{
 Typically, an application does not need to be notified when a session switch
 occurs. However, if the application needs to be aware when its desktop is
 current, it can register for session switch notifications. Applications that
 access the serial port or another shared resource on the computer should
 check for this. To register for a notification, use the following function:
}
 function WTSRegisterSessionNotification(
  hWnd: HWND , // Window handle
  dwFlags: DWORD // Flags
  ): Bool; // Return value
{
 The registered HWND receives the message WM_WTSSESSION_CHANGE
 through its WindowProc function.
 In dwFlags you can specify:
  a) NOTIFY_FOR_THIS_SESSION. A window is notified only about the session
  change events that affect the session to which window belongs.
  b) NOTIFY_FOR_ALL_SESSIONS. A window is notified for all session change
  events.
 The action happening on the session can be found in wParam code, which may
 contain one of the following flags.
 WTS_CONSOLE_CONNECT: A session was connected to the console session.
 WTS_CONSOLE_DISCONNECT: A session was disconnected from the console session.
 WTS_REMOTE_CONNECT: A session was connected to the remote session.
 WTS_REMOTE_DISCONNECT: A session was disconnected from the remote session.
 WTS_SESSION_LOGON: A user has logged on to the session.
 WTS_SESSION_LOGOFF: A user has logged off the session.
 WTS_SESSION_LOCK: A session has been locked.
 WTS_SESSION_UNLOCK: A session has been unlocked.
 WTS_SESSION_REMOTE_CONTROL: A session has changed its remote controlled status.

 lParam contains the sessionId for the session affected.
 When your process no longer requires these notifications or is terminating,
 it should call the following to unregister its notification.
}
 function WTSUnRegisterSesssionNotification(
  hWnd: HWND // window handle.
  ): Boolean; // Result
{
 The HWND values passed to WTSRegisterSessionNotification are reference
 counted, so you must call WTSUnRegisterSessionNotification exactly the same
 number of times that you call WTSRegisterSessionNotification.
 Applications can use the WTS_CONSOLE_CONNECT, WTS_CONSOLE_DISCONNECT,
 WTS_REMOTE_CONNECT, WTS_REMOTE_DISCONNECT messages to track their state, as
 well as to release and acquire console specific resources.
}

unit Wtsapi;
interface
{ (c) By Thomas Stutz 10. April 02 }
uses
 Windows;
const
 // The WM_WTSSESSION_CHANGE message notifies applications of changes in session state.
 WM_WTSSESSION_CHANGE = $2B1;
 // wParam values:
 WTS_CONSOLE_CONNECT = 1;
 WTS_CONSOLE_DISCONNECT = 2;
 WTS_REMOTE_CONNECT = 3;
 WTS_REMOTE_DISCONNECT = 4;
 WTS_SESSION_LOGON = 5;
 WTS_SESSION_LOGOFF = 6;
 WTS_SESSION_LOCK = 7;
 WTS_SESSION_UNLOCK = 8;
 WTS_SESSION_REMOTE_CONTROL = 9;
 // Only session notifications involving the session attached to by the window
 // identified by the hWnd parameter value are to be received.
 NOTIFY_FOR_THIS_SESSION = 0;
 // All session notifications are to be received.
 NOTIFY_FOR_ALL_SESSIONS = 1;

function RegisterSessionNotification(Wnd: HWND; dwFlags: DWORD): Boolean;
function UnRegisterSessionNotification(Wnd: HWND): Boolean;
function GetCurrentSessionID: Integer;
implementation
function RegisterSessionNotification(Wnd: HWND; dwFlags: DWORD): Boolean;
 // The RegisterSessionNotification function registers the specified window
 // to receive session change notifications.
 // Parameters:
 // hWnd: Handle of the window to receive session change notifications.
 // dwFlags: Specifies which session notifications are to be received:
 // (NOTIFY_FOR_THIS_SESSION, NOTIFY_FOR_ALL_SESSIONS)
type
 TWTSRegisterSessionNotification = function(Wnd: HWND; dwFlags: DWORD): BOOL; stdcall;
var
 hWTSapi32dll: THandle;
 WTSRegisterSessionNotification: TWTSRegisterSessionNotification;
begin
 Result := False;
 hWTSAPI32DLL := LoadLibrary('Wtsapi32.dll');
 if (hWTSAPI32DLL > 0) then
 begin
  try @WTSRegisterSessionNotification :=
  GetProcAddress(hWTSAPI32DLL, 'WTSRegisterSessionNotification');
  if Assigned(WTSRegisterSessionNotification) then
  begin
  Result:= WTSRegisterSessionNotification(Wnd, dwFlags);
  end;
  finally
  if hWTSAPI32DLL > 0 then
  FreeLibrary(hWTSAPI32DLL);
  end;
 end;
end;
function UnRegisterSessionNotification(Wnd: HWND): Boolean;
 // The RegisterSessionNotification function unregisters the specified window
 // Parameters:
 // hWnd: Handle to the window
type
 TWTSUnRegisterSessionNotification = function(Wnd: HWND): BOOL; stdcall;
var
 hWTSapi32dll: THandle;
 WTSUnRegisterSessionNotification: TWTSUnRegisterSessionNotification;
begin
 Result := False;
 hWTSAPI32DLL := LoadLibrary('Wtsapi32.dll');
 if (hWTSAPI32DLL > 0) then
 begin
  try @WTSUnRegisterSessionNotification :=
  GetProcAddress(hWTSAPI32DLL, 'WTSUnRegisterSessionNotification');
  if Assigned(WTSUnRegisterSessionNotification) then
  begin
  Result:= WTSUnRegisterSessionNotification(Wnd);
  end;
  finally
  if hWTSAPI32DLL > 0 then
  FreeLibrary(hWTSAPI32DLL);
  end;
 end;
end;
function GetCurrentSessionID: Integer;
 // Getting the session id from the current process
type
 TProcessIdToSessionId = function(dwProcessId: DWORD; pSessionId: DWORD): BOOL; stdcall;
var
 ProcessIdToSessionId: TProcessIdToSessionId;
 hWTSapi32dll: THandle;
 Lib : THandle;
 pSessionId : DWord;
begin
 Result := 0;
 Lib := GetModuleHandle('kernel32');
 if Lib <> 0 then
 begin
  ProcessIdToSessionId := GetProcAddress(Lib, '1ProcessIdToSessionId');
  if Assigned(ProcessIdToSessionId) then
  begin
  ProcessIdToSessionId(GetCurrentProcessId(), DWORD(@pSessionId));
  Result:= pSessionId;
  end;
 end;
end;
end.

unit Unit1;
interface
uses
 Windows, Messages, {...}, Wtsapi;
type
 TForm1 = class(TForm)
  procedure FormCreate(Sender: TObject);
  procedure FormDestroy(Sender: TObject);
 private
 { Private declarations }
  FRegisteredSessionNotification : Boolean;
  procedure AppMessage(var Msg: TMSG; var HAndled: Boolean);
 end;
var
 Form1: TForm1;
implementation
{$R *.DFM}
procedure TForm1.AppMessage(var Msg: TMSG; var Handled: Boolean);
var
 strReason: string;
begin
 Handled := False;
 // Check for WM_WTSSESSION_CHANGE message
 if Msg.Message = WM_WTSSESSION_CHANGE then
 begin
  case Msg.wParam of
  WTS_CONSOLE_CONNECT:
  strReason := 'WTS_CONSOLE_CONNECT';
  WTS_CONSOLE_DISCONNECT:
  strReason := 'WTS_CONSOLE_DISCONNECT';
  WTS_REMOTE_CONNECT:
  strReason := 'WTS_REMOTE_CONNECT';
  WTS_REMOTE_DISCONNECT:
  strReason := 'WTS_REMOTE_DISCONNECT';
  WTS_SESSION_LOGON:
  strReason := 'WTS_SESSION_LOGON';
  WTS_SESSION_LOGOFF:
  strReason := 'WTS_SESSION_LOGOFF';
  WTS_SESSION_LOCK:
  strReason := 'WTS_SESSION_LOCK';
  WTS_SESSION_UNLOCK:
  strReason := 'WTS_SESSION_UNLOCK';
  WTS_SESSION_REMOTE_CONTROL:
  begin
  strReason := 'WTS_SESSION_REMOTE_CONTROL';
  // GetSystemMetrics(SM_REMOTECONTROL);
  end;
  else
  strReason := 'WTS_Unknown';
  end;
  // Write strReason to a Memo
  Memo1.Lines.Add(strReason + ' ' + IntToStr(msg.Lparam));
 end;
end;
procedure TForm1.FormCreate(Sender: TObject);
begin
 // register the window to receive session change notifications.
 FRegisteredSessionNotification := RegisterSessionNotification(Handle, NOTIFY_FOR_THIS_SESSION);
 Application.OnMessage := AppMessage;
end;
procedure TForm1.FormDestroy(Sender: TObject);
begin
 // unregister session change notifications.
 if FRegisteredSessionNotification then
  UnRegisterSessionNotification(Handle);
end;
procedure TForm1.Button1Click(Sender: TObject);
begin
 // retrieve current session ID
 ShowMessage(Inttostr(GetCurrentSessionID));
end;