Alisson:

How to detect when the user locks/unlocks the screen in Windows 7?

I found this question which has an answer for C#, but I'd like to use it in Delphi 2009. I'd guess there is some windows message (like these) which could do the work. This is the code I tried, but it didn't work:

const
  NOTIFY_FOR_ALL_SESSIONS = 1;
  {$EXTERNALSYM NOTIFY_FOR_ALL_SESSIONS}
  NOTIFY_FOR_THIS_SESSION = 0;
  {$EXTERNALSYM NOTIFY_FOR_THIS_SESSION}

type

TfrmAlisson = class(TForm)
  lbl2: TLabel;
  procedure FormCreate(Sender: TObject);
  procedure FormDestroy(Sender: TObject);
public
  FLockedCount: Integer;
  procedure WndProc(var Message: TMessage); override;
  function WTSRegisterSessionNotification(hWnd: HWND; dwFlags: DWORD): bool; stdcall;
  function WTSUnRegisterSessionNotification(hWND: HWND): bool; stdcall;
end;

implementation

uses
  // my impl uses here

procedure TfrmAlisson.FormCreate(Sender: TObject);
begin
  if (WTSRegisterSessionNotification(Handle, NOTIFY_FOR_THIS_SESSION)) then
    ShowMessage('Nice')
  else
  begin
    lastError := GetLastError;
    ShowMessage(SysErrorMessage(lastError));
  end;
end;

procedure TfrmAlisson.FormDestroy(Sender: TObject);
begin
  WTSUnRegisterSessionNotification(Handle);
end;

procedure TfrmAlisson.WndProc(var Message: TMessage);
begin
  case Message.Msg of
    WM_WTSSESSION_CHANGE:
      begin
        if Message.wParam = WTS_SESSION_LOCK then
        begin
          Inc(FLockedCount);
        end;
        if Message.wParam = WTS_SESSION_UNLOCK then
        begin
          lbl2.Caption := 'Session was locked ' +
          IntToStr(FLockedCount) + ' times.';
        end;
      end;
  end;
  inherited;
end;

function TfrmAlisson.WTSRegisterSessionNotification(hWnd: HWND; dwFlags: DWORD): bool;
  external 'wtsapi32.dll' Name 'WTSRegisterSessionNotification';

function TfrmAlisson.WTSUnRegisterSessionNotification(hWND: HWND): bool;
  external 'wtsapi32.dll' Name 'WTSUnRegisterSessionNotification';

When FormCreate executes, WTSRegisterSessionNotification returns false and the last OS error returns Invalid Parameter.