This is a specification for a basic calculator with the implementation rounded out.

BasicCalculatorSpecification.cs

using System;
using Microsoft.VisualStudio.TestTools.UnitTesting;
using MSTestContrib.Specifications;

namespace Calculator.Core.Specifications
{
    [TestClass]
    [SpecificationDescription("As a user I want to perform mathematical calculations so that my head doesn't hurt.")]
    public class BasicCalculatorSpecification : Specification
    {
        public BasicCalculator Calculator { get; private set; }

        [TestMethod]
        [ScenarioDescription("Add two to a calculator with zero on the accumulator.")]
        public void AddTwoToACalculatorWithZeroOnTheAccumulator()
        {
            Given("a calculator with zero on the accumulator", CalculatorWithZeroOnTheAccumulator)
                .When("I add two to the accumulator", x => this.Calculator.Add(2))
                .Then("the accumulator should show two", x => this.Calculator.Accumulator == 2);
        }

        private void CalculatorWithZeroOnTheAccumulator(SpecificationContext context)
        {
            Calculator = new BasicCalculator();
        }
    }
}