C# OptionType

option.cs

using System;

namespace Options {

	public abstract class Option<T>
	{
		public class Some : Option<T>
		{
			public T Value { get; private set; }
			public Some(T value)
			{
				Value = value;
			}
			public bool HasValue { get { return Value != null; }}
		}

		public class None : Option<T>
		{
			public bool HasValue { get { return false; } }
		}

		public TOut Match<TOut>(Func<T, TOut> valueProjection, Func<TOut> alternativeProjection)
		{
			if (this is Some)
			{
				var t1 = (Some)this;
				return valueProjection(t1.Value);
			}
			else if (this is None)
			{
				None t2 = (None)this;
				return alternativeProjection();
			}
			else throw new ArgumentException();
		}
	}

	public static class OptionExtensions
	{
		public static Option<B> Bind<A, B>(this Option<A> a, Func<A, Option<B>> func)
		{
			return a is Option<A>.Some
				? func(((Option<A>.Some) a).Value)
				: new Option<B>.None();
		}

		public static Option<C> SelectMany<A, B, C>(this Option<A> a, Func<A, Option<B>> func, Func<A, B, C> select)
		{
			return a.Bind(aval => func(aval).Bind(bval => select(aval, bval).ToOption()));
		}

		public static Option<T> ToOption<T>(this T value) { return new Option<T>.Some(value); } 
	}

}