如何使用gzipstream将内存流压缩成内存流？

Question

Hello my problem is that I need to compress MemoryStream with GZipStream into MemoryStream if possible into the same MemoryStream that method gets as parameter. If this can't be achieved how can I make it so it is the most memory efficient.

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Here is my current method and it is giving me System.NotSupportedException: 'Stream does not support reading.' for compress.CopyTo

public static void GZipCompress(MemoryStream memoryStream)
{
    using (GZipStream compress = new GZipStream(memoryStream, CompressionMode.Compress))
    {
        compress.CopyTo(memoryStream);
    }
}

Answer 1

You can't copy the stream to itself, at least not without a lot of work. Just allocating a new MemoryStream for the compressed data is simple and reasonably efficient. eg

public MemoryStream GZipCompress(MemoryStream memoryStream)
{
    var newStream = new MemoryStream((int)memoryStream.Length / 2); //set to estimate of compression ratio

    using (GZipStream compress = new GZipStream(newStream, CompressionMode.Compress))
    {
        memoryStream.CopyTo(compress);
    }
    newStream.Position = 0;
    return newStream;
}

Here's an untested idea for how to perform in-place compression of a MemoryStream.

public void GZipCompress(MemoryStream memoryStream)
{
    var buf = new byte[1024 * 64];
    int writePos = 0;

    using (GZipStream compress = new GZipStream(memoryStream, CompressionMode.Compress))
    {
        while (true)
        {
            var br = compress.Read(buf, 0, buf.Length);
            if (br == 0) //end of stream
            {
                break;
            }
            var readPos = memoryStream.Position;
            memoryStream.Position = writePos;
            memoryStream.Write(buf, 0, br);
            writePos += br;

            if (memoryStream.Position > readPos)
            {
                throw new InvalidOperationException("Overlapping writes corrupted the stream");
            }
            memoryStream.Position = readPos;
        }
    }
    memoryStream.SetLength(writePos);
    memoryStream.Position = 0;
}