Yurik Yurik - 4 months ago 29
C# Question

Implementing async stream for producer/cosumer in C# / .NET

There is a lib that outputs its results into a given Stream object. I would like to begin consuming the results before the lib is done. The Stream should be blocking to simplify usage and avoid excessive memory consumption if producer runs ahead too far; thread safe to allow independent existence of producer and consumer.

Once the lib finishes, the producer thread should close the stream, hence notifying consumer that there is no more data.

I was thinking of using NetworkStream or PipeStream (anonymous), but both are probably slow as they send data through kernel.

Any recomendations?


var stream = new AsyncBlockingBufferedStream();

void ProduceData()
{
// In producer thread
externalLib.GenerateData(stream);
stream.Close();
}

void ConsumeData()
{
// In consumer thread
int read;
while ((read = stream.Read(...)) != 0)
{ ... }
}

Answer

Based on the previous answer by Chris Taylor, here's my own, revised, with much faster block based operations and corrected write completion notifications. Its marked as wiki now, so you can change it.

public class BlockingStream : Stream
{
    private readonly BlockingCollection<byte[]> _blocks;
    private byte[] _currentBlock;
    private int _currentBlockIndex;

    public BlockingStream(int streamWriteCountCache)
    {
        _blocks = new BlockingCollection<byte[]>(streamWriteCountCache);
    }

    public override bool CanTimeout { get { return false; } }
    public override bool CanRead { get { return true; } }
    public override bool CanSeek { get { return false; } }
    public override bool CanWrite { get { return true; } }
    public override long Length { get { throw new NotSupportedException(); } }
    public override void Flush() {}
    public long TotalBytesWritten { get; private set; }
    public int WriteCount { get; private set; }

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

    public override long Seek(long offset, SeekOrigin origin)
    {
        throw new NotSupportedException();
    }

    public override void SetLength(long value)
    {
        throw new NotSupportedException();
    }

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

        int bytesRead = 0;
        while (true)
        {
            if (_currentBlock != null)
            {
                int copy = Math.Min(count - bytesRead, _currentBlock.Length - _currentBlockIndex);
                Array.Copy(_currentBlock, _currentBlockIndex, buffer, offset + bytesRead, copy);
                _currentBlockIndex += copy;
                bytesRead += copy;

                if (_currentBlock.Length <= _currentBlockIndex)
                {
                    _currentBlock = null;
                    _currentBlockIndex = 0;
                }

                if (bytesRead == count)
                    return bytesRead;
            }

            if (!_blocks.TryTake(out _currentBlock, Timeout.Infinite))
                return bytesRead;
        }
    }

    public override void Write(byte[] buffer, int offset, int count)
    {
        ValidateBufferArgs(buffer, offset, count);

        var newBuf = new byte[count];
        Array.Copy(buffer, offset, newBuf, 0, count);
        _blocks.Add(newBuf);
        TotalBytesWritten += count;
        WriteCount++;
    }

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

    public override void Close()
    {
        CompleteWriting();
        base.Close();
    }

    public void CompleteWriting()
    {
        _blocks.CompleteAdding();
    }

    private static void ValidateBufferArgs(byte[] buffer, int offset, int count)
    {
        if (buffer == null)
            throw new ArgumentNullException("buffer");
        if (offset < 0)
            throw new ArgumentOutOfRangeException("offset");
        if (count < 0)
            throw new ArgumentOutOfRangeException("count");
        if (buffer.Length - offset < count)
            throw new ArgumentException("buffer.Length - offset < count");
    }
}
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