基于MemoryCache的缓存辅助类

背景：

　　1. 什么是MemoryCache？

　　　　memoryCache就是用电脑内存做缓存处理

　　2.使用范围？

　　　　可用于不常变的数据，进行保存在内存中，提高处理效率

代码：

        /// <summary>
        /// 基于MemoryCache的缓存辅助类
        /// </summary>
        public static class MemoryCacheHelper
        {
            private static readonly Object _locker = new object();

            public static T GetCacheItem<T>(String key, Func<T> cachePopulate, TimeSpan? slidingExpiration = null, DateTime? absoluteExpiration = null)
            {
                if (String.IsNullOrWhiteSpace(key)) throw new ArgumentException("Invalid cache key");
                if (cachePopulate == null) throw new ArgumentNullException("cachePopulate");
                if (slidingExpiration == null && absoluteExpiration == null) throw new ArgumentException("Either a sliding expiration or absolute must be provided");

                if (MemoryCache.Default[key] == null)
                {
                    lock (_locker)
                    {
                        if (MemoryCache.Default[key] == null)
                        {
                            var item = new CacheItem(key, cachePopulate());
                            var policy = CreatePolicy(slidingExpiration, absoluteExpiration);

                            MemoryCache.Default.Add(item, policy);
                        }
                    }
                }

                return (T)MemoryCache.Default[key];
            }

            private static CacheItemPolicy CreatePolicy(TimeSpan? slidingExpiration, DateTime? absoluteExpiration)
            {
                var policy = new CacheItemPolicy();

                if (absoluteExpiration.HasValue)
                {
                    policy.AbsoluteExpiration = absoluteExpiration.Value;
                }
                else if (slidingExpiration.HasValue)
                {
                    policy.SlidingExpiration = slidingExpiration.Value;
                }

                policy.Priority = CacheItemPriority.Default;

                return policy;
            }
        }