How to use it to detect files arriving in a directory, as well as files that are already there.

using System;
using System.Collections;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Diagnostics.Contracts;
using System.IO;
using System.Threading;

namespace Fws.Collections
{
    /// <summary>
    /// Detects the arrival of files in a directory and makes them available to a client class
    /// as an IEnumerable of fully pathed file names. Unlike the .NET FileSystemWatcher, this
    /// class yields files that exist when the object is constructed. Also, it is not an IDisposable.
    /// </summary>
    /// <remarks>
    /// <para>
    /// If a file arrives during the execution of this class's constructor, it may be reported more than
    /// once. Also, some programs write their files in such a way that the underlying FileSystemWatcher
    /// will fire a Create event more than once. In those cases, this class will yield the
    /// file multiple times.
    /// </para><para>
    /// Client code must account for these possibilities. It is envisioned that wrapping classes may
    /// refine the yielded files by waiting for them to quiesce, filtering out duplicates, etc.
    /// </para>
    /// <para>
    /// This class is thread-safe: more than one thread may enumerate the files presented by a
    /// single instance of this class, and each thread will get all the files.
    /// </para>
    /// </remarks>
    public sealed class CreatedFileCollection : IEnumerable<string>
    {
        #region Fields
        readonly string _directory;
        readonly string _filePattern;
        readonly CancellationToken _cancellationToken;
        #endregion

        #region Nested Class to Collect Results
        /// <summary>
        /// A queue of files found within one GetEnumerator call.
        /// </summary>
        private sealed class CreatedFileQueue : IDisposable
        {
            readonly ConcurrentQueue<string> _queue = new ConcurrentQueue<string>();
            readonly SemaphoreSlim _fileEnqueued = new SemaphoreSlim(0);

            /// <summary>
            /// Attempt to get a file from the queue.
            /// </summary>
            /// <param name="fileName">The name of the file, if one is immediately available.</param>
            /// <returns>True if got a file; false if not.</returns>
            public bool TryDequeue(out string fileName, CancellationToken cancellationToken)
            {
                fileName = null;
                // Avoid the OperationCanceledException if we can.
                if (cancellationToken.IsCancellationRequested)
                    return false;
                try
                {
                    _fileEnqueued.Wait(cancellationToken);
                    return _queue.TryDequeue(out fileName);
                }
                catch (OperationCanceledException)
                {
                    return false;
                }
            }

            /// <summary>
            /// Handles the Created event of the enclosing class's FileSystemWatcher.
            /// </summary>
            /// <param name="sender">This object.</param>
            /// <param name="e">Args for the new file.</param>
            public void FileCreated(object sender, FileSystemEventArgs e)
            {
                _queue.Enqueue(e.FullPath);
                _fileEnqueued.Release();
            }

            public void Dispose()
            {
                _fileEnqueued.Dispose();
            }
        }
        #endregion

        #region Constructor
        /// <summary>
        /// Constructor.
        /// </summary>
        /// <param name="cancellationToken">This class will terminate the enumeration of
        /// files when and only when the token enters the canceled state.</param>
        /// <param name="directory">The directory to watch.</param>
        /// <param name="filePattern">A pattern to match in the file name. Example: "*.txt".
        /// Null means all files.</param>
        /// <remarks>Duplicates may be returned on the queue. See remarks for the class.</remarks>
        public CreatedFileCollection(CancellationToken cancellationToken, string directory, string filePattern = null)
        {
            Contract.Requires(directory != null);
            Contract.Requires(cancellationToken != null);

            if (!Directory.Exists(directory))
                throw new ArgumentException(String.Format("Directory '{0}' does not exist.", directory));

            _directory = directory;
            _filePattern = filePattern ?? "*";
            _cancellationToken = cancellationToken;
        }
        #endregion

        #region Methods
        /// <summary>
        /// Get an enumerator that will yield files until the CanellationToken is canceled.
        /// </summary>
        /// <returns>Fully pathed file names.</returns>
        /// <remarks>
        /// It is possible for a file name to be returned from more than once.
        /// </remarks>
        public IEnumerator<string> GetEnumerator()
        {
            if (!_cancellationToken.IsCancellationRequested)
            {
                using (var watcher = new FileSystemWatcher(_directory, _filePattern))
                {
                    using (var queue = new CreatedFileQueue())
                    {
                        // Restrict the NotifyFilter to all that's necessary for Create events.
                        // This minimizes the likelihood that FileSystemWatcher's buffer will be overwhelmed.
                        watcher.NotifyFilter = NotifyFilters.FileName;

                        watcher.Created += queue.FileCreated;

                        watcher.EnableRaisingEvents = true;
                        // Note that if a file arrives during the following loop, it will be placed on the queue
                        // twice: once when the Create event is raised, and once by the loop itself.
                        foreach (var file in Directory.GetFiles(_directory, _filePattern, SearchOption.TopDirectoryOnly))
                        {
                            queue.FileCreated(this, new FileSystemEventArgs(WatcherChangeTypes.Created, _directory, Path.GetFileName(file)));
                        }

                        if (!_cancellationToken.IsCancellationRequested)
                        {
                            string fileName;
                            while (queue.TryDequeue(out fileName, _cancellationToken))
                                yield return fileName;
                        }
                    }
                }
            }
        }

        /// <summary>
        /// Required method for IEnumerable.
        /// </summary>
        /// <returns>The generic enumerator, but as a non-generic version.</returns>
        IEnumerator IEnumerable.GetEnumerator()
        {
            return GetEnumerator();
        }
        #endregion
    }
}