创建microsoft.extensions.caching.memory作为单顿类

Question

Im trying to use Microsoft.Extensions.Caching.Memory as singleton class in Asp.net MVC 5.

Problem is with each call the Instance Variable returns null and class reinstantiating it

self.(old value of Cache will删除）

这是我的缓存类：

public class CacheManagerHelper<TItem>
    {

  private static CacheManagerHelper<TItem> instance;
        private static MemoryCache memoryCache;
        private static object syncRoot = new();
        private CacheManagerHelper() { }


        public static CacheManagerHelper<TItem> Instance
        {
            get
            {
                        if (instance == null)
                        {

                            instance = new CacheManagerHelper<TItem>();
                            memoryCache = new MemoryCache(new MemoryCacheOptions()
                            {
                            });
                        }

                return instance;
            }
        }


        
        private static ConcurrentDictionary<object, SemaphoreSlim> _locks = new ConcurrentDictionary<object, SemaphoreSlim>();

        public async Task<TItem> GetOrCreate(object key, Func<Task<TItem>> createItem)
        {
            TItem cacheEntry;

            if (!memoryCache.TryGetValue(key, out cacheEntry))
            {
                SemaphoreSlim mylock = _locks.GetOrAdd(key, k => new SemaphoreSlim(1, 1));

                await mylock.WaitAsync();
                try
                {
                    if (!memoryCache.TryGetValue(key, out cacheEntry))
                    {
                        cacheEntry = await createItem();
                        var cacheEntryOptions = new MemoryCacheEntryOptions()
                   .SetPriority(Microsoft.Extensions.Caching.Memory.CacheItemPriority.NeverRemove)
                   .SetSlidingExpiration(TimeSpan.FromHours(24))
                   .SetAbsoluteExpiration(TimeSpan.FromHours(48));


                        memoryCache.Set(key, cacheEntry);
                    }
                }
                finally
                {
                    mylock.Release();
                }
            }
            return cacheEntry;
        }

        public T Get<T>(string key)
        {
            return (T)Convert.ChangeType(memoryCache.Get(key), typeof(T));
        }

        public void RemoveCache(string key)
        {
            memoryCache.Remove(key);
        }

        public static void ClearAllCacheObject()
        {
            IDictionaryEnumerator enumerator = HttpContext.Current.Cache.GetEnumerator();

            while (enumerator.MoveNext())
                HttpContext.Current.Cache.Remove(enumerator.Key.ToString());

        }
}