VideoPowerOn).CopyTo(vpm, 8);

                 ExtEscapeSet(hdc, SETPOWERMANAGEMENT, size, vpm, 0, IntPtr.Zero);
                 }

         [DllImport("coredll", EntryPoint="ExtEscape")]
         private static extern int ExtEscapeSet( 
                 IntPtr hdc, 
                 uint nEscape, 
                 uint cbInput, 
                 byte[] lpszInData,
                 int cbOutput, 
                 IntPtr lpszOutData
                 );

         [DllImport("coredll")]
         private static extern IntPtr GetDC(IntPtr hwnd);

}
         
         
public Enum VideoPowerStateenum VideoPowerState : uint
{
         VideoPowerOn = 1,
         VideoPowerStandBy,
         VideoPowerSuspend,
         VideoPowerOff
}