program OneInstance;

uses
  Windows, //General API Functions
  Forms, //For the Application objects
  Unit1 in 'Unit1.pas' {Form1};

{$R *.RES}

begin
 {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;
end.


{The code for unit1.pas}

unit Unit1;

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.