C# : The null object pattern and the poor man's Maybe Monad

Lots of people have re-invented the Maybe monad in C#, as a simple search will show, usually as a way of bypassing null checking if statements and to be able to write code like

	return Music.GetCompany("4ad.com")
	.IfNotNull(company => company.GetBand("Pixies"))
	.IfNotNull(band => band.GetMember("David"))
	.IfNotNull(member => member.Role);

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But we have the tools given to us already if we observe that null has no type -- as we can see when

	var x = new string[1]; // default null value
	var n = x.OfType<string>().Count();
	Console.WriteLine("Length = {0}", n);

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reports a length of zero!

So our Some<T> is just a length-1 IEnumerable<T>; and None<T> an empty one -- the null object pattern, in fact.

For the price of having to specify types at each stage -- as we would anyway have had to in the past for declaring intermediate the values to test against null -- we can use existing methods to finesse the null check. Returning to the borrowed example, it would be

	return new[] { Music.GetCompany("4ad.com") }
	.OfType<RecordCompany>().Select(company => company.GetBand("Pixies"))
	.OfType<Band>().Select(band => band.GetMember("David"))
	.OfType<Member>().Select(member => member.Role);

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The monadic return monad extraction operation is just FirstOrDefault(); which is fine for reference types. Value types are different in any case as we wouldn't be null checking those -- as the later steps in a chain though, stopping at the filtering to the value type and making an Any test may be preferred.

Looking at value types, switching to this example, the code looks like:

	public static IEnumerable<int> Div(this int numerator, int denominator)
	{
	//return denominator == 0
	// ? Enumerable.Empty<int>()
	// : new[] { numerator/denominator };
	if (denominator != 0) yield return numerator/denominator;
	}
	public static IEnumerable<int> DoSomeDivision(int denominator)
	{
	return from a in 12.Div(denominator)
	from b in a.Div(2)
	select b;
	}
	...
	var result =
	from a in new[] { "Hello World!" }
	from b in DoSomeDivision(2)
	from c in new[] { new DateTime(2010, 1, 14) }
	select (a + " " + b.ToString() + " " + c.ToShortDateString());
	Console.WriteLine(result.Any() ? result.First() : "Nothing");

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which leaks a little bit in that we have to do the monadic return monad extraction by hand at the end, but otherwise behaves exactly as the explicit monad implementation.