c# - GZipStream - 即使刷新也不写入所有压缩数据？

Question

我遇到了一个针对 .Net 3.5 的 gzipstream 的讨厌问题。这是我第一次使用 gzipstream，但是我已经模仿了许多教程，包括 here我仍然卡住了。

我的应用程序将数据表序列化为 xml 并插入数据库，将压缩数据存储到 varbinary(max) 字段以及未压缩缓冲区的原始长度。然后，当我需要它时，我检索这些数据并将其解压缩并重新创建数据表。解压缩似乎失败了。

编辑:遗憾的是，按照建议将 GetBuffer 更改为 ToArray 后，我的问题仍然存在。代码更新如下

压缩代码:

DataTable dt = new DataTable("MyUnit");
//do stuff with dt
//okay...  now compress the table
using (MemoryStream xmlstream = new MemoryStream())
{
    //instead of stream, use xmlwriter?
    System.Xml.XmlWriterSettings settings = new System.Xml.XmlWriterSettings();
    settings.Encoding = Encoding.GetEncoding(1252);
    settings.Indent = false;
    System.Xml.XmlWriter writer = System.Xml.XmlWriter.Create(xmlstream, settings);
    try
    {
        dt.WriteXml(writer);
        writer.Flush();
    }
    catch (ArgumentException)
    {
        //likely an encoding issue...  okay, base64 encode it
        var base64 = Convert.ToBase64String(xmlstream.ToArray());
        xmlstream.Write(Encoding.GetEncoding(1252).GetBytes(base64), 0, Encoding.GetEncoding(1252).GetBytes(base64).Length);
    }

    using (MemoryStream zipstream = new MemoryStream())
    {
        GZipStream zip = new GZipStream(zipstream, CompressionMode.Compress);
        log.DebugFormat("Compressing commands...");
        zip.Write(xmlstream.GetBuffer(), 0, xmlstream.ToArray().Length);
        zip.Flush();
        float ratio = (float)zipstream.ToArray().Length / (float)xmlstream.ToArray().Length;
        log.InfoFormat("Resulting compressed size is {0:P2} of original", ratio);

        using (SqlCommand cmd = new SqlCommand())
        {
            cmd.CommandText = "INSERT INTO tinydup (lastid, command, compressedlength) VALUES (@lastid,@compressed,@length)";
            cmd.Connection = db;
            cmd.Parameters.Add("@lastid", SqlDbType.Int).Value = lastid;
            cmd.Parameters.Add("@compressed", SqlDbType.VarBinary).Value = zipstream.ToArray();
            cmd.Parameters.Add("@length", SqlDbType.Int).Value = xmlstream.ToArray().Length;
            cmd.ExecuteNonQuery();

        }
    }

解压代码:

/* This is an encapsulation of what I get from the database
 public class DupUnit{
    public uint lastid;
    public uint complength;
    public byte[] compressed;
}*/
  //I have already retrieved my list of work to do from the database in a List<Dupunit> dupunits
foreach (DupUnit unit in dupunits)
{
    DataSet ds = new DataSet();
    //DataTable dt = new DataTable();
    //uncompress and extract to original datatable
    try
    {
        using (MemoryStream zipstream = new MemoryStream(unit.compressed))
        {
            GZipStream zip = new GZipStream(zipstream, CompressionMode.Decompress);
            byte[] xmlbits = new byte[unit.complength];
            //WHY ARE YOU ALWAYS 0!!!!!!!!
            int bytesdecompressed = zip.Read(xmlbits, 0, unit.compressed.Length);
            MemoryStream xmlstream = new MemoryStream(xmlbits);
            log.DebugFormat("Uncompressed XML against {0} is: {1}", m_source.DSN, Encoding.GetEncoding(1252).GetString(xmlstream.ToArray()));
            try{
               ds.ReadXml(xmlstream);
            }catch(Exception)
            {
                //it may have been base64 encoded...  decode first.
               ds.ReadXml(Encoding.GetEncoding(1254).GetString(
                 Convert.FromBase64String(
                 Encoding.GetEncoding(1254).GetString(xmlstream.ToArray())))
                 );
            }
            xmlstream.Dispose();
        }
    }
    catch (Exception e)
    {
        log.Error(e);
        Thread.Sleep(1000);//sleep a sec!
        continue;
    }

注意上面的评论...bytesdecompressed 始终为 0。有什么想法吗？我做错了吗？

编辑 2:

所以这很奇怪。我在解压例程中添加了以下调试代码:

   GZipStream zip = new GZipStream(zipstream, CompressionMode.Decompress);
   byte[] xmlbits = new byte[unit.complength];
   int offset = 0;
   while (zip.CanRead && offset < xmlbits.Length)
   {
       while (zip.Read(xmlbits, offset, 1) == 0) ;
       offset++;
   }

调试时，有时该循环会完成，但有时会挂起。当我停止调试时，它将位于 1616 字节中的第 1600 字节。我会继续，但它根本不会移动。

编辑 3: 该错误似乎出现在压缩代码中。无论出于何种原因，它都没有保存所有数据。当我尝试使用第三方 gzip 机制解压缩数据时，我只得到了部分原始数据。

我会开始赏金，但我现在真的没有多少声望可以给予 :-(

Answer 1

终于找到答案了。压缩数据不完整，因为 GZipStream.Flush() 完全没有做任何事情来确保所有数据都在缓冲区之外 - 您需要使用 GZipStream.Close() 作为 pointed out here .当然，如果压缩不好，一切都会变得糟糕 - 如果您尝试解压缩它，Read() 将始终返回 0。