Retrieves the number of files that the user has selected in a window opened by the GetOpenFileName Win32 C API function. It assumes that the OFN_EXPLORER and OFN_ALLOWMULTISELECT flags are set.

countFileNames.c

size_t FilesSelected(OPENFILENAME ofn, size_t bufferSize)
{
    // ofn.lpstrFile format:
    // - If the user picks one file: folder\file\0\0
    // - If the user picks several files: folder\0file1\0file2\0fileN\0\0
    // This function assumes that input parameters are valid and is used just after GetOpenFileName returns with success.

    size_t toret = 0;

    BOOL bBufferEnd = FALSE;
    BOOL bFoundNullPreviously = FALSE;
    for (size_t i = 0; i < bufferSize - 2 && !bBufferEnd; i += 2)
    {
        TCHAR cur = ofn.lpstrFile[i];
        TCHAR next = ofn.lpstrFile[i + 1];
        TCHAR nextNext = ofn.lpstrFile[i + 2];

        BOOL bFoundNull = cur == '\0' || next == '\0';
        bBufferEnd = cur == '\0' && next == '\0' || next == '\0' && nextNext == '\0';

        if (bFoundNull && (!bBufferEnd || !bFoundNullPreviously))
        {
            // We found another file if we find a null character, and one of the following conditions applies:
            // 1. The buffer doesn't end (i.e., a null character is followed by another null character).
            //    E.g., a buffer with two files would have two \0, including the one which separates the folder from the first file.
            //    We don't want to count the last null on the buffer, followed by another null, because it signals the buffer end.
            // 2. It's the first null character we find. This would count a first null no matter where it is.
            //    It allows to handle the case where the user only selects one file correctly.
            ++toret;
        }

        bFoundNullPreviously = bFoundNullPreviously || bFoundNull;
    }

    return toret;
}