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 (var outStream = new MemoryStream()) {     using (var tinyStream = new GZipStream(outStream, CompressionMode.Compress))     using (var 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 (var inStream = new MemoryStream(compressed)) using (var bigStream = new GZipStream(inStream, CompressionMode.Decompress)) using (var bigStreamOut = new MemoryStream()) {     bigStream.CopyTo(bigStreamOut);     output = Encoding.UTF8.GetString(bigStreamOut.ToArray()); }  // “output” now contains the uncompressed string. Console.WriteLine(output);