How do I use GZipStream with System.IO.MemoryStream?

Question

I am having an issue with this test function where I take an in memory string, compress it, and decompress it. The compression works great, but I can't seem to get the decompression to work.

//Compress
System.IO.MemoryStream outStream = new System.IO.MemoryStream();                
GZipStream tinyStream = new GZipStream(outStream, CompressionMode.Compress);
mStream.Position = 0;
mStream.CopyTo(tinyStream);

//Decompress    
outStream.Position = 0;
GZipStream bigStream = new GZipStream(outStream, CompressionMode.Decompress);
System.IO.MemoryStream bigStreamOut = new System.IO.MemoryStream();
bigStream.CopyTo(bigStreamOut);

//Results:
//bigStreamOut.Length == 0
//outStream.Position == the end of the stream.

I believe that bigStream out should at least have data in it, especially if my source stream (outStream) is being read. is this a MSFT bug or mine?

Answer 1

What happens in your code is that you keep opening streams, but you never close them.

• In line 2, you create a GZipStream. This stream will not write anything to the underlying stream until it feels it’s the right time. You can tell it to by closing it.

• However, if you close it, it will close the underlying stream (outStream) too. Therefore you can’t use mStream.Position = 0 on it.

You should always use using to ensure that all your streams get closed. Here is a variation on your code that works.

var inputString = "“ ... ”";
byte[] compressed;
string output;

using (MemoryStream outStream = new MemoryStream())
{
    using (GZipStream tinyStream = new GZipStream(outStream, CompressionMode.Compress))
    using (MemoryStream mStream = new MemoryStream(Encoding.UTF8.GetBytes(inputString)))
        mStream.CopyTo(tinyStream);
    compressed = outStream.ToArray();
}

// “compressed” now contains the compressed string.
// Also, all the streams are closed and the above is a self-contained operation.

using (MemoryStream inStream = new MemoryStream(compressed))
using (GZipStream bigStream = new GZipStream(inStream, CompressionMode.Decompress))
using (MemoryStream bigStreamOut = new MemoryStream())
{
    bigStream.CopyTo(bigStreamOut);
    output = Encoding.UTF8.GetString(bigStreamOut.ToArray());
}

// “output” now contains the uncompressed string.
Console.WriteLine(output);

Answer 2

This is a known issue: http://blogs.msdn.com/b/bclteam/archive/2006/05/10/592551.aspx

I have changed your code a bit so this one works:

         MemoryStream mStream = new MemoryStream(new byte[100]);
        System.IO.MemoryStream outStream = new System.IO.MemoryStream();
        using (GZipStream tinyStream = new GZipStream(outStream, CompressionMode.Compress))
        {
            mStream.CopyTo(tinyStream);           
        }
        byte[] bb = outStream.ToArray();

        //Decompress                
        GZipStream bigStream = new GZipStream(new MemoryStream(bb), CompressionMode.Decompress);
        System.IO.MemoryStream bigStreamOut = new System.IO.MemoryStream();
        bigStream.CopyTo(bigStreamOut);