BDD-style specifications using NUnit

BDD.cs

using System;
using NUnit.Framework;

namespace Bdd
{
	[TestFixture]
	public abstract class Specification
	{
		protected Exception CaughtException = new ThereWasNoExceptionButOneWasExpectedException();

		public virtual void Given()
		{
		}

		public abstract void When();

		[SetUp]
		public void Init()
		{
			Given();
			try
			{
				When();
			}
			catch (Exception exception)
			{
				CaughtException = exception;
			}
		}
	}

	public class ThenAttribute : TestAttribute
	{
	}

	public class ThereWasNoExceptionButOneWasExpectedException : Exception
	{
		public ThereWasNoExceptionButOneWasExpectedException()
			: base("There was no exception but one was expected")
		{
		}
	}
}

CalculatorSpecs.cs

public class Calculator
{
	public int Add(int a, int b)
	{
		return a + b;
	}
}

public class CalculatorTests
{
	public class When_adding_two_numbers : Specification
	{
		protected Calculator Calculator;
		protected int Sum;

		public override void Given()
		{
			Calculator = new Calculator();
		}

		public override void When()
		{
			Sum = Calculator.Add(2, 3);
		}

		[Then]
		public void It_should_sum_the_numbers_correctly()
		{
			Assert.That(Sum, Is.EqualTo(5));
		}

		/// <remarks>
		/// Just demonstrating multiple tests per scenario
		/// </remarks>
		[Then]
		public void It_should_be_positive()
		{
			Assert.That(Sum, Is.GreaterThan(0));
		}
	}
}