How to use the 7z SDK to compress and decompress a file

Question

According to this link How do I create 7-Zip archives with .NET? , WOPR tell us how to compress a file with LMZA (7z compression algorithm) using 7z SDK ( http://www.7-zip.org/sdk.html )

using SevenZip.Compression.LZMA;
private static void CompressFileLZMA(string inFile, string outFile)
{
   SevenZip.Compression.LZMA.Encoder coder = new SevenZip.Compression.LZMA.Encoder();

   using (FileStream input = new FileStream(inFile, FileMode.Open))
   {
      using (FileStream output = new FileStream(outFile, FileMode.Create))
      {
          coder.Code(input, output, -1, -1, null);
          output.Flush();
      }
   }
}

But how to decompress it?

I try :

private static void DecompressFileLZMA(string inFile, string outFile)
        {
            SevenZip.Compression.LZMA.Decoder coder = new SevenZip.Compression.LZMA.Decoder();
            using (FileStream input = new FileStream(inFile, FileMode.Open))
            {
                using (FileStream output = new FileStream(outFile, FileMode.Create))
                {
                    coder.Code(input, output, input.Length, -1, null);
                    output.Flush();
                }
            }
        }

but without success.

Do you have a working example?

Thanks

PS: According to an other code http://www.koders.com/csharp/fid43E85EE5AE7BB255C69D18ECC3288285AD67A4A4.aspx?s=zip+encoder#L5 , it seems that the decoder needs a header, a dictionary at the beginning of the file to work. This file generated by Koders is not a 7z archive.

   public static void Decompress(Stream inStream, Stream outStream)
    {
        byte[] properties = new byte[5];
        inStream.Read(properties, 0, 5);
        SevenZip.Compression.LZMA.Decoder decoder = new SevenZip.Compression.LZMA.Decoder();
        decoder.SetDecoderProperties(properties);
        long outSize = 0;
        for (int i = 0; i < 8; i++)
        {
            int v = inStream.ReadByte();
            outSize |= ((long)(byte)v) << (8 * i);
        }
        long compressedSize = inStream.Length - inStream.Position;
        decoder.Code(inStream, outStream, compressedSize, outSize, null);
    }

The outSize is computed the same way than their Compress method. But how to compute the output size otherwise?

Answer 1

Similar to @Dominic Grenier solution. Decompressor must know which properties to use to decompress the payload. This is your responsibility to serialize/deserialize them.

public static void 
Compress7Zip(this Stream input, Stream output) {
    var coder = new SevenZip.Compression.LZMA.Encoder();
    coder.WriteCoderProperties(output);
    output.WriteInt64(input.Length);
    coder.Code(input, output, -1, -1, null);
}

public static void 
Decompress7Zip(this Stream input, Stream output) {
    var coder = new SevenZip.Compression.LZMA.Decoder();
    var properties = new byte[5];
    input.Read(properties, 0, 5);
    var payloadLength = input.ReadInt64();
    coder.SetDecoderProperties(properties);
    coder.Code(input, output, input.Length, payloadLength, null);
}

Answer 2

I needed LZMA compression for sending images over network, not sure it's the best alternative but at least it works in my ecosystem! So here is something that should work right away for that purpose.

using System;
using System.IO;
using SevenZip;

  public class LZMA{
    public static byte[] Compress(byte[] toCompress)
      {
        SevenZip.Compression.LZMA.Encoder coder = new SevenZip.Compression.LZMA.Encoder();

        using(MemoryStream input = new MemoryStream(toCompress))
        using(MemoryStream output = new MemoryStream()){

          coder.WriteCoderProperties(output);

          for (int i = 0; i < 8; i++) {
            output.WriteByte((byte)(input.Length >> (8 * i)));
          }

          coder.Code(input, output, -1, -1, null);
          return output.ToArray();
        }
      }

    public static byte[] Decompress(byte[] toDecompress)
    {
        SevenZip.Compression.LZMA.Decoder coder = new SevenZip.Compression.LZMA.Decoder();

        using(MemoryStream input = new MemoryStream(toDecompress))
        using(MemoryStream output = new MemoryStream()){

          // Read the decoder properties
          byte[] properties = new byte[5];
          input.Read(properties, 0, 5);


          // Read in the decompress file size.
          byte [] fileLengthBytes = new byte[8];
          input.Read(fileLengthBytes, 0, 8);
          long fileLength = BitConverter.ToInt64(fileLengthBytes, 0);

          coder.SetDecoderProperties(properties);
          coder.Code(input, output, input.Length, fileLength, null);

          return output.ToArray();
        }
    }
  }

Answer 3

This question is a little old, but google fails to provide a satisfactory answer so this is for those like me still seeking it out. If you look into the LMZAAlone folder of the SDK there is code that compresses and decompresses files. Using it as an example it would seem you need to write and read the encoder properties and decompresses file size to your output file:

private static void CompressFileLZMA(string inFile, string outFile)
    {
        SevenZip.Compression.LZMA.Encoder coder = new SevenZip.Compression.LZMA.Encoder();
        FileStream input = new FileStream(inFile, FileMode.Open);
        FileStream output = new FileStream(outFile, FileMode.Create);

        // Write the encoder properties
        coder.WriteCoderProperties(output);

        // Write the decompressed file size.
        output.Write(BitConverter.GetBytes(input.Length), 0, 8);

        // Encode the file.
        coder.Code(input, output, input.Length, -1, null);
        output.Flush();
        output.Close();
    }

    private static void DecompressFileLZMA(string inFile, string outFile)
    {
        SevenZip.Compression.LZMA.Decoder coder = new SevenZip.Compression.LZMA.Decoder();
        FileStream input = new FileStream(inFile, FileMode.Open);
        FileStream output = new FileStream(outFile, FileMode.Create);

        // Read the decoder properties
        byte[] properties = new byte[5];
        input.Read(properties, 0, 5);

        // Read in the decompress file size.
        byte [] fileLengthBytes = new byte[8];
        input.Read(fileLengthBytes, 0, 8);
        long fileLength = BitConverter.ToInt64(fileLengthBytes, 0);

        coder.SetDecoderProperties(properties);
        coder.Code(input, output, input.Length, fileLength, null);
        output.Flush();
        output.Close();
    }

Note that the files created this way can be extracted by the 7zip program as well but will not retain their filename or any other metadata.

Answer 4

I highly recommend managed-lzma:
https://github.com/weltkante/managed-lzma

It preserves file info and directory structure in file encoding.