using System;

namespace LongCacheTest
{
    public class Program
    {
        public static void Main(string[] args)
        {
            var cacher = new Cacher.Cacher<string>();

            cacher.Add("first", SimulateSlowLoadingContentRetrieval);
            cacher.Add("second", SimulateSlowLoadingContentRetrieval);

            Console.ReadLine();
        }

        private static string SimulateSlowLoadingContentRetrieval(string key)
        {
            var end = DateTime.Now.AddSeconds(10);

            while (DateTime.Now < end) ;

            return $"content:{key}";
        }
    }
}

using System;
using System.Runtime.Caching;
using System.Threading.Tasks;

namespace Cacher
{
    public class Cacher<T>
    {
        public delegate T CacheEntryGet(string key);
        private MemoryCache Cache { get; set; } = new MemoryCache("long");

        private CacheItemPolicy ImmediatePolicy => new CacheItemPolicy
        {
            AbsoluteExpiration = new DateTimeOffset(DateTime.Now.AddSeconds(3)),
            Priority = CacheItemPriority.Default,
            RemovedCallback = DidRemove
        };

        private CacheItemPolicy RegularPolicy => new CacheItemPolicy
        {
            AbsoluteExpiration = new DateTimeOffset(DateTime.Now.AddSeconds(20)),
            Priority = CacheItemPriority.Default,
            RemovedCallback = DidRemove
        };

        private CacheItemPolicy NoRemovePolicy => new CacheItemPolicy
        {
            Priority = CacheItemPriority.NotRemovable
        };

        private async Task GetFreshContent(CacheEntryRemovedArguments args)
        {
            await Task.Run(() =>
            {
                Cache.Set(args.CacheItem, NoRemovePolicy);

                var item = args.CacheItem.Value as Cacheable<T>;

                item.Obj = item.Getter(args.CacheItem.Key);

                var policy = RegularPolicy;

                Cache.Set(args.CacheItem, policy);
            });
        }

        private void DidRemove(CacheEntryRemovedArguments args)
        {
            GetFreshContent(args);
        }

        public void Add(string key, CacheEntryGet getter)
        {
            var item = new Cacheable<T>
            {
                Getter = getter
            };

            Cache.Set(key, item, ImmediatePolicy);
        }

        public T Get(string key) => (Cache[key] as Cacheable<T>).Obj;
    }
}