MaybeMonad.cs

 public abstract class Maybe<T>
    {
        public abstract T Value { get; }

        public virtual string Failure => null;

        public Maybe<V> SelectMany<U, V>(Func<T, Maybe<U>> f, Func<T, U, V> g)
        {
            var that = f(this.Value);

            if (that.Failure != null)
                return new Failed<V>(that.Failure);

            return new Success<V>(g(this.Value, that.Value));
        }

        public sealed class Success<U> : Maybe<U>
        {
            readonly U value;

            public override U Value => value;

            public Success(U value) => this.value = value;
        }

        public sealed class Failed<U> : Maybe<U>
        {
            readonly string failure;

            public override string Failure { get; }

            public override U Value => throw new InvalidOperationException();

            public Failed(string s) => this.failure = s;
        }