關於ConcurrentQueue–一個隊列緩衝的設計

using System;using System.Collections.Generic;using System.Linq;using System.Web;using System.Collections.Concurrent;namespace PDAJob.PDAService.Service{    public class CacheList<T>    {        private int MaxSize = 10000;        private int ShrinkLen = 2000;        private ConcurrentQueue<CacheItem<T>> _Queue = new ConcurrentQueue<CacheItem<T>>();        public CacheList(int maxSize,int shrinkLen)        {            this.MaxSize = maxSize;            this.ShrinkLen = shrinkLen;        }        public void Add(T item)        {            #region 超過則收縮            if (_Queue.Count >= MaxSize)            {                lock (this)                {                    if (_Queue.Count >= MaxSize)                    {                        CacheItem<T> outItem;                        for (int i = 0; i < ShrinkLen; i++)                        {                            _Queue.TryDequeue(out outItem);                        }                    }                }            }            #endregion            _Queue.Enqueue(new CacheItem<T>() { Data = item, AddTime = DateTime.Now });        }        public ConcurrentQueue<CacheItem<T>> Queue        {            get            {                return _Queue;            }        }    }    /// <summary>    /// 緩衝內容    /// </summary>    /// <typeparam name="T"></typeparam>    public class CacheItem<T>    {        public DateTime AddTime { get; set; }        public T Data { get; set; }    }}

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在多線程環境下可能多個Thread讀到Queue.Count>MaxSize，雖然Queue是Thread安全的，但是上面的代碼可造成_Queue被重複移空，所以要加lock