Memoizer.cs

public class MyCalculator
{
    private Memoizer<MyCalculator> _memoizer;

    public MyCalculator()
    {
        _memoizer = new Memoizer(this);
    }

    private string MyNotOptomizedMethod(string arg1, string arg2, int arg3)
    {
        //calculating...
    }

    public string MyMethod(string arg1, string arg2, int arg3) =>
        _memoizer.Memo<string>(nameof(MyNotOptomizedMethod), arg1, arg2, arg3);
}

public class Memoizer<TState>
    where TState: class, new()
{
    private TState _state;

    private Type _stateType;

    private Dictionary<(string methodName, object[] args), object> _cache = new Dictionary<(string methodName, object[] args), object>();

    public Memoizer(TState state)
    {
        _state = state;
        _stateType = state.getType();
    }

    public TResult Memo<TResult>(string methodName, params object[] args)
    {
        if (_cache.TryGetValue((methodName, args), out var result)
        {
            return (TResult)result;
        }

        var result = _stateType.GetMethod(methodName).Invoke(_state, args);

        _cache[(methodName, args)] = result;

        return result;
    }
}