使用“保护工作簿结构”选项确定XLS文件是否受密码保护

// check for Excel header
const string workbookName = "W\0o\0r\0k\0b\0o\0o\0k" + afterNamePadding;
headerOffset = bufferString.IndexOf(workbookName, StringComparison.InvariantCulture);
if (headerOffset >= 0)
{
    sectionId = BitConverter.ToInt32(buffer, headerOffset + sectionIdOffset);
    int sectionOffset = coBaseOffset + sectionId * sectionSize;
    const int streamScanSize = 0x100;
    if (sectionOffset < 0 || sectionOffset + streamScanSize > stream.Length)
       return false; // invalid document
    var workbookStream = new byte[streamScanSize];
    stream.Seek(sectionOffset, SeekOrigin.Begin);
    ReadFromStream(stream, workbookStream);
    short record = BitConverter.ToInt16(workbookStream, 0);
    short recordSize = BitConverter.ToInt16(workbookStream, sizeof(short));
    const short bofMagic = 0x0809;
    const short eofMagic = 0x000A;
    const short filePassMagic = 0x002F;
    if (record != bofMagic)
       return false; // invalid BOF
    // scan for FILEPASS record until the end of the buffer
    int offset = sizeof(short) * 2 + recordSize;
    int recordsLeft = 16; // simple infinite loop check just in case
    do
    {
        record = BitConverter.ToInt16(workbookStream, offset);
        if (record == filePassMagic)
           return true;
        recordSize = BitConverter.ToInt16(workbookStream, sizeof(short) + offset);
        offset += sizeof(short) * 2 + recordSize;
        recordsLeft--;
    } while (record != eofMagic && recordsLeft > 0);
}