DirectShow CLR having issue with global variables - c++

Question

I am trying to code up GMFBridge and DirectShow in CLR C++. I am trying to compare its performance against the GMFBridgeLib and the DirectShowLib in the same solution to see which is more efficient.
Right now I am following the GMFBridge source code for setting up C++ capture. One issue I am having is in objects that need to be global so that they can be accessed across the GUI buttons. The GMFBridge code does that as follows:
private:
IGMFBridgeControllerPtr m_pBridge;
that is then used in the setup code as follows:
HRESULT hr = m_pBridge.CreateInstance(__uuidof(GMFBridgeController));
if (FAILED(hr))
{
return hr;
}
// init to video-only, in discard mode (ie when source graph
// is running but not connected, buffers are discarded at the bridge)
hr = m_pBridge->AddStream(true, eMuxInputs, true);
My current problem is that CLR states that any global has to be a pointer of some form, * or ^ depending on managed or unmanaged. It will not just let me add in a global variable such as the GMFBridge source code does. If I create a pointer:
IGMFBridgeControllerPtr* pBridge2;
and try to use that in my GUI code:
(*pBridge2).CreateInstance(__uuidof(GMFBridgeController));
(*pBridge2).AddStream(true, eMuxInputs, true);
It does compile, but when i run it, the code crashes with
An unhandled exception of type 'System.NullReferenceException' occurred in Program.exe.
Addidional information: Object reference not set to an instance of an object.
on the block of code
void _Release() throw()
{
if (m_pInterface != NULL) { <--------------
m_pInterface->Release();
}
}
in comip.h line 823 called from:
HRESULT CreateInstance(const CLSID& rclsid, IUnknown* pOuter = NULL, DWORD dwClsContext = CLSCTX_ALL) throw()
{
HRESULT hr;
_Release();
if (dwClsContext & (CLSCTX_LOCAL_SERVER | CLSCTX_REMOTE_SERVER)) { <----------
IUnknown* pIUnknown;
hr = CoCreateInstance(rclsid, pOuter, dwClsContext, __uuidof(IUnknown), reinterpret_cast<void**>(&pIUnknown));
if (SUCCEEDED(hr)) {
hr = OleRun(pIUnknown);
if (SUCCEEDED(hr)) {
hr = pIUnknown->QueryInterface(GetIID(), reinterpret_cast<void**>(&m_pInterface));
}
pIUnknown->Release();
}
}
else {
hr = CoCreateInstance(rclsid, pOuter, dwClsContext, GetIID(), reinterpret_cast<void**>(&m_pInterface));
}
if (FAILED(hr)) {
// just in case refcount = 0 and dtor gets called
m_pInterface = NULL;
}
return hr;
}
comip.h line 626 called from this line of code
(*pBridge2).CreateInstance(__uuidof(GMFBridgeController));
the only thing that seems to work is creating a local variable that is not a point object, but then i cant set it to a global, or use it across GUI objects.
if I make it local:
IGMFBridgeControllerPtr pBridge;
pBridge.CreateInstance(__uuidof(GMFBridgeController));
that works.

Answer 1

The problem seems to be that you do not assign anything to the pointer you declare:
IGMFBridgeControllerPtr* pBridge2;
You have to do something like:
pBridge2 = &m_pBridge;
Or just skip the use of pBridge2 completely and use &m_pBridge instead.