Howto Track down variable Corruption

Question

I have a MFC class derived from CStdioFile declared as follows

// Datafile.h
class CDataFile : public CStdioFile
{
 public:
 CDataFile(void);
 ~CDataFile(void);

 int OpenFile(LPCWSTR FileName);
}

After my OpenFile function is called the FileName variable is being corrupted.

int CDataFile::OpenFile(LPCWSTR FileName)
    {

m_OpenFlags = CFile::modeNoTruncate | CFile::modeReadWrite;   

// Before open. FileName = "c:\afile.txt"

     if (!Open(FileName, m_OpenFlags, NULL)) 

         {
            return GetLastError();
         }

//After open. FileName = ""ﻮﻮﻮﻮﻮﻮﻮﻮﻮﻮﻮﻮﻮﻮﻮﻮﻮﻮﻮﻮﻮﻮﻮﻮﻮﻮﻮﻮﻮﻮﻮﻮﻮﻮﻮﻮﻮﻮﻮﻮﻮﻮﻮﻮﻮﻮﻮﻮވĚᗸ÷ᘸ÷㼠碞­"

// other stuff
}

}

But if I change the FileName to

WCHAR FileName[] = _T("c:\\afile.txt");

Before opening the File the variable Filename remains untouched. I have seen this behaviour before with the MFC/Winapi and always worked around it be by using character arrays instead of LPCWSTR or CString. Why does this happen? and what is the best way to track down problems such as this with the VS Debugger. The corruption appears to happen here in the MFC file Filecore.cpp

if (!CFile::Open(lpszFileName, (nOpenFlags & ~typeText), pException))
  return FALSE;

Answer 1

Take a look at using a data breakpoint (also known as a hardware breakpoint.) You can break when the memory is modified.