AsyncMultiFileReader.cs

using System;
using System.IO;
using System.Runtime.ExceptionServices;
using System.Threading;
using System.Threading.Tasks;

namespace DropboxEncrypedUploader
{
    public class AsyncMultiFileReader : IDisposable
    {
        public byte[] CurrentBuffer { get; private set; }
        public (string Name, object Tag) NextFile { get; set; }
        public (string Name, object Tag) CurrentFile { get; private set; }

        readonly Func<string, object, Stream> _asyncFileStreamFactory;
        byte[] _buffer1;
        byte[] _buffer2;
        Stream _stream;
        Task<(int read, Stream stream, string fileName, object tag)> _preOpenedFile;
        Task<int> _latestReadBytesCount;

        public AsyncMultiFileReader(int bufferSize, Func<string, object, Stream> asyncFileStreamFactory = null)
        {
            _asyncFileStreamFactory = asyncFileStreamFactory
                ?? ((fileName, _) => new FileStream(fileName, FileMode.Open, FileAccess.Read, FileShare.ReadWrite, _buffer1.Length, true));
            _buffer1 = new byte[bufferSize];
            _buffer2 = new byte[bufferSize];
            CurrentBuffer = _buffer1;
        }

        (int read, Stream stream, string fileName, object tag) OpenAndRead(string fileName, object tag, byte[] buffer)
        {
            Stream f = null;
            try
            {
                f = _asyncFileStreamFactory(fileName, tag);
                return (f.Read(buffer, 0, buffer.Length), f, fileName, tag);
            }
            catch
            {
                f?.Dispose();
                throw;
            }
        }

        public int ReadNextBlock()
        {
            if (_stream == null) return 0;

            int read = _latestReadBytesCount.Result;

            if (_stream.Position < _stream.Length)
            {
                _latestReadBytesCount = _stream.ReadAsync(_buffer2, 0, _buffer2.Length);
            }
            else
            {
                _latestReadBytesCount = null;
                // in threadpool thread we shouldn't block for another threadpool task
                if ((NextFile.Name != null) && !Thread.CurrentThread.IsThreadPoolThread)
                {
                    var fn2 = NextFile;
                    var b2 = _buffer2;
                    _preOpenedFile = Task.Run(() => OpenAndRead(fn2.Name, fn2.Tag, b2));
                }

                _stream.Dispose();
                _stream = null;
            }

            CurrentBuffer = _buffer1;

            var swap = _buffer2;
            _buffer2 = _buffer1;
            _buffer1 = swap;

            return read;
        }

        /// <summary>
        /// 
        /// </summary>
        /// <exception cref="AggregateException"></exception>
        /// <exception cref="IOException"></exception>
        public void OpenNextFile()
        {
            if (_stream != null)
            {
                _stream.Dispose();
                _stream = null;
            }

            CurrentFile = (null, null);
            _latestReadBytesCount = null;

            try
            {

                int read;
                string opened;
                object openedTag;
                var nextFile = NextFile;

                (read, _stream, opened, openedTag) = _preOpenedFile?.Result ?? OpenAndRead(nextFile.Name, nextFile.Tag, _buffer1);

                if ((opened != nextFile.Name) || (openedTag != nextFile.Tag))
                {
                    _stream.Dispose();
                    (read, _stream, _, _) = OpenAndRead(nextFile.Name, nextFile.Tag, _buffer1);
                }

                CurrentFile = nextFile;;
                _latestReadBytesCount = Task.FromResult(read);
            }
            finally
            {
                NextFile = (null, null);
                _preOpenedFile = null;
            }
        }

        public void Dispose()
        {
            _stream?.Dispose();
            try
            {
                _preOpenedFile?.Result.stream.Dispose();
            }
            catch
            {
            }
        }
    }
}