Problem with Call Back Capture function

Hello.
I am a Delphi developer.
I am testing demo version af Lead Tools.
I have studied demo project OcrUtilityDemo and then I have created a little project to automate scanning more pages and proccess them using LTWAINBITMAPCALLBACK.
There is a problem when the call back function is called because the pUserData parameter remains null.
I tried to put my code in the demo project OCRUtilityDemo and there it run correctly; I have done many test, but without understanding what is the cause of the different running.
My code is:

interface
type
TMainFrm = class(TForm)
Button1: TButton;
procedure FormCreate(Sender: TObject);
procedure FormDestroy(Sender: TObject);
procedure Button1Click(Sender: TObject);
private
_hBitmap: BITMAPHANDLE;
_bTwnExist: Boolean;
_hZonePen: HPEN;
_hSelZonePen: HPEN;
Procedure InitTwain();
Function InitOcrEngine(): Boolean;
public
end;

function MyTwainCallBack(hSession: HTWAINSESSION; pBitmap: pBITMAPHANDLE; pUserData: L_PVOID): L_INT;

implementation

procedure TMainFrm.FormCreate(Sender: TObject);
var
nRet: L_INT;
rAppData: APPLICATIONDATA;
begin
UNLOCKSUPPORT;
hDocument := 0;
_bTwnExist:= L_IsTwainAvailable(Handle);
if(_bTwnExist)then begin
InitTwain();
end;
InitOcrEngine();
L_InitBitmap(@_hBitmap, sizeof(BITMAPHANDLE), 0, 0, 0);
end;

procedure TMainFrm.FormDestroy(Sender: TObject);
begin
L_DocShutDown(@hDocument);
if (IsAllocated(@_hBitmap)) then L_FreeBitmap(@_hBitmap);
end;

procedure TMainFrm.Button1Click(Sender: TObject);
begin
L_TwainAcquire(hTwnSession,
nil,
sizeof(BITMAPHANDLE),
@MyTwainCallBack,
LTWAIN_SHOW_USER_INTERFACE + LTWAIN_MODAL_USER_INTERFACE,
Nil,
Self);
end;

function MyTwainCallBack(hSession: HTWAINSESSION; pBitmap: pBITMAPHANDLE; pUserData: L_PVOID): L_INT;
var
mForm: TMainFrm;
begin
mForm:= TMainFrm(pUserData);
if(IsAllocated(@mForm._hBitmap))then begin
L_FreeBitmap(@mForm._hBitmap);
L_InitBitmap(@mForm._hBitmap, sizeof(BITMAPHANDLE), 0, 0, 0);
end;
L_CopyBitmap(@mForm._hBitmap, pBitmap, sizeof(BITMAPHANDLE));
// rest of my code ...
end;

What LEADTOOLS version are you using?

What does the InitTwain() function do exactly?

Can
you please try to isolate the problem in a very small sample project (not your full application) and post it here or send it to support@leadtools.com.

If
you are going to post it here please make sure that you don't hit Preview when attaching.

Dear Adam Boulad
I have sent my code to support@leadtools.com.
The code is an extract of the OCRUtilityDemo included in demo of Lead Tools, with some my implementation.
The problem occurs in execution of MyTwainCallBack: hSession, pBitmap and pUserData are null.
Many thanks for your help.
Franco

I have received your email and I downloaded the attachment. I will continue to work with you through emails.
I will post here the results when we reach final results.

After tracing the customer's project, I found that the Callback calling convention is not complete.
Modify the project as follows:
function MyTwainCallBack(hSession: HTWAINSESSION; pBitmap: pBITMAPHANDLE;
pUserData: L_PVOID): L_INT;

will become:

function MyTwainCallBack(hSession: HTWAINSESSION; pBitmap: pBITMAPHANDLE;
pUserData: L_PVOID): L_INT; stdcall;

That seemed to solve the issue.