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;

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