programming in C++ : problem for accessing block properties

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here is a small function to query the name of each block in the current drawing

void CDrvAcad::GetBlockList(CStringList *pList)
{
IAcadBlocksPtr pBlocksCollection = m_acadDoc->GetBlocks();
IAcadBlockPtr pBlock;
_variant_t index;
index.vt = VT_I4;
long count = pBlocksCollection->GetCount();
CString str;

for (int i = 0; i< count; i++)
{
index.lVal = i;
pBlock = pBlocksCollection->Item(index);
str = pBlock->GetName()
}
}

I know that the pBlock is valid, because i can access properties as blockID, blockHandle, ....
but pBlock->GetName() will stop the execution (&quot;access violation&quot. Another strange thing : if i have 2 Autocad applications running at the same time, it works !