BufferedEnumerator.cs

using System;
using System.Collections;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Threading.Tasks;

namespace CoreTechs.Common
{
    public class BufferedEnumerable<T> : IEnumerable<T>
    {
        private readonly IEnumerable<T> _source;
        private readonly int _bufferSize;

        public BufferedEnumerable(IEnumerable<T> source, int bufferSize = 2)
        {
            if (bufferSize < 2)
                throw new ArgumentOutOfRangeException("bufferSize", "bufferSize must not be less than 2");

            _source = source;
            _bufferSize = bufferSize;
        }

        public IEnumerator<T> GetEnumerator()
        {
            using (var buffer = new BlockingCollection<T>(_bufferSize))
            using (var producer = Task.Run(() =>
            {
                try
                {
                    foreach (var item in _source)
                        buffer.Add(item);
                }
                finally
                {
                    buffer.CompleteAdding();
                }

            }))
            {
                foreach (var item in buffer.GetConsumingEnumerable())
                    yield return item;

                // wait to expose exceptions
                producer.Wait();
            }
        }

        IEnumerator IEnumerable.GetEnumerator()
        {
            return GetEnumerator();
        }
    }
}