关于ConcurrentQueue--一个队列缓存的设计

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 超过则收缩
            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>
    /// 缓存内容
    /// </summary>
    /// <typeparam name="T"></typeparam>
    public class CacheItem<T>
    {
        public DateTime AddTime { get; set; }
        public T Data { get; set; }

    }
}

在多线程环境下可能多个Thread读到Queue.Count>MaxSize，虽然Queue是Thread安全的，但是上面的代码可造成_Queue被重复移空，所以要加lock