GZipStream 和 DeflateStream 不会解压缩所有字节 [英] GZipStream And DeflateStream will not decompress all bytes

问题描述

我需要一种在 .net 中压缩图像的方法，所以我研究了使用 .net GZipStream 类(或 DeflateStream).但是我发现解压并不总是成功，有时图像可以解压得很好，有时我会收到 GDI+ 错误，表明某些东西已损坏.

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.

在调查这个问题后，我发现解压没有返回它压缩的所有字节.所以如果我压缩了 2257974 个字节，我有时只会得到 2257870 个字节(实数).

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).

最有趣的是有时它会起作用.所以我创建了这个只压缩 10 个字节的小测试方法，现在我什么也没有得到.

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.

我用 GZipStream 和 DeflateStream 两种压缩类都尝试过，我仔细检查了我的代码是否有可能的错误.我什至尝试将流定位到 0 并刷新所有流，但没有运气.

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.

这是我的代码:

    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();
    }

推荐答案

添加所有要压缩的数据后，需要Close() ZipStream；它在内部保留了一个需要写入的未写入字节缓冲区(即使您 Flush()).

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.

更一般地说，Stream 是 IDisposable，所以你也应该 using 每个......(是的，我知道 MemoryStream 不会丢失任何数据，但如果你不养成这个习惯，它会被其他 Stream 咬你.

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())
    { ... }
}

不要使用诸如MemoryStream之类的东西——这总是一个有趣的事情，围栏两边都有很多选票:但最终......

[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...

(修辞——我们都知道答案...) MemoryStream 是如何实现的?它是一个字节 [](由 .NET 拥有)吗?它是内存映射文件(由操作系统拥有)吗?

(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)?

您不使用的原因是因为您让内部实现细节的知识改变了您针对公共 API 进行编码的方式 - 即您刚刚违反了封装定律.公共 API 说:我是 IDisposable;你欠我的;因此，当你完成后，Dispose() 我是你的工作.

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.

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