Where does MFC COleControl::DoPropExchange store persistent properties? - c++

Question

I have taken over maintenance of a legacy MFC OCX control in C++. The project is now in VS2013. I'm trying to understand the functioning of the DoPropExchange method. This method appears to be calling PX_?????(member) for nearly all the data members in the control where ???? is the type (Bool, Short, Long ...) My understanding is these are called for the purpose of providing persistent storage of properties. But from my understanding of the operation of the OCX control there are no persistent properties. Would there be any other reason to be calling PX_???? for all data members in DoPropExchange other than to support persistent properties? I'm also trying to understand where these persistent properties are loaded/stored. Where is the serialized file for loading/storing persistent property values specified?
Here is the source for DoPropExchange
// CSigPlusCtrl::DoPropExchange - Persistence support
void CSigPlusCtrl::DoPropExchange(CPropExchange* pPX)
{
DWORD Version;
long BaudRate;
short ComPort;
BOOL Rv;
LOG(("DoPropExchange Entry"));
ExchangeVersion(pPX, MAKELONG(_wVerMinor, _wVerMajor));
COleControl::DoPropExchange(pPX);
Version = pPX->GetVersion();
if (pPX->IsLoading())
{
LoadDefaultProperties();
LoadIniParameters();
}
if ((Version & 0xFFFF0000) == (DWORD)MAKELONG(0, _wVerMajor))
{
Rv = PX_Short(pPX, _T("ImageFileFormat"), ImageFileFormat, 0);
Rv = PX_Short(pPX, _T("ImageXSize"), ImageXSize, 0);
Rv = PX_Short(pPX, _T("ImageYSize"), ImageYSize, 0);
Rv = PX_Short(pPX, _T("ImagePenWidth"), ImagePenWidth, 1);
. . .
Rv = PX_Short(pPX, _T("ZoomY"), ZoomY, 0);
Rv = PX_Short(pPX, _T("ZoomPower"), ZoomPower, 1);
if (pPX->IsLoading())
{
if (SigBlob != NULL)
{
GlobalFree(SigBlob);
SigBlob = NULL;
}
}
else
{
if (SigBlob == NULL)
{
SigBlobType* SigBlobPtr;
SigBlob = GlobalAlloc(GMEM_MOVEABLE, sizeof(DWORD));
SigBlobPtr = (SigBlobType*)GlobalLock(SigBlob);
SigBlobPtr->Size = 0;
GlobalUnlock(SigBlob);
}
}
if ((Version & 0xFFFF) == Version223)
{
Rv = PX_Blob(pPX, _T("SigBlob"), SigBlob, NULL);
}
if ((Version & 0xFFFF) >= Version224)
{
CString SigStr;
if (!pPX->IsLoading())
{
SigStr = BlobToString();
}
Rv = PX_String(pPX, _T("SigStringStored"), SigStr, _T(""));
if (pPX->IsLoading())
{
BlobFromString(SigStr);
}
}
}
else
{
SigMessageBox("Warning Incompatable Versions of SigPlus Control");
}
LoadTabletParameters();
LOG(("DoPropExchange Exit"));
}
EDIT Added 6-21-2018
Running in the debugger I observe that when DoPropExchange is called, VS2013 shows the stack with a message that stack frames below may be incorrect. And the one frame just above, that calls DoPropExchange, is from mfc120d.dll which does not have symbol file available mfc120d.i386.pdb.
This Microsoft Forum Post seems to indicate that the symbol file is not available for VS2015 and I'm wondering if that is also the case for VS2013. So far I have not been able to find place to download MFC120 symbols for debug.
Starting a bounty today to find someone who can explain how and where properties are normally serialized for OLE controls and what methods are used to specify the serialized data storage location/media. This is of concern because this control runs in a Citrix ZenDesk network environment in a Terminal Aware program and if properties are being stored somewhere then each client needs to specify a location unique to that client.

Answer 1

The DoPropExchange is used to implement control attribute persistence mainly between design and run-time. The actual destination sink is passed by the client of the OCX.
In VC the the settings are stored in RC file while in VB in frm and frx files. If you open FRM in notepad you would probably see the section with the properties of this control.
As a side note, there is a similar implementation in case the control is used in HTML with inline settings in he html itself.
Unless your VB clients saves the settings externally via direct calls to Property bag function it is not likely you have a issue here since the above properties are not stored at runtime.