如何分阶段创建zip文件？

Question

I'm writing data into ZIP file in a following way:

    private static void ZipStringToFile(string value)
    {
        using (MemoryStream ms = new MemoryStream())
        {
            using (GZipStream compressedzipStream = new GZipStream(ms, CompressionMode.Compress, true))
            {
                byte[] queryData = _encUtf8.GetBytes(value);
                compressedzipStream.Write(queryData, 0, queryData.Length);
                compressedzipStream.Write(_dataEndOfLine, 0, _dataEndOfLine.Length);
            }
            using (FileStream outfile = new FileStream("TestZip.zip", FileMode.Append))
            {
                byte[] buffer2 = new byte[ms.Length];
                ms.Seek(0, SeekOrigin.Begin);
                ms.Read(buffer2, 0, buffer2.Length);
                outfile.Write(buffer2, 0, buffer2.Length);
            }
        }
    }

That works quite well. But if I use the same method once again:

        ZipStringToFile("String #1");
        ZipStringToFile("\r\n");
        ZipStringToFile("String #2");

I have bigger zip file (so I believe file contains both strings), but after unpacking using "TotalCommander" embedded archiver I only see "String #2".

I believe that structure of zip-file itself doesn't allow "appending" so I have something principally wrong in my approach.

Should I just implement serializing everything into file first and zip IN THE END? Or there is proper way of appending data into zip archive?

Thanks.

Answer 1

You are using the term "zip" incorrectly. gzip is not zip. gzip is a compressed data format for a single stream. zip is an archive format for multiple files. You are generating gzip streams.

It is possible concatenate gzip streams to make a larger, valid gzip stream. A compliant gzip decoder should deliver all of the data from the concatenated streams. So either you are not concatenating them properly, or you are not delivering the result of the concatenation properly to "TotalCommander", or "TotalCommander" is not a compliant gzip decoder.

Should I just implement serializing everything into file first and zip IN THE END?

In general, yes, you should. The reason is that you will get lousy compression if you only feed gzip little strings. In order to get good compression, you need to give gzip a lot more data to work with.

Answer 2

If you were using the ASP.NET ZipArchive library, my suggestion would be to try:

1. check to see if the zip exists
2. If it exists, open it.
3. Use the GetEntry function to open the existing archive entry
4. Open() the entry to a stream, then write your new data to the stream
5. Save the archive back to disk.

Otherwise you're probably overwriting the existing archive each time.