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;
             }
         }