How to receive session switch notifications (XP) ?

Problem/Question/Abstract:

How can I receive the WM_WTSSESSION_CHANGE message when a session switch occurs ?

Answer:

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:

NOTIFY_FOR_THIS_SESSION. A window is notified only about the session change events that affect the session to which window belongs.
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
********************************************************************}

unit Wtsapi;

interface

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 := -1;
  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.

{********************************************************************
  Example:
********************************************************************}

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;