如何从任务异步返回WCF响应中的流？

Question

我有一个类，该类对大量数据进行长时间运行的处理，并将输出写入我提供的流中。 我正在尝试将WCF放在此前面（使用命名管道），但是在弄清楚如何返回流时遇到了麻烦。 到目前为止，我有这样的事情：

interface IProcessor { Stream GetStream(); }

class Host {
  static void Main(string[] args) {
    using (ServiceHost sh = new ServiceHost(typeof(Processor), new Uri[]{new Uri("net.pipe://localhost")})) {
      var binding = new NetNamedPipeBinding();
      binding.TransferMode = TransferMode.StreamedResponse;
      sh.AddServiceEndpoint(typeof(IProcessor), binding, "foo");
      sh.Open();
      Console.WriteLine("Waiting...");
      Console.ReadLine();
      sh.Close();
    }
  }
}

class Processor : IProcessor {
  Stream GetStream() {
    var SourceProcessor = new SourceProcessor(...);
    var s = new MemoryStream();
    new Task(() => { SourceProcessor.Run(s); }).Start();
    return s;
  }
}

class Client {
  static void Main(string[] args) {
    Console.WriteLine("Starting...");
    var binding = new NetNamedPipeBinding();
    binding.TransferMode = TransferMode.StreamedResponse;
    ChannelFactory<IProcessor> f = new ChannelFactory<IProcessor>(binding, new EndpointAddress("net.pipe://localhost/foo"));
    Console.WriteLine("Creating channel...");
    IProcessor eps = f.CreateChannel();
    Console.WriteLine("Getting stream.");
    Stream s = eps.GetStream();
    StreamReader sr = new StreamReader(s);
    while (!sr.EndOfStream) Console.WriteLine(sr.ReadLine());
    Console.ReadLine();
  }
}

一切都会进行，但当然没有源数据可以传递给客户端。 我对如何做到这一点感到困惑（也许我做不到），因为我既需要返回流并运行任务，又可能等待任务完成。 如果我不调用任务就直接调用SourceProcessor.Run，​​它会阻塞和缓冲，但是，我不确定如何等到任务完成后再返回流以供客户端读取...

Answer 1

问题是，如果WCF调用Read(且调用返回0字节，WCF会认为流“完成” MemoryStream会很乐意这样做，如果没有可用数据，它将不阻止读取。

问题的根源是WCF正在读取MemoryStream速度比您编写它的速度快，并认为它已“完成”，解决该问题的方法是，您需要返回阻塞的另一种类型的Stream ，而不是在出现这种情况时返回0没有可用数据。 .NET没有内置可实现此目的的东西，您将需要找到一个第三方类或创建自己的类（它可能很简单，就像从MemoryStream派生并覆盖Read以阻止读取，直到出现“ Done”标志为止设置（有关类似行为，请参见BlockingCollection<T>及其CompleteAdding()方法）。

出于乐趣，我将其组合在一起，虽然未经测试，但可以满足您的需要。

using System;
using System.Collections.Concurrent;
using System.IO;

namespace Example
{
    public class BufferStream : Stream
    {
        public BufferStream()
        {
            _data = new BlockingCollection<byte[]>();
        }


        /// <param name="boundedCapacity">The maximum number of calls to <see cref="Write"/> that can be made without
        /// the buffer being drained.</param>
        public BufferStream(int boundedCapacity)
        {
            _data = new BlockingCollection<byte[]>(boundedCapacity);
        }

        private readonly BlockingCollection<byte[]> _data;
        private byte[] _currentBlock = null;
        private int _currentBlockIndex = 0;

        public int BoundedCapacity { get { return _data.BoundedCapacity; } }

        public int BufferedWrites { get { return _data.Count; } }

        public bool IsAddingCompleted
        {
            get { return _data.IsAddingCompleted; }
        }

        public bool IsCompleted
        {
            get { return _data.IsCompleted; }
        }

        public void CompleteAdding()
        {
            _data.CompleteAdding();
        }

        public override void Write(byte[] buffer, int offset, int count)
        {
            var localArray = new byte[count];

            //Copy the data in to a new buffer of exactly the count size.
            Array.Copy(buffer, offset, localArray, 0, count);

            _data.Add(localArray);
        }


        public override int Read(byte[] buffer, int offset, int count)
        {
            if (_currentBlock == null || _currentBlockIndex == _currentBlock.Length)
            {
                if (!GetNextBlock()) 
                    return 0;
            }

            int minCount = Math.Min(count, _currentBlock.Length - _currentBlockIndex);

            Array.Copy(_currentBlock, _currentBlockIndex, buffer, offset, minCount);
            _currentBlockIndex += minCount;

            return minCount;
        }

        /// <summary>
        /// Loads the next block in to <see cref="_currentBlock"/>.
        /// </summary>
        /// <returns>True if the next block was retrieved.</returns>
        private bool GetNextBlock()
        {
            if (!_data.TryTake(out _currentBlock))
            {
                //The TryTake failed, the collection is empty.

                //See if we are in the completed state.
                if (_data.IsCompleted)
                {
                    return false;
                }

                //Wait for more data to show up.
                try
                {
                    _currentBlock = _data.Take();
                }
                catch (InvalidOperationException)
                {
                    //If the blocking collection was marked complete while we where waiting Take throws a InvalidOperationException
                    return false;
                }
            }

            _currentBlockIndex = 0;
            return true;
        }

        #region Constant functions

        public override bool CanRead
        {
            get { return true; }
        }

        public override bool CanSeek
        {
            get { return false; }
        }

        public override bool CanWrite
        {
            get { return true; }
        }

        public override void Flush()
        {
        }

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

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

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

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

        #endregion
    }
}

这种从MemoryStream派生的优点是，一旦WCF读取了一个值，它就不再需要保留在内存中（返回Stream而不是byte[]的整个过程）