using System;
using System.Runtime.Caching;

namespace MemoryCacheHelpSpace
{
    public static class MemoryCacheHelp
    {
        private static Object m_objLocker = new object();

        public static T GetCacheItem<T>(String strKey,
            Func<T> cachePopulate,
            TimeSpan
 
? slidingExpiration = null,
            DateTime? absoluteExpiration = null)
        {
            if (string.IsNullOrWhiteSpace(strKey)) throw new ArgumentException("無效鍵");
            if (cachePopulate == null) throw new ArgumentException("委託不能為null");
            if (slidingExpiration == null && absoluteExpiration == null
 
) throw new ArgumentException("間隔或時間至少有一個");
            if (MemoryCache.Default[strKey] == null)
            {
                lock (m_objLocker)
                {
                    var item = new CacheItem(strKey, cachePopulate());
                    var policy = CreatePolicy(slidingExpiration, absoluteExpiration);
                    MemoryCache.Default.Add(item, policy);
                }
            }
            
 
return (T)MemoryCache.Default[strKey];
        }

        public static void CacheRemove(string strKey)
        {
            if (string.IsNullOrWhiteSpace(strKey)) throw new ArgumentException("無效鍵");
            if (MemoryCache.Default[strKey] != null)
            {
                lock (m_objLocker)
                {
                    MemoryCache.Default.Remove(strKey);
                }
            }
        }

        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;
        }
    }
}