LRUCacheReaderWriterLock.cs

using System.Collections.Generic;
using System.Threading;

namespace Blog.LRUCacheThreadSafe
{
    public class LRUCacheReaderWriterLock<T>
    {
        private readonly Dictionary<int, LRUCacheItem<T>> _records = new Dictionary<int, LRUCacheItem<T>>();
        private readonly LinkedList<int> _freq = new LinkedList<int>();
        private readonly ReaderWriterLockSlim _rw = new ReaderWriterLockSlim();
        private readonly int _capacity;


        public LRUCacheReaderWriterLock(int capacity)
        {
            _capacity = capacity;
        }

        public int Capacity => _capacity;

        public object Get(int key)
        {
            try
            {
                _rw.EnterUpgradeableReadLock();
                var keyNotExists = !_records.ContainsKey(key);
                
                if (keyNotExists) return null;

                _rw.EnterWriteLock();

                _freq.Remove(key);
                _freq.AddLast(key);

                _rw.ExitWriteLock();

                return _records[key].CacheValue;
            }
            finally
            {
                _rw.ExitUpgradeableReadLock();
            }
        }

        public void Set(int key, T val)
        {
          ...
        }
    }
}