How can I modify a queue collection in a loop?

Question

I have a scenario where I need to remove an item for the queue as soon as been processed. I understand I cannot remove an item from a collection whilst in loop but was wondering if something could be done with the Enumerator etc...

This is just a basic example throwing an error "Collection was modified after the enumerator was instantiated."

Any suggestions? Thanks a lot!!!

Code is as follows:

     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

Change your foreach to:

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

Edit:

To reprocess if save fails do something like:

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);
    }
}

Edit:

John K mentions thread safety which is a valid concern if you have multiple threads accessing the same Queue. See http://ccutilities.codeplex.com/SourceControl/changeset/view/40529#678487 for a ThreadSafeQueue class that covers simple thread safety issues.

---

Edit: Here's the thread safety example I keep pointing everyone to :-)

Here's an example of the thread safety issues mentioned. As shown the default Queue can "miss" items while still decreasing the count.

Updated: To better represent the problem. I never add a null item to the Queue but the standard Queue.Dequeue() returns several null values. This alone would be fine but in doing so a valid item is removed from the internal collection and the Count decreases. It's a safe assumption, in this specific example, that every null item returned from a Queue.Dequeue() operation represents a valid item that was never processed.

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
        }

    }
}