memoi.cs

public class CourseTests
    {
        [Fact]
        public void PlayIncreaseViewers()
        {
            Func<int, int> doubleX = x => x*2;
            var memoized = doubleX.Memoize();
            var result = memoized(4);
            var obj = memoized(1);
            Assert.Equal(result, obj);
        }
    }

    public static class MemoizeExtension
    {
        public static Func<A, R> Memoize<A, R>(this Func<A, R> f)
        {
            var cache = new ConcurrentDictionary<A, R>();
            return a => cache.GetOrAdd(a, f);
        }
    }