如何在循环中修改队列集合？

Question

我有一个场景,我需要在处理后尽快删除队列中的项目.我知道我不能在循环中从一个集合中删除一个项目但是想知道是否可以使用枚举器等完成某些事情......

这只是一个基本的例子,抛出错误"在实例化枚举数后修改了集合".

有什么建议？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

将你的foreach改为:

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

编辑:

要重新处理如果保存失败,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,如果你有多个线程访问它,这是一个有效的问题Queue.有关涵盖简单线程安全问题的类,请参见http://ccutilities.codeplex.com/SourceControl/changeset/view/40529#678487ThreadSafeQueue.

编辑:这是我一直指着每个人的线程安全示例:-)

这是一个提到的线程安全问题的例子.如图所示,默认Queue可以"遗漏"项目,同时仍然减少计数.

更新:更好地代表问题.我从不添加空项,Queue但标准Queue.Dequeue()返回几个空值.仅此一项就可以了,但这样做会从内部集合中删除有效项目并Count减少.在这个具体示例中,这是一个安全的假设,即null从Queue.Dequeue()操作返回的每个项目都代表一个从未处理过的有效项目.

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
        }

    }
}