Handle for a DLL with system service

Well... look, I have done for you an example of this with two applications: one receiver and other sender:

The Receiver application with a custom Allocated Window:

unit rUnit1;

interface

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

const
 WM_CUSTOM_MSG = WM_USER + $1000;

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

type
 TShared = record
   OtherStuff : integer;
   WinHandle  : hwnd;
 end;
 PShared      = ^TShared;

var
  Form1      : TForm1;
  Shared     : PShared;
  FicheroM   : THandle;
  TheWindow  : Hwnd;

implementation

{$R *.dfm}

procedure TForm1.FormCreate(Sender: TObject);
begin
  //First, allocate a Window for to have a handle and a
  //winproc for catch the received messages:
  TheWindow := AllocateHWnd(HookWindowProc);


  {Create the memory mapped file}
  FicheroM:=CreateFileMapping( $FFFFFFFF,
                              nil,
                              PAGE_READWRITE,
                              0,
                              SizeOf(TShared),
                              'MyFile');
    if FicheroM=0 then
     raise Exception.Create( 'Error while create file');

   {Adrress our record in the memory mapped file}
   Shared:=MapViewOfFile(FicheroM,FILE_MAP_WRITE,0,0,0);

   {Write data in the memory shared zone:}
   Shared^.OtherStuff:=666;  //Just an example...
   Shared^.WinHandle:=TheWindow;
end;

procedure TForm1.HookWindowProc(var M :TMessage);
begin
  case M.msg of
   WM_CUSTOM_MSG:
    begin
      Caption:='Custom Message received...';
    end;
  else
    m.Result := DefWindowProc(TheWindow , m.Msg, m.WParam, m.LParam);
  end;
end;

procedure TForm1.FormDestroy(Sender: TObject);
begin
  //Deallocate the receiver window
  if (TheWindow<>0) then DeAllocateHWnd(TheWindow);

  {Close the MapView}
  if Assigned(Shared) then UnmapViewOfFile(Shared);

  {Close the file}
  if FicheroM<>0 then CloseHandle(FicheroM);
end;

end.

An the Sender Application:

- Just have a TButton called Button1

unit sUnit1;

interface

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

const
 WM_CUSTOM_MSG = WM_USER + $1000;


type
  TForm1 = class(TForm)
    Button1: TButton;
    procedure FormCreate(Sender: TObject);
    procedure FormDestroy(Sender: TObject);
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

type
 TShared = record
   OtherStuff : integer;
   WinHandle  : hwnd;
 end;
 PShared      = ^TShared;


var
  Form1     : TForm1;
  Shared    : PShared;
  FicheroM  : THandle;

implementation

{$R *.dfm}

procedure TForm1.FormCreate(Sender: TObject);
begin
  {Open the memory mapped file}
  FicheroM:=OpenFileMapping(FILE_MAP_WRITE,False,'MyFile');

  if FicheroM=0 then
     raise Exception.Create( 'Error while open file');

  {Adrress our record in the memory mapped file}
  Shared:=MapViewOfFile(FicheroM,FILE_MAP_WRITE,0,0,0);
end;

procedure TForm1.FormDestroy(Sender: TObject);
begin
  {Close the MapView}
  if Assigned(Shared) then UnmapViewOfFile(Shared);

  {Close the file}
  if FicheroM<>0 then CloseHandle(FicheroM);
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  //Send the message to the receiver application:
  SendMessage(Shared^.WinHandle,WM_CUSTOM_MSG,0,0);
end;

end.


Working explanation:

The receiver makes this operations:

- Define a new user message
const
 WM_CUSTOM_MSG = WM_USER + $1000;
- Allocate a Window for get a handle and a WindProc with AllocateWindow
- Create a Memory mapped file
- Address a record (Shared) to the memory shared zone (with MapViewOfFile)
- Write the handle of the window allocated as receptor of the messages sended by the sender application

The sender application makes:

- Define the same user message
- Open the memory mapped file
- Adrress the same structure to the same memory zone
- Read the hwnd of the allocated window in the receiver application
- Send a message to that readed handle

I hope that with this example you can make work your application... :)

This tecnique of allocate window is also used in Non visual components when it's neccessary receive messages...