使用 GZipStream 对 MemoryStream 进行编程压缩/解压缩

Question

I built (based on a CodeProject article) a wrapper class (C#) to use a GZipStream to compress a MemoryStream . It compresses fine but doesn't decompress. I've looked at many other examples that have the same problem, and I feel like I'm following what's said but still am getting nothing when I decompress. Here's the compression and decompression methods:

public static byte[] Compress(byte[] bSource)
{

    using (MemoryStream ms = new MemoryStream())
    {
        using (GZipStream gzip = new GZipStream(ms, CompressionMode.Compress, true))
        {
            gzip.Write(bSource, 0, bSource.Length);
            gzip.Close();
        }

        return ms.ToArray();
    }
}


public static byte[] Decompress(byte[] bSource)
{

    try
    {
        using (MemoryStream ms = new MemoryStream())
        {
            using (GZipStream gzip = new GZipStream(ms, CompressionMode.Decompress, true))
            {
                gzip.Read(bSource, 0, bSource.Length);
                gzip.Close();
            }

            return ms.ToArray();
        }
    }
    catch (Exception ex)
    {
        throw new Exception("Error decompressing byte array", ex);
    }
}

Here's an example of how I use it:

string sCompressed = Convert.ToBase64String(CompressionHelper.Compress("Some Text"));
// Other Processes
byte[] bReturned = CompressionHelper.Decompress(Convert.FromBase64String(sCompressed));
// bReturned has no elements after this line is executed

Answer 1

There is a bug in Decompress method.

The code does not read content of bSource . On the contrary, it overrides its content wile reading from empty gzip, created based on empty memory stream.

Basically what your version of code is doing:

//create empty memory
using (MemoryStream ms = new MemoryStream())

//create gzip stream over empty memory stream
using (GZipStream gzip = new GZipStream(ms, CompressionMode.Compress, true))

// write from empty stream to bSource
gzip.Write(bSource, 0, bSource.Length);

The fix could look like this:

public static byte[] Decompress(byte[] bSource)
{
    using (var inStream = new MemoryStream(bSource))
    using (var gzip = new GZipStream(inStream, CompressionMode.Decompress))
    using (var outStream = new MemoryStream())
    {
        gzip.CopyTo(outStream);
        return outStream.ToArray();
    }
}

Answer 2

The OP said in an edit, now rolled back:

Thanks to Alex's explanation of what was going wrong, I was able to fix the Decompress method. Unfortunately, I'm using.Net 3.5, so I wasn't able to implement the Stream.CopyTo method he suggested. With his explanation, though, I was able to figure out a solution. I made the appropriate changes to the Decompress method below.

    public static byte[] Decompress(byte[] bSource)
    {
        try
        {
            using (var instream = new MemoryStream(bSource))
            {
                using (var gzip = new GZipStream(instream, CompressionMode.Decompress))
                {
                    using (var outstream = new MemoryStream())
                    {
                        byte[] buffer = new byte[4096];

                        while (true)
                        {
                            int delta = gzip.Read(buffer, 0, buffer.Length);

                            if (delta > 0)
                                outstream.Write(buffer, 0, delta);

                            if (delta < 4096)
                                break;
                        }
                        return outstream.ToArray();
                    }
                }
            }
        }
        catch (Exception ex)
        {
            throw new Exception("Error decompressing byte array", ex);
        }
    }