.NET GZipStream解压缩生成空流

Question

我正在尝试序列化和压缩WPF FlowDocument,然后反向 - 解压缩字节数组并反序列化以重新创建FlowDocument - 使用.NET GZipStream类.我正在关注MSDN上描述的示例,我有以下测试程序:

var flowDocumentIn = new FlowDocument();
flowDocumentIn.Blocks.Add(new Paragraph(new Run("Hello")));
Debug.WriteLine("Compress");
byte[] compressedData;
using (var uncompressed = new MemoryStream())
{
    XamlWriter.Save(flowDocumentIn, uncompressed);
    uncompressed.Position = 0;
    using (var compressed = new MemoryStream())
    using (var compressor = new GZipStream(compressed, CompressionMode.Compress))
    {
        Debug.WriteLine(" uncompressed.Length: " + uncompressed.Length);
        uncompressed.CopyTo(compressor);
        Debug.WriteLine(" compressed.Length: " + compressed.Length);
        compressedData = compressed.ToArray();
    }
}

Debug.WriteLine("Decompress");
FlowDocument flowDocumentOut;
using (var compressed = new MemoryStream(compressedData))
using (var uncompressed = new MemoryStream())
using (var decompressor = new GZipStream(compressed, CompressionMode.Decompress))
{
    Debug.WriteLine(" compressed.Length: " + compressed.Length);
    decompressor.CopyTo(uncompressed);
    Debug.WriteLine(" uncompressed.Length: " + uncompressed.Length);
    flowDocumentOut = (FlowDocument) XamlReader.Load(uncompressed);
}

Assert.AreEqual(flowDocumentIn, flowDocumentOut);

但是XamlReader.Load,由于调试输出告诉未压缩的流的长度为零,因此在线路上出现异常是正常的.

Compress
 uncompressed.Length: 123
 compressed.Length: 202
Decompress
 compressed.Length: 202
 uncompressed.Length: 0

为什么最终uncompressed流不包含原始的123字节？

(请忽略"压缩"字节数组大于"未压缩"字节数组的事实 - 我通常会使用更大的流文档)

Answer 1

您需要GZipStream在从内存流中获取压缩字节之前关闭它.在这种情况下,Dispose由于使用,关闭由被调用者处理.

using (var compressed = new MemoryStream())
{
    using (var compressor = new GZipStream(compressed, CompressionMode.Compress))
    {
        uncompressed.CopyTo(compressor);
    }
    // Get the compressed bytes only after closing the GZipStream
    compressedBytes = compressed.ToArray();
}

这可以工作,你甚至可以删除它using,MemoryStream因为它将被处理,GZipStream除非你使用构造函数重载,允许你指定底层流应该保持打开.这意味着使用该代码调用ToArray已处理的流,但这是允许的,因为字节仍然可用,这使得处理内存流有点奇怪,但如果你不这样做,FXCop会惹恼你.

Answer 2

若昂的回答起到了作用。我复制了下面的完整工作示例。我在输出中添加了一行compressedData.Length。有趣的是，它输出 218 字节，而compressedStream.Length仅输出 202 字节。如果您在读取字节数组之前不关闭 GZipStream，则为compressedData.Length202。我不确定为什么关闭 GZipStream 会给您带来额外的 16 个字节。

var flowDocumentIn = new FlowDocument();
flowDocumentIn.Blocks.Add(new Paragraph(new Run("Hello")));

Debug.WriteLine("Compress");

byte[] compressedData;

using (var uncompressedStream = new MemoryStream())
{
    XamlWriter.Save(flowDocumentIn, uncompressedStream);
    uncompressedStream.Position = 0;
    using (var compressedStream = new MemoryStream())
    {
        using (var gZipCompressor = new GZipStream(compressedStream, CompressionMode.Compress))
        {
            Debug.WriteLine(" uncompressedStream.Length: " + uncompressedStream.Length);
            uncompressedStream.CopyTo(gZipCompressor);
            Debug.WriteLine(" compressedStream.Length: " + compressedStream.Length);
        }
        compressedData = compressedStream.ToArray();
    }
}

Debug.WriteLine(" compressedData.Length: " + compressedData.Length);

Debug.WriteLine("Decompress");

FlowDocument flowDocumentOut;

using (var compressedStream = new MemoryStream(compressedData))
using (var uncompressedStream = new MemoryStream())
{
    using (var gZipDecompressor = new GZipStream(compressedStream, CompressionMode.Decompress))
    {
        Debug.WriteLine(" compressedStream.Length: " + compressedStream.Length);
        gZipDecompressor.CopyTo(uncompressedStream);
        Debug.WriteLine(" uncompressedStream.Length: " + uncompressedStream.Length);
    }
    uncompressedStream.Position = 0;
    flowDocumentOut = (FlowDocument)XamlReader.Load(uncompressedStream);
}

调试输出：

Compress
 uncompressedStream.Length: 123
 compressedStream.Length: 202
 compressedData.Length: 218
Decompress
 compressedStream.Length: 218
 uncompressedStream.Length: 123

uncompressedStream.Position = 0;另请注意调用 to 之前的附加内容XamlReader.Load。