Question

I was in need of a way to compress images in .net so i looked into using the .net GZipStream class (or DeflateStream). However i found that decompression was not always successful, sometimes the images would decompress fine and other times i would get a GDI+ error that something is corrupted.

After investigating the issue i found that the decompression was not giving back all the bytes it compressed. So if i compressed 2257974 bytes i would sometimes get back only 2257870 bytes (real numbers).

The most funny thing is that sometimes it would work. So i created this little test method that compresses only 10 bytes and now i don’t get back anything at all.

I tried it with both compression classes GZipStream and DeflateStream and i double checked my code for possible errors. I even tried positioning the stream to 0 and flushing all the streams but with no luck.

Here is my code:

    public static void TestCompression()     {         byte[] test = new byte[] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };          byte[] result = Decompress(Compress(test));          // This will fail, result.Length is 0         Debug.Assert(result.Length == test.Length);     }      public static byte[] Compress(byte[] data)     {         var compressedStream = new MemoryStream();         var zipStream = new GZipStream(compressedStream, CompressionMode.Compress);         zipStream.Write(data, 0, data.Length);         return compressedStream.ToArray();     }      public static byte[] Decompress(byte[] data)     {         var compressedStream = new MemoryStream(data);         var zipStream = new GZipStream(compressedStream, CompressionMode.Decompress);         var resultStream = new MemoryStream();          var buffer = new byte[4096];         int read;          while ((read = zipStream.Read(buffer, 0, buffer.Length)) > 0) {             resultStream.Write(buffer, 0, read);         }          return resultStream.ToArray();     } 

Answer 1

You need to Close() the ZipStream after adding all the data you want to compress; it retains a buffer of unwritten bytes internally (even if you Flush()) that needs to be written.

More generally, Stream is IDisposable, so you should also be using each… (yes, I know that MemoryStream isn’t going to lose any data, but if you don’t get into this habit, it will bite you with other Streams).

public static byte[] Compress(byte[] data) {     using (var compressedStream = new MemoryStream())     using (var zipStream = new GZipStream(compressedStream, CompressionMode.Compress))     {         zipStream.Write(data, 0, data.Length);         zipStream.Close();         return compressedStream.ToArray();     } }  public static byte[] Decompress(byte[] data) {     using(var compressedStream = new MemoryStream(data))     using(var zipStream = new GZipStream(compressedStream, CompressionMode.Decompress))     using (var resultStream = new MemoryStream())     { ... } } 

[edit : updated re comment] Re not using things like MemoryStream – this is always a fun one, with lots of votes on either side of the fence: but ultimatey…

(rhetorical – we all know the answer…) How is MemoryStream implemented? is it a byte[] (owned by .NET)? is it a memory-mapped file (owned by the OS)?

The reason you aren’t using it is because you are letting knowledge of internal implementation details change how you code against a public API – i.e. you just broke the laws of encapsulation. The public API says: I am IDisposable; you ‘own’ me; therefore, it is your job to Dispose() me when you are through.