Option Monad example for C#

Option.cs

using System;
namespace Functional {
	public class GetOnEmptyOptionException : Exception { }
	public interface Some { }
	public interface None { }
	public abstract class Option {
		public static bool operator ==(Option a, Option b) =>
			!ReferenceEquals(a, null) && !ReferenceEquals(b, null) && a.Equals(b);
		public static bool operator !=(Option a, Option b) => !(a == b);
		public static Option<A> None<A>() => new None<A>();
		public static Option<A> Some<A>(A a) => new Some<A>(a);
	}
	public static class OptionOps {
		public static Option<B> map<A, B>(this Option<A> oa, Func<A, B> f) => bind(oa, a => Option.Some(f(a)));
		public static Option<B> bind<A, B>(this Option<A> oa, Func<A, Option<B>> f) {
			if (oa.isEmpty) return Option.None<B>();
			else return f(oa.get);
		}
	}
	public abstract class Option<A> : Option {
		public A get { get; }
		public bool isDefined { get; }
		public bool isEmpty { get { return !isDefined; } }
		public Option() { isDefined = false; }
		public Option(A a) { this.get = a; isDefined = true; }
		public override string ToString() {
			if (isEmpty) return "None";
			else return $"Some({get.ToString()})";
		}
		public override bool Equals(object obj) {
			var that = (Option<A>)obj;
			return
				(this.isEmpty && that.isEmpty) ||
				(this.isDefined && that.isDefined && this.get.Equals(that.get));
		}
		public override int GetHashCode() {
			return base.GetHashCode();
		}
	}
	public class Some<A> : Option<A>, Some {
		public Some(A a) : base(a) {}
	}
	public class None<A> : Option<A>, None { }
}

Program.cs

using System;
using static Functional.Option;
               
namespace Functional {
	class MainClass {
		public static void Main(string[] args) {
			var o1 = Some("hello");
			var o2 = o1.map(s => s.ToUpper());
			Console.WriteLine(o1);
			Console.WriteLine(o2);
			Console.WriteLine(o2 == Some("HELLO"));
			Console.WriteLine(o1.bind(s => countChars('l', s)));
			Console.WriteLine(o1.bind(s => countChars('r', s)));
			Console.WriteLine(o1.bind(count));
			Console.ReadKey();
		}

		public static Option<int> count(String s) {
			if (s == String.Empty) return None<int>();
			else return Some(s.Length);
		}

		public static Option<int> countChars(Char c, String s) {
			var count = 0;
			foreach (var cc in s)
				if (cc == c) count++;
			if (count == 0)
				return None<int>();
			else 
				return Some(count);
		}
	}
}

zzoutput

Some(hello)
Some(HELLO)
True
Some(2)
None
Some(5)