LRUCacheReaderWriterLock.cs

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)
    {
        _rw.EnterReadLock();
        
        var keyNotExists = !_records.ContainsKey(key);
        if (keyNotExists) return null;
        
        _rw.ExitReadLock();

        _rw.EnterWriteLock();

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

        _rw.ExitWriteLock();

        try
        {
            _rw.EnterReadLock();
            return _records[key].CacheValue;
        }
        finally
        {
            _rw.ExitReadLock();
        }
    }

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