public override void Write( 
   byte[] buffer,
   int offset,
   int count
)

using System.IO;
using Xceed.Compression;
using Xceed.Compression.Formats;

using( FileStream sourceStream = new FileStream( @"d:\data.txt", FileMode.Open ) )
{
      // Because the GZipCompressedStream will automatically close the destination
      // memory stream, there is no need to declare the memory stream within a using
      // statement or to call Close once we are done with the stream.
      //
      // If you do not want the inner stream to be closed by the GZipCompressedStream
      // then set the GZipCompressedStream's Transient property to true.
      
      MemoryStream destinationStream = new MemoryStream();
      
      using( GZipCompressedStream gzip = new GZipCompressedStream( destinationStream ) )
      { 
            // 32K at a time.
            byte[] buffer = new byte[ 32768 ];
            int bytesRead = 0;
            
            // Loop until we have nothing more to read from the source stream.
            while( ( bytesRead = sourceStream.Read( buffer, 0, buffer.Length  ) ) > 0 )
            {
                  gzip.Write( buffer, 0, bytesRead );
            }
      }
      
      // To get access to the MemoryStream's compressed data, you can use
      // byte[] compressedData = destinationStream.ToArray(); 
      // ToArray() works even when the memory stream is closed.
}