LRUCache.cs

using System.Collections.Generic;

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

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

        public int Capacity => _capacity;

        public object Get(int key)
        {
            if (!_records.ContainsKey(key)) return null;

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

            return _records[key].CacheValue;
        }

        public void Set(int key, T val)
        {
            if (_records.ContainsKey(key))
            {
                _records[key].CacheValue = val;
                _freq.Remove(key);
                _freq.AddLast(key);
                return;
            }

            if (_records.Count >= _capacity)
            {
                _records.Remove(_freq.First.Value);
                _freq.RemoveFirst();
            }

            _records.Add(key, new LRUCacheItem<T> { CacheKey = key, CacheValue = val });
            _freq.AddLast(key);
        }
    }
}