How do I use CoMarshalInterThreadInterfaceInStream to wrap an IHTMLElement?

I need to access an IHTMLElement from another thread, in my Browser Helper Object.  Unfortunately, I've been unable to get it working without causing a segmentation fault when put_innerHTML is called.

Does anyone know how to (correctly) use CoMarshalInterThreadInterfaceInStream in a BHO?
// *********** This is the calls that are made in the main thread ************
       LPSTREAM st;
       CoMarshalInterThreadInterfaceInStream(IID_IHTMLElement, Status, &st);
       m_downloadParam.statusElement = reinterpret_cast<IHTMLElement*>(st);
 
       // Create a thread to download it
       m_pDownloadThread = CreateThread(NULL,
                                                                               0,
                                                                               StartYAIPPDownload,
                                                                               &m_downloadParam,
                                                                               CREATE_SUSPENDED,
                                                                               NULL);
 
// *********** This is the calls that are made in the second thread for downloading ************
DWORD WINAPI StartYAIPPDownload(PVOID pParam)
{
       CBhoApp::DOWNLOADPARAM* parameters =
reinterpret_cast<CBhoApp::DOWNLOADPARAM*>(pParam);
       IHTMLElement* Status = parameters->statusElement;
       BSTR URLToDownload = parameters->strURL;
       YAIPPDownloader* yaippdownloader = new YAIPPDownloader(Status);
       yaippdownloader->Status = Status; // pass the HTML element
       yaippdownloader->AddRef();
       HRESULT hr = URLDownloadToFile(NULL,"http://www.roket-games.com/games/19/gdownload"/*URLToDownload*/,"C:\\test.htm",0,yaippdownloader);
 
       if (hr)
       {
               YAIPPInProgress=false;
               YAIPPStatusText=L"Could Not Download File";
       }
       else
       {
               YAIPPInProgress=false;
               YAIPPStatusText=L"Downloaded File";
       }
       return S_OK;
}
 
// *********** This is the calls that are made in the downloader class's OnProgress function (note that the two Status variables ARE different) ************
if (Status != NULL)
	{
		char ulProgressBuffer [33];
		char ulProgressPercBuffer [33];
		counter++;
		if (ulProgressMax!=0)
			itoa((100/ulProgressMax*ulProgress),ulProgressPercBuffer,10);
		else
			itoa(counter,ulProgressPercBuffer,10);
		itoa(ulProgress,ulProgressBuffer,10);
		std::wstring text;
		text = L"Downloading... ";
		if (ulProgressMax!=0)
			text += ConvertToWideChar(ulProgressBuffer);
		else
			text += ConvertToWideChar(ulProgressPercBuffer);
		BSTR StatText;
		StatText = W2BSTR((text).c_str());
 
		YAIPPStatusText=StatText;
	
		// ************* SEGMENTATION FAULT ON NEXT LINE ****************
		Status->put_innerHTML(YAIPPStatusText);
	}

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hach-queAsked:
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HalfAsleepCommented:
Where do you do the
CoInitializeEx(NULL, COINIT_MULTITHREADED);

and the

CoGetInterfaceAndReleaseStream  ?

Without these in the other thread, the other thread still has not "received" the marshalled stream.

In one thread you need to to the

CoMarshalInterThreadInterfaceInStream ( longest method name ever)

In the other thread,  you need to use the
CoInitialize and the CoGetInterfaceAndReleaseStream, or the other one that does not release the stream, if you are doing it more than once.


thread 1:
 
_IHTMLElement* pIC;
HRESULT hr = QueryInterface(IID__IHTMLElement, (LPVOID*)&pIC);
 
hRunnerThread = CreateThread(NULL, 0, Runner, pIS, 0, &dwThreadId);
 
IStream* pIS;
hr = CoMarshalInterThreadInterfaceInStream(IID__IHTMLElement, pIC, &pIS);
 
Thread 2 (runner):
CoInitializeEx(NULL, COINIT_MULTITHREADED);
DWORD RetValue = S_OK;
_IHTMLElement* pIC;
HRESULT hr = CoGetInterfaceAndReleaseStream((IStream*) lParam, IID__IHTMLElement, (LPVOID*)&pIC);
_ASSERT(SUCCEEDED(hr));
 
// now you have a valid pointer to interface, the pIC.
pIC->put_innerHTML(YAIPPStatusText);

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HalfAsleepCommented:
In my code, the CoMarshal should be above the CreateThread..sorry about that.
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HalfAsleepCommented:
Also, remember to call pIC->Release() and
CoUninitialize(); before you exit your second thread, where you did the CoInitialize
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jkrCommented:
Take a look at http://svn.openqa.org/fisheye/browse/~raw,r=57/floyd/trunk/src/cpp/ie/IEBrowserObject.cpp - they do that like
void IEBrowserObject::click(IEHTMLElement *element, int button)
{
    COMAutoPtr<IHTMLDOMNode> comObj = element->getCOMObject();
 
	if (comObj != NULL)
	{
        COMAutoPtr<IHTMLElement2> elementObj = comObj.castTo<IHTMLElement2>(IID_IHTMLElement2);
 
	    if (elementObj != NULL)
	    {
            IStream * stream     = NULL;
            IUnknown *unknownPtr = NULL;
 
            elementObj->QueryInterface(IID_IUnknown, (void **)&unknownPtr);
            if (unknownPtr != NULL)
            {
                CoMarshalInterThreadInterfaceInStream(__uuidof(IHTMLElement2), unknownPtr, &stream);
                unknownPtr->Release();
 
                DWORD result = 0;
 
                sendMessageToThread(MSG_CLICK, (WPARAM)button, (LPARAM)stream, &result);
            }
            else
            {
                LOG4CXX_WARN(_logger, L"Cannot retrieve IDispatch pointer for the element object");
            }
        }
        else
        {
            throw IEBrowserException(L"Could not access the IHTMLElement2 interface for the element");
        }
	}
    else
    {
        throw IEBrowserException(L"Could not access the COM object of the element");
    }
}

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HalfAsleepCommented:
Although you can use a windows handle and the windows message queue to communicate with the other thread, it is a slight cheat.

Don't get me wrong, it is a well known shortcut, and it is even documented on microsoft's own msdn webpages, but it is still not the "correct" way of doing it.

I had first hand experience of this, when I inherited the job of maintaining an old ActiveX automation server.  This old control relied indeed on a windows handle to communicate to the main thread from all its worker threads.  This made the code fairly straightforward, but it had some drawbacks.

First, the component needed to visually be added to a form (it needs a windows handle, remember?).

Second, and this is the reason we had to do it with "proper" marshaling, some environments will not let you cheat with a windows handle like that.  When we wanted to get our old control to work with WinXP/.NET, we discovered that .NET refused to give us such a windows handle (unmanaged code etc? I'm not sure).  We found that the only way to get around this, was to properly implement marshaling.  It was a bit tricky to get right at first, and the other method/cheat sure is easier to find on the net.  But once we had the proper marshaling code into place, we were not dependent on the windows handle and the windows message queue/pump any more.  The control now works in any environment we throw at it, be it .NET, delphi etc, and it does not even need to be tied to a form, it can just be referenced/created in code.
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HalfAsleepCommented:
I used to have a good link to this stuff at work.  I will see if I can find it tomorrow.
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hach-queAuthor Commented:
Thank you so much!  This stumped me for ages.

For anyone reading this answer, make sure you replace _IHTMLElement and IID__IHTMLElement with IHTMLElement and IID_IHTMLElement respectively.
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