問題描述

我需要一種在 .net 中壓縮圖像的方法，所以我研究了使用 .net GZipStream 類(或 DeflateStream).但是我發現解壓并不總是成功，有時圖像可以解壓得很好，有時我會收到 GDI+ 錯誤，表明某些東西已損壞.

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.

在調查這個問題后，我發現解壓沒有返回它壓縮的所有字節.所以如果我壓縮了 2257974 個字節，我有時只會得到 2257870 個字節(實數).

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).

最有趣的是有時它會起作用.所以我創建了這個只壓縮 10 個字節的小測試方法，現在我什么也沒有得到.

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 兩種壓縮類都嘗試過，我仔細檢查了我的代碼是否有可能的錯誤.我什至嘗試將流定位到 0 并刷新所有流，但沒有運氣.

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.

這是我的代碼:

    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();
    }

推薦答案

添加所有要壓縮的數據后，需要Close() ZipStream；它在內部保留了一個需要寫入的未寫入字節緩沖區(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.

更一般地說，Stream 是 IDisposable，所以你也應該 using 每個......(是的，我知道 MemoryStream 不會丟失任何數據，但如果你不養成這個習慣，它會被其他 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())
    { ... }
}

不要使用諸如MemoryStream之類的東西——這總是一個有趣的事情，圍欄兩邊都有很多選票:但最終......

[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...

(修辭——我們都知道答案...) MemoryStream 是如何實現的?它是一個字節 [](由 .NET 擁有)嗎?它是內存映射文件(由操作系統擁有)嗎?

(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為您讓內部實現細節的知識改變了您針對公共 API 進行編碼的方式 - 即您剛剛違反了封裝定律.公共 API 說:我是 IDisposable;你欠我的;因此，當你完成后，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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