Variable corrupted when calling C++ COM from C#

Question

I'm calling C++ COM interface from C# code using a dll. At the C++ side, i have a WCHAR* global variable which is updated through a method with a BSTR parameter.

The problem is that, when i first call the C++ wrapper method from C# to change the variable, everything works fine, but at the moment that i call another C++ wrapper method from C#, unexplainably the WCHAR* global variable points to a different memory position and its value gets corrupted.

Some code:

    //THE C# side:

    capture.filename = PATH + "\\" + DIRECTORY_NAME + "\\";
    capture.MaxMinutesPerFile = MAX_MINUTE_PER_FILE;

"capture" is an object of the C++ wrapper class (i think it is autogenerated when building the C++ code to a DLL. Not my code). "filename" property calls a "put_FileName" C++ method and "MaxMinutesPerFile" a "put_MaxMinutesPerFile" method.

//C++ code    

WCHAR *m_bstFileName = L"None";

(...)     

STDMETHODIMP CCaptureMF::put_FileName(BSTR PathName)
{
    EnterCriticalSection(&m_critsec);

   HRESULT hr = S_OK;

   m_bstFileName = PathName;

   LeaveCriticalSection(&m_critsec);
   return hr;
}

STDMETHODIMP CCaptureMF::put_MaxMinutesPerFile(LONG Minutes)
{   

    MaxMinutes= Minutes;

    return S_OK;
}

So, after calling "put_FileName", "m_bstFileName" is updated correctly with the "PathName" value, but just after calling "MaxMinutesPerFile" (or any other interface wrapper method), "m_bstFileName" gets corrupted pointing to a different memory position and fulfilled with strange data.

Thank you.

EDIT:

To make a buffer of "m_bstFileName" and then copy the "PathName" data, i used the following code, taking in mind that "m_bstFileName" size can change at runtime:

m_bstFileName = (wchar_t*)malloc(sizeof(PathName));
wcscpy(m_bstFileName, PathName);

That code works fine, but the rest of the program behaves bad. I´m not sure why, i should investigate more, but for now, could you analyze that pice of code and tell me if it is correct or if i should implement it in other way?

SOLUTION:

Ok, following your recomendations i have finally implemented the following code, which works perfect for the whole application:

CComBSTR m_bstFileName = L"None";

(...)

STDMETHODIMP CCaptureMF::put_FileName(BSTR PathName)
{
    EnterCriticalSection(&m_critsec);

    HRESULT hr = S_OK;

    m_bstFileName = PathName;

    if (g_pCapture)
    {
        g_pCapture->SetPath(m_bstFileName);
    }
    LeaveCriticalSection(&m_critsec);
    return hr;
}

If you think that this can be implemented better, just tell.

Thank you for your help!

The caller will destroy the BSTR after the call completes. You have to deep-copy the string to preserve it. — Hans Passant
– Hans Passant, Commented Nov 27, 2014 at 16:38
Declare m_bstFileName as a CComBSTR, the assignment will then copy the BSTR passed to it. — AndersK
– AndersK, Commented Nov 28, 2014 at 8:43
Remember, sizeof doesn't work on pointers. @CyberSpock already gave the right answer, but if you're going manual you should calloc (wcslen(PathName)+1, sizeof *m_bstFileName). — Medinoc
– Medinoc, Commented Nov 28, 2014 at 10:10

Nathan · Accepted Answer · 2014-11-27 16:24:22Z

1

In the most basic case you'll need to make a buffer to do a string copy into. Same operation could be accomplished via assignment with cstring, ccombstr, std::string, etc. depending on the framework you're using.

answered Nov 27, 2014 at 16:24

Nathan

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2 Comments

Resource Over a year ago

Forgot the buffer. My C++ is a bit rusty.

MorgoZ Over a year ago

Than you. Could you please put a sample with a properly way of creating the buffer at "m_bstFileName"? It is not a fixed size since this variable can change through the code at runtime. Thanks again.

Resource · Accepted Answer · 2014-11-27 16:20:14Z

0

You need to copy the string to m_bstFileName, not just assign it. Use something like

strcpy(m_bstFileName, PathName);

answered Nov 27, 2014 at 16:20

Resource

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2 Comments

Steve Wellens Over a year ago

AFTER you allocate enough room in the destination string.

Nathan Over a year ago

Don't forget to allocate (on the stack or heap) someplace to copy PayhName into.

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