FIX: Access Violation When Calling COleVariant::Clear()

   #ifndef _UNICODE
      if (vt == VT_BSTRT)
             ::SysFreeString(bstrVal);
      else
   #endif
      {
             VERIFY(::VariantClear(this) == NOERROR);
      }
				

MORE INFORMATION

The GetRows() ADO sample, which is provided with the OLE DB 1.1 SDK, contains code that can cause the access violation when calling Clear().

Here is the DoGetRows() function from GETRDLG.CPP:

   HRESULT CGetRowsDlg::DoGetRows()
   {
      HRESULT         hr;
      COleVariant      vBookmark, rgvFields;
      COleVariant      cRows;
      COleVariant      varField, varNewField;
      CString         strLBRow;
      LONG         lNumOfCol, lNumRecords;
      LONG         lIndex[2];
      CListBox      *pListBox =
                               (CListBox *)GetDlgItem(IDD_GETROWSLIST);

      //Perform GetRows on Employee table.

      //Start from the current place
      vBookmark.vt = VT_ERROR;
      vBookmark.scode = DISP_E_PARAMNOTFOUND;

      //Get all columns.
      rgvFields.vt = VT_ERROR;
      rgvFields.scode = DISP_E_PARAMNOTFOUND;

   if (!m_piEmpRecordSet)
      return E_NOINTERFACE;

      hr = m_piEmpRecordSet->GetRows(   adGetRowsRest,
                                           vBookmark,
                                           rgvFields,
                                           &cRows
                                         );
      if (FAILED(hr)) goto ErrorExit;


      //Find out how many records were actually retrieved,
      //(SafeArrays are 1-based)
      lNumOfCol = 2;
      SafeArrayGetUBound(cRows.parray, 2, &lNumRecords);

      //Clear the listbox
      pListBox->ResetContent();

      for (lIndex[1] = 0; lIndex[1] <= lNumRecords; lIndex[1]++)
      {
             strLBRow.Empty();//Clear the string
             for (lIndex[0] = 0; lIndex[0] <= lNumOfCol; lIndex[0]++)
             {
               SafeArrayGetElement(cRows.parray, &lIndex[0], &varField);
               hr = VariantChangeType(&varNewField, &varField, 0, VT_BSTR);

               if(hr == S_OK)
               {
                 strLBRow += (LPCWSTR)varNewField.bstrVal;
                 strLBRow += _T(", ");
               }
               varField.Clear();
               varNewField.Clear();
             }
             pListBox->AddString(strLBRow);
           }

      return hr;

   ErrorExit:
      TCHAR szBuf[256];
      wsprintf(szBuf, _T("Error: %d \n"), hr);
      AfxMessageBox(szBuf);

      return hr;
   }
				

NOTE: If the column type returned from the ADO source is VT_DECIMAL, the varField.Clear() function will crash because COleVariant::Clear() calls SysFreeString() on data that is not a string.

To reproduce the error, add the following lines of code to the function CGetRowsDlg::DoGetRows():

   COleVariant var10;

   hr = VariantChangeType(&var10, &varField, 0, VT_DECIMAL);
   if(hr!=S_OK)
        AfxMessageBox("Can't VariantChangeType to decimal");
   else
       var10.Clear();
				

Add the code just before the line:

    varField.Clear();