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GeneralRe: IDirect3DSurface9 Problem after Windows 10 Update Creator Pin
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QuestionC++ ActiveX project worked with VB6, broken in later versions Pin
Burl Rice22-Jul-17 10:30
Burl Rice22-Jul-17 10:30 
I have an ActiveX control (written originally in VC6, and later upgraded to VC 2010). When we first released it, I included a VB6 demo program to show how to use it. This all still works.

Recently, a customer requested sample code in VB 2017, and I have found that it crashes in this environment. I have also checked VB 2015 and VB 2010, and they also crash in the same place.

Here is a simplified overview of the ActiveX library:

library BoxWriter
{
    interface ILine : IDispatch 
    {
        [propget, id(8), helpstring("property m_lCount")] HRESULT m_lCount([out, retval] long *pVal);
    };
    interface IPrinter : IDispatch
    {
        [propget, id(1), helpstring("property m_line")] HRESULT m_line([optional, defaultvalue ("")] BSTR strLine, [out, retval] ILine* *pVal);
    };
};


Here is a VB code snippet that causes the crash:

Dim printer As New BoxWriter.PRINTER
printer.Connect("192.168.1.3")
MsgBox(m_printer.m_line.m_lCount)    ‘ <—crashes here


This is the error it gives:
An unhandled exception of type 'System.AccessViolationException' occurred in demo.exe
Additional information: Attempted to read or write protected memory. This is often an indication that other memory is corrupt.



The reference returned by CPrinter::get_m_line seems to be what it is complaining about. When I debug, I see the reference is set correctly by CPrinter::get_m_line, but after the function returns, something is calling the destructor for the CLine class.

STDMETHODIMP CPrinter::get_m_line(BSTR bstrLine, ILine **pVal)
{ 
    AFX_MANAGE_STATE(AfxGetStaticModuleState())

    CString strLine = (LPCTSTR)_bstr_t (bstrLine);
    CComObject <CLine>* pLine = GetLine (strLine);

    if (!pLine) {
        return E_INVALIDARG;
    }

    * pVal = pLine;

    return S_OK;
}


I have tried creating a new ActiveX project in VC2010, and it also exhibits the same behavior. I tried returning the reference in several different ways (property vs method), and still have not been able to solve this. Surely I am not the first person to encounter this. Can anyone suggest how to fix this?
AnswerRe: C++ ActiveX project worked with VB6, broken in later versions Pin
Richard MacCutchan22-Jul-17 20:29
mveRichard MacCutchan22-Jul-17 20:29 
GeneralRe: C++ ActiveX project worked with VB6, broken in later versions Pin
Burl Rice23-Jul-17 2:32
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GeneralRe: C++ ActiveX project worked with VB6, broken in later versions Pin
Richard MacCutchan23-Jul-17 2:54
mveRichard MacCutchan23-Jul-17 2:54 
GeneralRe: C++ ActiveX project worked with VB6, broken in later versions Pin
Burl Rice23-Jul-17 9:12
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QuestionClass Shape Pin
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AnswerRe: Class Shape Pin
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SuggestionEducational: my buffer scroll C++ working code. Pin
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