c# - 如何在循环中修改队列集合？

Question

我有一个场景，我需要在处理完后立即从队列中删除一个项目。 我知道我不能在循环中从集合中删除一个项目，但想知道是否有什么 可以用枚举器等来完成...

这只是一个抛出错误的基本示例 “在实例化枚举器后修改了集合。”

有什么建议吗？非常感谢!!!

代码如下:

     class Program
        {
            static void Main()
            {

                Queue<Order> queueList = GetQueueList();

                foreach (Order orderItem in queueList)
                {
                    Save(orderItem);
                    Console.WriteLine("Id :{0} Name {1} ", orderItem.Id, orderItem.Name);
                    queueList.Dequeue();
                }
                Console.Read();

            }

            private static void Save(Order orderItem)
            {
               //we are pretending to save or do something.
            }

            private static Queue<Order>GetQueueList()
            {
                Queue<Order> orderQueue = new Queue<Order>();
                orderQueue.Enqueue(new Order { Id = 1, Name = "Order 1" });
                orderQueue.Enqueue(new Order { Id = 1, Name = "Order 2" });
                orderQueue.Enqueue(new Order { Id = 2, Name = "Order 3" });
                orderQueue.Enqueue(new Order { Id = 3, Name = "Order 4" });
                orderQueue.Enqueue(new Order { Id = 4, Name = "Order 5" });
                return orderQueue;
            }
        }

        public  class Order
        {
            public int Id { get; set; }
            public string Name { get; set; }
        }

Answer 1

将您的 foreach 更改为:

while (queueList.Count > 0)
{
    Order orderItem = queueList.Dequeue();
    Save(orderItem);
    Console.WriteLine("Id :{0} Name {1} ", orderItem.Id, orderItem.Name);
}

编辑:

要在保存失败时重新处理，请执行以下操作:

while (queueList.Count > 0)
{
    Order orderItem = queueList.Dequeue();

    if (!Save(orderItem))
    {
        queueList.Enqueue(orderItem); // Reprocess the failed save, probably want more logic to prevent infinite loop
    }
    else
    {
        Console.WriteLine("Successfully saved: {0} Name {1} ", orderItem.Id, orderItem.Name);
    }
}

编辑:

John K 提到了线程安全，如果您有多个线程访问同一个 Queue，这是一个值得关注的问题。参见 http://ccutilities.codeplex.com/SourceControl/changeset/view/40529#678487对于涵盖简单线程安全问题的 ThreadSafeQueue 类。

---

编辑:这是我一直向大家指出的线程安全示例:-)

下面是提到的线程安全问题的一个例子。如图所示，默认的 Queue 可以“错过”项目，同时仍然减少计数。

更新:为了更好地展示问题。我从不向 Queue 添加 null 项，但标准 Queue.Dequeue() 会返回多个 null 值。这本身就很好，但这样做会从内部集合中删除一个有效项目，并且 Count 会减少。在此特定示例中，可以安全地假设从 Queue.Dequeue() 操作返回的每个 null 项都代表一个从未处理过的有效项。

using System;
using System.Collections.Generic;
using System.Threading;

namespace SO_ThreadSafeQueue
{
    class Program
    {
        static int _QueueExceptions;
        static int _QueueNull;
        static int _QueueProcessed;

        static int _ThreadSafeQueueExceptions;
        static int _ThreadSafeQueueNull;
        static int _ThreadSafeQueueProcessed;

        static readonly Queue<Guid?> _Queue = new Queue<Guid?>();
        static readonly ThreadSafeQueue<Guid?> _ThreadSafeQueue = new ThreadSafeQueue<Guid?>();
        static readonly Random _Random = new Random();

        const int Expected = 10000000;

        static void Main()
        {
            Console.Clear();
            Console.SetCursorPosition(0, 0);
            Console.WriteLine("Creating queues...");

            for (int i = 0; i < Expected; i++)
            {
                Guid guid = Guid.NewGuid();
                _Queue.Enqueue(guid);
                _ThreadSafeQueue.Enqueue(guid);
            }

            Console.SetCursorPosition(0, 0);
            Console.WriteLine("Processing queues...");

            for (int i = 0; i < 100; i++)
            {
                ThreadPool.QueueUserWorkItem(ProcessQueue);
                ThreadPool.QueueUserWorkItem(ProcessThreadSafeQueue);
            }

            int progress = 0;

            while (_Queue.Count > 0 || _ThreadSafeQueue.Count > 0)
            {
                Console.SetCursorPosition(0, 0);

                switch (progress)
                {
                    case 0:
                        {
                            Console.WriteLine("Processing queues... |");
                            progress = 1;
                            break;
                        }
                    case 1:
                        {
                            Console.WriteLine("Processing queues... /");
                            progress = 2;
                            break;
                        }
                    case 2:
                        {
                            Console.WriteLine("Processing queues... -");
                            progress = 3;
                            break;
                        }
                    case 3:
                        {
                            Console.WriteLine("Processing queues... \\");
                            progress = 0;
                            break;
                        }
                }

                Thread.Sleep(200);
            }

            Console.SetCursorPosition(0, 0);
            Console.WriteLine("Finished processing queues...");
            Console.WriteLine("\r\nQueue Count:           {0} Processed: {1, " + Expected.ToString().Length + "} Exceptions: {2,4} Null: {3}", _Queue.Count, _QueueProcessed, _QueueExceptions, _QueueNull);
            Console.WriteLine("ThreadSafeQueue Count: {0} Processed: {1, " + Expected.ToString().Length + "} Exceptions: {2,4} Null: {3}", _ThreadSafeQueue.Count, _ThreadSafeQueueProcessed, _ThreadSafeQueueExceptions, _ThreadSafeQueueNull);

            Console.WriteLine("\r\nPress any key...");
            Console.ReadKey();
        }

        static void ProcessQueue(object nothing)
        {
            while (_Queue.Count > 0)
            {
                Guid? currentItem = null;

                try
                {
                    currentItem = _Queue.Dequeue();
                }
                catch (Exception)
                {
                    Interlocked.Increment(ref _QueueExceptions);
                }

                if (currentItem != null)
                {
                    Interlocked.Increment(ref _QueueProcessed);
                }
                else
                {
                    Interlocked.Increment(ref _QueueNull);
                }

                Thread.Sleep(_Random.Next(1, 10)); // Simulate different workload times
            }
        }

        static void ProcessThreadSafeQueue(object nothing)
        {
            while (_ThreadSafeQueue.Count > 0)
            {
                Guid? currentItem = null;

                try
                {
                    currentItem = _ThreadSafeQueue.Dequeue();
                }
                catch (Exception)
                {
                    Interlocked.Increment(ref _ThreadSafeQueueExceptions);
                }

                if (currentItem != null)
                {
                    Interlocked.Increment(ref _ThreadSafeQueueProcessed);
                }
                else
                {
                    Interlocked.Increment(ref _ThreadSafeQueueNull);
                }

                Thread.Sleep(_Random.Next(1, 10)); // Simulate different workload times
            }
        }

        /// <summary>
        /// Represents a thread safe <see cref="Queue{T}"/>
        /// </summary>
        /// <typeparam name="T"></typeparam>
        public class ThreadSafeQueue<T> : Queue<T>
        {
            #region Private Fields
            private readonly object _LockObject = new object();
            #endregion

            #region Public Properties
            /// <summary>
            /// Gets the number of elements contained in the <see cref="ThreadSafeQueue{T}"/>
            /// </summary>
            public new int Count
            {
                get
                {
                    int returnValue;

                    lock (_LockObject)
                    {
                        returnValue = base.Count;
                    }

                    return returnValue;
                }
            }
            #endregion

            #region Public Methods
            /// <summary>
            /// Removes all objects from the <see cref="ThreadSafeQueue{T}"/>
            /// </summary>
            public new void Clear()
            {
                lock (_LockObject)
                {
                    base.Clear();
                }
            }

            /// <summary>
            /// Removes and returns the object at the beggining of the <see cref="ThreadSafeQueue{T}"/>
            /// </summary>
            /// <returns></returns>
            public new T Dequeue()
            {
                T returnValue;

                lock (_LockObject)
                {
                    returnValue = base.Dequeue();
                }

                return returnValue;
            }

            /// <summary>
            /// Adds an object to the end of the <see cref="ThreadSafeQueue{T}"/>
            /// </summary>
            /// <param name="item">The object to add to the <see cref="ThreadSafeQueue{T}"/></param>
            public new void Enqueue(T item)
            {
                lock (_LockObject)
                {
                    base.Enqueue(item);
                }
            }

            /// <summary>
            /// Set the capacity to the actual number of elements in the <see cref="ThreadSafeQueue{T}"/>, if that number is less than 90 percent of current capactity.
            /// </summary>
            public new void TrimExcess()
            {
                lock (_LockObject)
                {
                    base.TrimExcess();
                }
            }
            #endregion
        }

    }
}