No filenames in pdb program database

I have a .pdb program database from a C++ application compiled in debug on Windows VS2005. I use the DIA SDK to find function names but I can't seem to retrieve the filenames for the symbols.

Is there some switch I need to turn on? Does this work?!

So the answer to this appears to be that you find the line number first and then the source file?!

E.g.

virtual IProgramSymbol^ getSymbolFromAddress(UInt32 address)
{
    // Find the symbol with the virtual address we were given
    IDiaSymbol ^ sym = nullptr;
    m_session->findSymbolByVA( address, SymTagEnum::SymTagFunction, sym );

    if (sym != nullptr)
    {
        // Get source code information via lines numbers. Odd, but this seems to be the way
        // to do it.
        String ^ srcFile = "unknown";
        int line = 0;

        UInt64 startAdr = sym->virtualAddress;

        // Find line numbers from start_address to start_address+1, which should be 1 symbol!
        IDiaEnumLineNumbers ^ lines;
        m_session->findLinesByVA(startAdr, 1, lines);

        if (lines != nullptr)
        {
            // get the line number
            IDiaLineNumber ^ lnb = lines->Item(0);
            line = lnb->lineNumber;

            // get the source file from the line number (weird!)
            IDiaSourceFile ^ sf = lnb->sourceFile;
            if (sf != nullptr)
            {
                srcFile = sf->fileName;
            }
        }

        return gcnew DiaSymbol(sym, srcFile, line);    // found a function
    }