MemoryStream CopyTo只是部分写入

Question

I'm about to lose my freaking mind. I've been trying to get GzipStream to compress a string for the past hour, but for whatever reason, it refuses to write the entire byte array to the memory stream. At first I thought it had something to do with the using statements, but even after removing them it didn't seem to make a difference.

Initial config:

var str = "Here is a relatively simple string to compress";
byte[] compressedBytes;
string returnedData;

var bytes = Encoding.UTF8.GetBytes(str);

Works correctly (writes 64 length byte array):

using (var msi = new MemoryStream(bytes))
using (var mso = new MemoryStream()) {
   using (var gs = new GZipStream(mso, CompressionMode.Compress)) {
       msi.CopyTo(gs);
   }

   compressedBytes = mso.ToArray();
}

Fails (writes 10 length byte array):

using(var mso = new MemoryStream())
using(var msi = new MemoryStream(bytes))
using(var zip = new GZipStream(mso, CompressionMode.Compress))
{
    msi.CopyTo(zip);
    compressedBytes = mso.ToArray();
}

Also fails (writes 10 length byte array):

var mso = new MemoryStream();
var msi = new MemoryStream(bytes);
var zip = new GZipStream(mso, CompressionMode.Compress);

msi.CopyTo(zip);
compressedBytes = mso.ToArray();

Can somebody explain why the first one works but in the other two I'm getting these incomplete arrays? Is something getting disposed out from under me? For that matter, is there a way for me to avoid using two memorystreams?

Thanks, Zoombini

Answer 1

System.IO.Compression.GZipStream has to be closed (disposed) before you can use the underlying stream, because

1. It works block oriented
2. It has to write the footer, including the checksum (see the file format description on Wikipedia )

Answer 2

You're trying to get the the compressed data before GZipStream is closed. This doesn't return the full data, as you've seen. The reason the first one works is because you're calling compressedBytes = mso.ToArray(); after GZipStream has been disposed. So, untested but in theory, you should be able to modify your second code slightly like this to get it to work.

using(var mso = new MemoryStream())
{
   using(var msi = new MemoryStream(bytes))
   using(var zip = new GZipStream(mso, CompressionMode.Compress))
   {
       msi.CopyTo(zip);
   }
   compressedBytes = mso.ToArray();
}

Answer 3

As others have said, you need to close the GZipStream before you can get the full data. A using statement will cause the Dispose method to be called on the stream at the end of the block, which will close the stream if it is not already closed. All of your examples above will work as expected if you place zip.Close(); after msi.CopyTo(zip); .

You can eliminate one of the MemoryStreams if you write it this way:

using (MemoryStream mso = new MemoryStream())
{
    using (GZipStream zip = new GZipStream(mso, CompressionMode.Compress))
    {
        zip.Write(bytes, 0, bytes.Length);
    }
    compressedBytes = mso.ToArray();
}