how can I delete memory allocated by a legacy C++ DLL

Question

Hi, I am having a problem deleting memory allocated from a legacy C++ DLL. The DLL uses PostMessage to send data to a parent App, and expects the parent app to delete the memory when it has finished with it. I have created a new C++ DLL to interface the calls from my new .NET application as one of the functions in the legacy DLL required a CWnd* parameter. I am trapping the message posted by the legacy DLL by overriding the WndProc of my .NET form, and I am collecting the data from the memory pointer in the posted message with Marshal.PtrToStructure(m.LParam, new myStruct().GetType()). I then tried calling a function in my new C++ DLL, passing m.LParam as the pointer to the memory I need to delete, but I get an error - _CrtIsValidHeapPointer(pUserData) thrown up from the Visual C++ debugger. I have also tried calling Marshal.FreeHGlobal(m.LParam) from my .NET form, but this doesn't work either. Can anyone please help me?