System.IO.InvalidDataException压缩文件异常!

  byte[] xmlbyte = Encoding.UTF8.GetBytes(xml);       //xml字符串
            MemoryStream stream = new MemoryStream(xmlbyte);
            Gzip1.Decompress(stream);

   public static MemoryStream Decompress(Stream input)
        {
            int length = 65536;

            if (input.CanSeek)
            {
                //流可定位 取出压缩流长度 Gzip最后4位
                input.Position = input.Length - 4;
                byte[] bytes = new byte[4];
                input.Read(bytes, 0, 4);
                length = BitConverter.ToInt32(bytes, 0);
                input.Position = 0;
            }
            else
            {
                //如果输入流有Length 长度为Length * 30 因为压缩率为3%
                try { length = (int)input.Length * 20; }
                catch { length = 65536; }
            }


            using (GZipStream compressedzipStream = new GZipStream(input, CompressionMode.Decompress))
            {
                MemoryStream ms = new MemoryStream(length);
                byte[] block = new byte[65536];

                int byteRead = 0;
                do
                {
                    byteRead = compressedzipStream.Read(block, 0, block.Length);
                    ms.Write(block, 0, byteRead);
                }
                while (byteRead == block.Length);

                ms.Position = 0;
                return ms;
            }