Restrict an Delphi application to only one instance

procedure EnsureSingleInstance;

var

  Wnd: HWnd;

  WndClass, WndText: array[0..255] of char;

begin

{$ifdef Win32}

  { Try and create a semaphore. If we succeed, then check }

  { if the semaphore was already present. If it was }

  { then a previous instance is floating around. }

  { Note the OS will free the returned semaphore handle }

  { when the app shuts so we can forget about it }

  if (CreateSemaphore(nil, 0, 1,

        PChar(ExtractFileName(Application.ExeName))) <> 0) and

     (GetLastError = Error_Already_Exists) then

{$else}

  if HPrevInst <> 0 then

{$endif}

  begin

    Wnd := GetWindow(Application.Handle, gw_HWndFirst);

    while Wnd <> 0 do

    begin

      { Look for the other TApplication window out there }

      if Wnd <> Application.Handle then

      begin

        { Check it's definitely got the same class and caption }

        GetClassName(Wnd, WndClass, Pred(SizeOf(WndClass)));

        GetWindowText(Wnd, WndText, Succ(Length(Application.Title)));

        if (StrPas(WndClass) = Application.ClassName) and

           (StrPas(WndText) = Application.Title) then

        begin

          { This technique is used by the VCL: post }

          { a message then bring the window to the }

          { top, before the message gets processed }

          PostMessage(Wnd, wm_SysCommand, sc_Restore, 0);

{$ifdef Win32}

          SetForegroundWindow(Wnd);

{$else}

          BringWindowToTop(Wnd);

{$endif}

          Halt

        end

      end;

      Wnd := GetWindow(Wnd, gw_HWndNext)

    end

  end

end;