Delphi: Restricting a program to running only once, using mutexes

program DD44;

uses
  Forms,
  DD44u1 in 'DD44u1.pas' {Form1};

{$R *.RES}

  Application.Initialize;
  Application.CreateForm(TForm1, Form1);
  Application.Run;

 {Attempt to create a named mutex}
  CreateMutex(nil, false, 'MyApp');
 {if it failed then there is another instance}
  if GetLastError = ERROR_ALREADY_EXISTS then begin
   {Send all windows our custom message - only our other}
   {instance will recognise it, and restore itself}
    SendMessage(HWND_BROADCAST,
                RegisterWindowMessage('MyApp'),
                0,
                0);
   {Lets quit}
    Halt(0);
  end;
  Application.Initialize;
 {Tell Delphi to un-hide it's hidden application window}
 {This allows our instance to have a icon on the task bar}
  Application.ShowMainForm := true;
  ShowWindow(Application.Handle, SW_RESTORE);
  Application.CreateForm(TForm1, Form1);
  Application.Run;

unit DD44u1;

interface

uses
  Windows, Messages, SysUtils, Classes, Graphics, Controls,
  Forms, Dialogs;

type
  TForm1 = class(TForm)
    procedure FormCreate(Sender: TObject);
    procedure FormDestroy(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.DFM}

var
  OldWindowProc : Pointer; {Variable for the old windows proc}
  MyMsg : DWord; {custom systemwide message}

function NewWindowProc(WindowHandle : hWnd;
                       TheMessage   : LongInt;
                       ParamW       : LongInt;
                       ParamL       : LongInt) : LongInt stdcall;
begin
  if TheMessage = MyMsg  then begin
   {Tell the application to restore, let it restore the form}
    SendMessage(Application.handle, WM_SYSCOMMAND, SC_RESTORE, 0);
    SetForegroundWindow(Application.Handle);
   {We handled the message - we are done}
    Result := 0;
    exit;
  end;
 {Call the original winproc}
  Result := CallWindowProc(OldWindowProc,
                           WindowHandle,
                           TheMessage,
                           ParamW,
                           ParamL);
end;


procedure TForm1.FormCreate(Sender: TObject);
begin
 {Register a custom windows message}
  MyMsg := RegisterWindowMessage('MyApp');
 {Set form1's windows proc to ours and remember the old window proc}
  OldWindowProc := Pointer(SetWindowLong(Form1.Handle,
                                         GWL_WNDPROC,
                                         LongInt(@NewWindowProc)));
end;

procedure TForm1.FormDestroy(Sender: TObject);
begin
 {Set form1's window proc back to it's original procedure}
  SetWindowLong(Form1.Handle,
                GWL_WNDPROC,
                LongInt(OldWindowProc));
end;

begin
 {Tell Delphi to hide it's hidden application window for now to avoid}
 {a "flash" on the taskbar if we halt due to another instance}
  ShowWindow(Application.Handle, SW_HIDE);
end.

var
  OldWindowProc : Pointer; {Variable for the old windows proc}
  MyMsg : DWord; {custom systemwide message}

function NewWindowProc(WindowHandle : hWnd;
                       TheMessage   : LongInt;
                       ParamW       : LongInt;
                       ParamL       : LongInt) : LongInt stdcall;
begin
  if TheMessage = MyMsg  then begin
   {Tell the application to restore, let it restore the form}
    SendMessage(Application.handle, WM_SYSCOMMAND, SC_RESTORE, 0);
    SetForegroundWindow(Application.Handle);
   {We handled the message - we are done}
    Result := 0;
    exit;
  end;
 {Call the original winproc}
  Result := CallWindowProc(OldWindowProc,
                           WindowHandle,
                           TheMessage,
                           ParamW,
                           ParamL);
end;

begin
 {Register a custom windows message}
  MyMsg := RegisterWindowMessage('MyApp');
 {Set form1's windows proc to ours and remember the old window proc}
  OldWindowProc := Pointer(SetWindowLong(Form1.Handle,
                                         GWL_WNDPROC,
                                         LongInt(@NewWindowProc)));

begin
 {Set form1's window proc back to it's original procedure}
  SetWindowLong(Form1.Handle,
                GWL_WNDPROC,
                LongInt(OldWindowProc));

begin
 {Tell Delphi to hide it's hidden application window for now to avoid}
 {a "flash" on the taskbar if we halt due to another instance}
  ShowWindow(Application.Handle, SW_HIDE);

Ad from page's editor: Yes.. I do enjoy compiling these things for you... hope they are helpful. However.. this doesn't pay my bills!!! If you find this stuff useful, (and you run an MS-DOS or Windows pc) please visit my freeware and shareware page, download something, and circulate it for me? Links on your page to this page would also be appreciated!