[TestClass]
public class when_foo_is_invoked : ContextSpecification {

   protected FooService    Service;
   protected string        Result;
   
   //"arrange"
   protected override void Context() {
       var repository = new Mock<IFooRepository>();
       .. mock setup code goes here ..
       Service = new FooService(repository);
   }

   //"act"
   protected override void Because() {
       Result = Service.Foo();
   }

   //"assert"
   [TestMethod]
   public void should_return_foo() {
       Assert.AreEqual(expected: "Foo", actual: Result);
   }
}