converting managed type to void* and back

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C++/CLI

I have a managed class ASwHTTPMgr and an unmanaged class ASwHTTPMgrCpp. I want to get the instanse of the ASwHTTPMgr and convert it into unmanaged void* and construct the ASwHTTPMgrCpp.
I have an extern C function "platformHTTPwifiNetworkAvailable(void* userData)" defined in the classes and implemented as below.
In that function I want to unwrap the void* and convert it back to the ASwHTTPMgr.
The ASwHttpMgrCpp->wifiNetworkAvailable()function is called at several places in the program.
At the first call platformHTTPwifiNetworkAvailable(), I could see valid data in userData and I could convert it back to managed ASwHTTPMgr without any issues. But the next call to it, I could see the same valid address for the userData, but h.Target is invalid and the call fails. It seems like the pointer got garbage collected at some point.
How could I fix this. Please help.

//////////ASwHTTPMgr.cpp////////////
...
extern "C" {
    bool platformHTTPwifiNetworkAvailable(void* userData) {
	ASwHTTPMgr^ mgr = nullptr;
	Object ^obj = nullptr;
	GCHandle h = GCHandle::FromIntPtr(IntPtr(safe_cast<IntPtr>(userData))); 
	try { 
		obj = safe_cast<Object^>(h.Target); 
		if(obj->GetType() == ASwHTTPMgr::typeid)
		{
		   mgr = safe_cast<ASwHTTPMgr^>(obj);
		   return mgr->wifiNetworkAvailable();   
		}			
	} 
	finally { 				
	    h.Free();
	} 			              
}
.........
ASwHTTPMgr::ASwHTTPMgr()
{
      ASwHTTPMgr^ reftype = this;
      void* opaqueObj = GCHandle::ToIntPtr(GCHandle::Alloc(reftype)).ToPointer();
      _httpMgrCpp = new ASwHttpMgrCpp(opaqueObj);
          
}
//////////////////////////////////////
definition of _httpMgrCpp is

static ASwHttpMgrCpp* _httpMgrCpp;

-----------------------------------------------------------------------------------
//////////////ASwHttpMgrCpp.cpp//////////////////
extern "C" {
    extern bool platformHTTPwifiNetworkAvailable(void* userData);    
}
ASwHttpMgrCpp::ASwHttpMgrCpp(void* userData){
    mUserData = userData;
}
bool ASwHttpMgrCpp::wifiNetworkAvailable() {
	return platformHTTPwifiNetworkAvailable(mUserData);
}
/////////////////////////////////////////////////////
mUserData is defined as
protected:
    void* mUserData;

Thanks.