Testing DSLs

custom-assertions.cs

int even = 2, odd = 3;
// meh
Assert.That(even % 2, Is.EqualTo(0));
// slightly better (message)
Assert.That(odd % 2, Is.Not.EqualTo(0), "Number must be odd");
// better
Assert.That(odd, new EvenContraint());
// best
Assert.That(even, Must.Be.Even());

////////////////////////////////////////

public class EvenContraint : NUnit.Framework.Constraints.Constraint
{
	public override string Description => "an even number";

	public override ConstraintResult ApplyTo<TActual>(TActual actual)
	{
		return new Result(this, actual, isEven(Convert.ToInt32(actual)));
	}
	
	private static bool isEven(int number)
	{
		bool even = number % 2 == 0;
		return even;
	}

	class Result : ConstraintResult
	{
		private readonly object _actual;
		
		public Result(NUnit.Framework.Constraints.IConstraint constraint, object actual, bool success) :
			base(constraint, actual, success)
		{
			_actual = actual;
		}

		public override void WriteActualValueTo(MessageWriter writer)
		{
			writer.Write("odd (");
			writer.WriteValue(_actual);
			writer.Write(")");
		}
	}
}
// using Testing.Commons for Must entry points
public static class MustExtensions
{
	public static EvenContraint Even(this Must.BeEntryPoint entry)
	{
		return new EvenContraint();
	}
}