Creating a Single instance of an Application

https://www.experts-exchange.com/questions/20425481/How-to-create-single-instance-application-in-Delphi.html

program PGLIWMan;

uses
  Forms,
  SysUtils,
  Windows,
  Messages,
  VTDevManFM in 'VTDevManFM.pas' {ManFRM};

{$R *.res}

var
  hFileMap, Handle:    Cardinal;
  FMapPt:              Pointer;

begin
  hFileMap := CreateFileMapping($FFFFFFFF,     // $FFFFFFFF get's memory file
                                nil,           // no security attributes
                                PAGE_READWRITE,// read/write access
                                0,             // size: high 32-bits
                                2048,          // size: low 32-bits
                                'Q_21080749'); // name of map object
  if (GetLastError = ERROR_ALREADY_EXISTS) then
  begin
    if (ParamCount > 0) then
    begin
      FMapPt := MapViewOfFile(hFileMap, FILE_MAP_WRITE, 0, 0, 0);
      if FMapPt <> nil then
      begin
        Handle := Cardinal(FMapPt^);
        if FileExists(ParamStr(1)) then
        StrCopy(Pointer(Integer(FMapPt)+4), PChar(ParamStr(1)));
        UnmapViewOfFile(FMapPt);
        SendMessage(Handle, WM_USER+432, 0, 0);
      end;
    end;
    CloseHandle(hFileMap);
    Exit;
  end;

  Application.Initialize;
  Application.Title := 'Single Instance Application';
  Application.CreateForm(TManFRM, ManFRM);
  Application.Run;
  CloseHandle(hFileMap);
end.

//........

unit VTDevManFM;

interface

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

type
  TManFRM = class(TForm)
    Edit1: TEdit;
  private   { Private declarations }
    procedure GetFileName(var Msg : TMessage); message WM_USER + 432;
  public    { Public declarations }
  end;

var
  ManFRM: TManFRM;

implementation

{$R *.dfm}

procedure TManFRM.GetFileName(var Msg : TMessage);
var
  hFileMap: Cardinal;
  FMapPt: Pointer;
  AryChar2: Array[0..2043] of Char;
begin
  hFileMap := OpenFileMapping(FILE_MAP_WRITE, False, 'Q_21080749');
  FMapPt := MapViewOfFile(hFileMap, FILE_MAP_WRITE, 0, 0, 0);
    if (FMapPt <> nil) then
    begin
      StrCopy(@AryChar2[0],Pointer(Integer(FMapPt)+4));
//      OpenFile(AryChar2);  {OpenFile is your apps procedure to open files}
      UnmapViewOfFile(FMapPt);
    end;
  CloseHandle(hFileMap);
end;

end.

Hi thanks for the responses

Workshop_alex do you have example of the code what you taled about?

Another point the code for bring the window forward has worked before in other apps, but not now any ideas?
 

Hi all realised with a couple of yours help what I did wrong the

I like the look of Hotrod's suggestion it is simple and effective, but it did not stop muliple instances

but it did inspire me write my own...


var
  Hmutex:Thandle;
  HWindow:Thandle;

begin
  HMutex:= CreateMutex(nil,false,'ManFRM');
  HWindow:= FindWindow('TManFrm');
  if WaitForSingleObject(hmutex,0) <> wait_TimeOut then
  begin
    Application.Initialize;
    Application.Title := 'VT Manager';
    Application.CreateForm(TManFRM,ManFRM);
    Application.Run;
  end
  else
  begin
    setForegroundWindow(HWindow);
  end;
end.


So I have split the point to those who has suggested FindWindow.

Repost...(Sorry)

var
  Hmutex:Thandle;
  HWindow:Thandle;

begin
  HMutex:= CreateMutex(nil,false,'ManFRM');
  //forgot to add nil as paramtre
  HWindow:= FindWindow('TManFrm',nil);
  if WaitForSingleObject(hmutex,0) <> wait_TimeOut then
  begin
    Application.Initialize;
    Application.Title := 'VT Manager';
    Application.CreateForm(TManFRM,ManFRM);
    Application.Run;
  end
  else
  begin
    setForegroundWindow(HWindow);
  end;
end.

a better approach would be:

  // Will try to create a named mutex
  CreateMutex(nil, false, ManFRM);
  // if it failed then there is another instance running
  if GetLastError = ERROR_ALREADY_EXISTS then begin
    // There is an error, so it exist another ManFRM instance running
    // Send all windows our custom message - only our other
    // instance will recognise it, and restore itself
    SendMessage(HWND_BROADCAST, RegisterWindowMessage('ManFRM'), 0, 0);
    Halt(0);
  end;

  Application.Initialize;
  Application.Title := 'VT Manager';
  Application.CreateForm(TManFRM,ManFRM);
  Application.Run;