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Asked: 2026-05-22T17:22:59+00:00

I have this code in my VB6 app: Private Declare Function FileGetParentFolder Lib Z-FileIO.dll

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I have this code in my VB6 app:

Private Declare Function FileGetParentFolder Lib "Z-FileIO.dll" _
(ByVal path As String) As String

Output.AddItem FileGetParentFolder(FileText.Text)

Output is a list, FileText is a text field containing a file path. My C++ DLL contains this function:

extern "C" BSTR ZFILEIO_API FileGetParentFolder(Path p)
{
    try {
        return SysAllocString(boost::filesystem::path(p).parent_path().c_str());
    } catch (...) {
        return SysAllocString(L"");
    }
}

where Path is typedef’d as LPCSTR. The argument comes into my DLL perfectly, but whatever I try to pass back, the VB6 app shows only garbage. I tried several different methods with SysAllocStringByteLength, casting the SysAllocString argument to LPCWSTR and other variants. Either, I only see the first letter of the string, or I see only Y’s with dots, just not the real string. Does anyone know what the real method is for creating and passing valid BSTRs from C++ to VB6?

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1 Answer

  1. Editorial Team

    Hopefully this will point you in the right direction. From memory…

    VB6 uses COM BSTRs (2-byte wide character strings) internally, but when communicating with external DLLs it uses single- or multi-byte strings. (Probably UTF-8, but I don’t remember for sure.) Your Path typedef to LPCSTR is an ANSI string, and that’s why you can receive it correctly. The return value you generate is a wide-character string, but VB is expecting an ANSI string. You’ll need to use WideCharToMultiByte to convert your return value before returning it.

    Seems a little odd that VB does this implicit conversion, but that’s the way it is. (As far as I remember.)

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