Apply GZip compression to a stream used by a method that pulls data

Question

Prerequisites

I have a stream rawStream and a method which takes a stream and reads it to the end, let's say something like this:

Task UploadFile(Stream stream) { ... }

Currently this method is successfully being used like this:

await UploadFile(rawStream);

What I want to do

Now I need to apply GZip compression to that stream. I wish I could write something like this:

using (var compressedStream = new GZipStream(rawStream, CompressionLevel.Fastest))
{
    await UploadFile(compressedStream);
}

But this doesn't work because the stream parameter of GZipStream is an output stream, so it's the wrong way round.

Question

How can I wrap my original stream in a compressing stream and still let my consuming function pull the data out of the stream?

---

Note

The above example is actually simplyfied, because I also need to apply base64 encoding. So what I actually want is something like this:

using (var compressedStream = new GZipStream(rawStream, CompressionLevel.Fastest))
using (var encodedStream = new CryptoStream(compressedStream, new ToBase64Transform(), CryptoStreamMode.Read))
{
    await UploadFile(encodedStream);
}

But I guess if someone can explain to me how it works for the compression part, I can figure out how to implement the entire chain.

Answer 1

None of the approaches presented so far really convinced me. So I went ahead and wrote the following customize stream which allows pulling the GZipped and Base64 encoded data out of the stream.

I did some tests and it seems to work fine.

I think this pattern can be useful in other contexts as well for turning a "push pipeline" into a "pull pipeline".

public sealed class GzipBase64Stream : Stream
{

    #region constructor / cleanup

    public GzipBase64Stream(Stream inputStream)
    {
        try
        {
            InputStream = inputStream;
            ToBase64Transform = new ToBase64Transform();
            OutputStream = new MemoryStream();
            Base64Stream = new CryptoStream(OutputStream, ToBase64Transform, CryptoStreamMode.Write);
            GzipStream = new GZipStream(Base64Stream, CompressionLevel.Fastest, true);
        }
        catch
        {
            Cleanup();
            throw;
        }
    }

    private void Cleanup()
    {
        GzipStream?.Dispose();
        Base64Stream?.Dispose();
        OutputStream?.Dispose();
        ToBase64Transform?.Dispose();
        InputStream?.Dispose();
    }

    #endregion

    #region private variables

    private bool EndOfInputStreamReached = false;

    private readonly Stream InputStream;
    private readonly ToBase64Transform ToBase64Transform;
    private readonly MemoryStream OutputStream;
    private readonly CryptoStream Base64Stream;
    private readonly GZipStream GzipStream;

    #endregion

    #region stream overrides

    public override bool CanRead => true;

    public override bool CanSeek => false;

    public override bool CanWrite => false;

    public override long Length => 0;

    public override long Position { get => throw new NotSupportedException(); set => throw new NotSupportedException(); }

    public override void SetLength(long value) => throw new NotSupportedException();

    public override long Seek(long offset, SeekOrigin origin) => throw new NotSupportedException();

    public override void Write(byte[] buffer, int offset, int count) => throw new NotSupportedException();

    public override void Flush() => throw new NotSupportedException();

    public override int Read(byte[] buffer, int offset, int count)
    {

        while ((OutputStream.Position >= OutputStream.Length) && !EndOfInputStreamReached)
        {
            // No unread data available in the output buffer
            // -> release memory of output buffer and read new data from the source and feed through the pipeline
            OutputStream.SetLength(0);
            var inputBuffer = new byte[1024];
            var readCount = InputStream.Read(inputBuffer, 0, inputBuffer.Length);
            if (readCount == 0)
            {
                EndOfInputStreamReached = true;
                GzipStream.Flush();
                GzipStream.Dispose(); // because Flush() does not actually flush...
            }
            else
            {

                GzipStream.Write(inputBuffer, 0, readCount);
            }
            OutputStream.Position = 0;
        }

        return OutputStream.Read(buffer, offset, count);
    }

    protected override void Dispose(bool disposing)
    {
        base.Dispose(disposing);
        if (disposing)
            Cleanup();
    }

    #endregion

}