COM and Services - delphi

Question

I'm trying to create a service to communicate with a COM object that controls on BarCode Scanner (model Motorola Symbol LS9208).
I created the TLB unit from ocx installed with scanner driver application.
In Delphi, I had create a DataModule that make all the work I put the start and stop code of scanner in ServiceExecute procedure:
Initialize the TScanner COM object (declared in TLB)
Set some scanner properties, "claim" the control and set one TScanner event with one of the procedures in DataModule Service to fire when barcode is readed
After all, before close, release and free the TScanner object
procedure TInteliPEDCheckService.ServiceExecute(Sender: TService);
begin
debugLog( 'initialization of service thread...' );
CheckInitParams;
//DataModule_Create;
startScanner;
//loopback
while (not Terminated) do
begin
Sleep(100);
ServiceThread.ProcessRequests(False);
end;
debugLog( 'end of service loop...' ); //only reached if stop service before read codbar
stopScanner;
//DataModule_Free;
debugLog( 'finalization of service thread...' );
end;
procedure TInteliPEDCheckService.startScanner;
var HR : HRESULT;
begin
//CoInitialize( nil );
//CoInitializeEx( nil, COINIT_MULTITHREADED );
//CoInitializeEx( nil, COINIT_MULTITHREADED or COINIT_DISABLE_OLE1DDE );
//CoInitializeEx( nil, COINIT_APARTMENTTHREADED );
CoInitializeEx( nil, COINIT_APARTMENTTHREADED or COINIT_DISABLE_OLE1DDE );
// I've tried all options above, but without success
HR := CoInitializeSecurity(
nil, -1, nil, nil,
1, // RPC_C_AUTHN_LEVEL_NONE
1, // RPC_C_IMP_LEVEL_ANONYMOUS
nil,
0, // EOAC_NONE
nil);
debugLog( 'HRESULT of CoInitializeSecurity: ' + IntToStr( HR ) );
// the log always shows 0 as HRESULT
Scanner := TScanner.Create( nil );
Scanner.Open( 'STI_USBSCANNER' );
if Scanner.Claim(0) = 0 then
begin
Scanner.OnDataEvent := ScannerDataEvent;
//Scanner.OnDirectIOEvent := ScannerDirectIOEvent;
Scanner.DeviceEnabled := TRUE;
Scanner.DataEventEnabled := TRUE;
Scanner.FreezeEvents := FALSE;
end
else
begin
Scanner.DeviceEnabled := FALSE;
Scanner.DataEventEnabled := FALSE;
Scanner.FreezeEvents := FALSE;
end;
// Caption := IfThen( Scanner.Claimed, 'OK', 'FAIL' );
end;
procedure TInteliPEDCheckService.stopScanner;
begin
if ( Scanner <> nil ) then
begin
Scanner.Release;
Scanner.Free;
CoUninitialize();
end;
end;
procedure TInteliPEDCheckService.ScannerDataEvent(ASender: TObject; lStatus: Integer);
begin
debugLog( 'will fire...' ); //never reached
debugLog( 'FIRED: ' + Scanner.ScanData );
//Scanner.DeviceEnabled := FALSE;
//Sleep(1000);
Scanner.DeviceEnabled := TRUE;
Scanner.DataEventEnabled := TRUE;
end;
When I create the DataModule run this code into an EXE application, everything works fine. But now I'm migrating this funcionality to a Windows Service.
As first problem, the error of "CoInitialize not called" fired and nothing works.
So, I tried to CoInitialize() in service's DPR, before Application.Initialize but don't solve, and error keep raising.
The error only gone when I put CoInitialize() just before TScanner.Create(nil), so the scanner starts and I could see the laser ready to read barcodes.
Now the scanner starts fine with the service start, and stop fine too when service stops.
But if I read some barcode, and the event OnDataEvent is about to be triggered, the service is halted, and no data is received, an (of course) I lost the control of the scanner, needing to restart the service.
I read some instructions of how to CoInitialize the COM objects (https://msdn.microsoft.com/en-us/library/windows/desktop/ff485844(v=vs.85).aspx) and found some mention about marshalling technique to transport data between COM objects and other threads. But I'm not sure if this is the problem... and I don't know how to do that.
The service always halt when I scan some barcode. If not, starts and stops without errors, and the laser lights on when start, and light off when stop, demonstrating normal and correct command and operation.
Please, some one can help me to solve this problem?
Thanks in advance!

Answer 1

I once solved COM trouble within an NT-service by calling CoInitializeSecurity right after CoInitialize. I wrote more about it here. Apart from that it is important to get the threading right. So be sure by debugging the 'normal' application version and using GetCurrentThreadId that the incoming data from the scanner is handled by the exact thread you expect it on.