C# DeflateStream.Read() giving an InvalidDataException

Question

I want to unzip a compressed string so that the string of characters would become usable.

I successfully extract in an array of bytes the content of a downloaded file with the function WebClient.DownloadData(String).

The characters are logically compressed with the deflate method, because 7-Zip is giving this information when I download the file (in varying extensions) :

Compressed file informations

Thus, I am supposed to use the DeflateStream class to be able to decode the string, with the function Read(byte[] array, int offset, int count) ; reading a MemoryStream.

I use a simple function which I can find online :

public string UnzipString2(byte[] byteArrayCompressedContent)
        {
            try
            {
                using (var memoryStream = new MemoryStream())
                {
                    int dataLength = BitConverter.ToInt32(byteArrayCompressedContent, 0);
                    memoryStream.Write(byteArrayCompressedContent, 4, byteArrayCompressedContent.Length - 4);
                    memoryStream.Position = 0;

                    var buffer = new byte[dataLength];

                    using (var deflateStream = new DeflateStream(memoryStream, System.IO.Compression.CompressionMode.Decompress))
                    {
                        deflateStream.Read(buffer, 0, buffer.Length);
                        deflateStream.Close();
                    }
                    return Encoding.UTF8.GetString(buffer);
                }
            }
            catch (Exception e)
            {
                return "";
            }
        }

When we call the Read() function, it is giving an InvalidDataException : Found in valid data while decoding ; with the stacktrace

at System.IO.Compression.Inflater.DecodeDynamicBlockHeader()\r\n at System.IO.Compression.Inflater.Decode()\r\n at System.IO.Compression.Inflater.Inflate(Byte[] bytes, Int32 offset, Int32 length)\r\n at System.IO.Compression.DeflateStream.Read(Byte[] array, Int32 offset, Int32 count)\r\n

However, I still think that the information are compressed with the "deflate method". Is there a different/better/working way to read the data and decompress it in a String?

legrojan legrojan · Accepted Answer · 2018-02-08T15:18:05

Compressing a string onto a MemoryStream is not the same as compressing a file as a zip or similar. Even in the compressing algorithm is the same, a compressed file has headers that can be used by the decoder to get information, not only about the compression itself, but also about the files inside of it.

Therefore, if you are trying to deflate a file with the DeflateStream function, the header will not have the expected format, which is exactly what the DecodeDynamicBlockHeader error is telling you.

C# DeflateStream.Read() giving an InvalidDataException

1 Answers