Example.cs

using System;

static class TimeKeeper
{
	public static readonly Func<DateTime> DefaultTimeKeeper = () => DateTime.UtcNow;

	private static readonly object LockObject = new object();

	private static Func<DateTime> CurrentTimeKeeper = DefaultTimeKeeper;

	public static DateTime Current
	{
		get
		{
			lock (LockObject)
			{
				return CurrentTimeKeeper();
			}
		}
	}

	public static void ChangeTimeKeeper(Func<DateTime> newTimeKeeper)
	{
		if (newTimeKeeper == null)
		{
			throw new ArgumentNullException("newTimeKeeper");
		}

		lock (LockObject)
		{
			CurrentTimeKeeper = newTimeKeeper;
		}
	}
}

class Address : IEquatable<Address>
{
	private readonly string _city;
	private readonly string _country;
	private readonly string _state;
	private readonly string _zipCode;
	private readonly string _street;

	public Address(string city, string country, string state, string zipCode, string street)
	{
		Validate(city, "city");
		Validate(country, "country");
		Validate(state, "state");
		Validate(zipCode, "zipCode");
		Validate(street, "street");

		_city = city;
		_country = country;
		_state = state;
		_zipCode = zipCode;
		_street = street;
	}

	public string City { get { return _city; } }

	public string Country { get { return _country; } }

	public string State { get { return _state; } }

	public string ZipCode { get { return _zipCode; } }

	public string Street { get { return _street; } }

	public bool Equals(Address other)
	{
		if (other == null)
		{
			return false;
		}

		var result = other.City == City &&
			other.Country == Country &&
			other.State == State &&
			other.ZipCode == ZipCode &&
			other.Street == Street;

		return result;
	}

	private static void Validate(string value, string name)
	{
		if (string.IsNullOrEmpty(value))
		{
			throw new ArgumentNullException(name);
		}
	}
}

class Student
{
	public Student(string firstName, string lastName)
	{
		Rename(firstName, lastName);
		EnrolledOn = TimeKeeper.Current;
	}

	public int Id { get; private set; }

	public string FirstName { get; private set; }

	public string LastName { get; private set; }

	public string CNP { get; private set; }

	public DateTime EnrolledOn { get; private set; }

	public Address Address { get; private set; }

	public void Rename(string firstName, string lastName)
	{
		if (string.IsNullOrEmpty(firstName))
		{
			throw new ArgumentNullException("firstName");
		}

		if (string.IsNullOrEmpty(lastName))
		{
			throw new ArgumentNullException("lastName");
		}

		FirstName = firstName;
		LastName = lastName;
	}

	public void ChangeAddress(Address address)
	{
		if (address == null)
		{
			throw new ArgumentNullException("address");
		}

		Address = address;
	}

	public void ChangeCNP(string cnp)
	{
		if (string.IsNullOrEmpty(cnp))
		{
			throw new ArgumentNullException("cnp");
		}

		CNP = cnp;
	}
}

// NUnit tests
[TestFixture]
class WhenSavingStudents
{
	private DbContext _context;

	[SetUp]
	public void SetUp()
	{
		_context = new DbContext("your connection string");
	}

	[TearDown]
	public void TearDown()
	{
		_context.Dispose();
	}

	[Test]
	public void ShouldGenerateId()
	{
		// Arrange
		var student = new Student("TEST", "TESTER");

		// Act
		_context.Students.Insert(student);
		_context.SaveChanges();

		// Assert
		Assert.AreNotEqual(0, student.Id);
	}

	[Test]
	public void ShouldSaveFirstName()
	{
		// Arrange
		var student = new Student("TEST", "TESTER");

		// Act
		_context.Students.Insert(student);
		_context.SaveChanges();

		var studentFromDatabase = _context.Students.First(x => x.Id == student.Id);

		// Assert
		Assert.AreEqual(student.FirstName, studentFromDatabase.FirstName);
	}

	[Test]
	public void ShouldSaveLastName()
	{
		// Arrange
		var student = new Student("TEST", "TESTER");

		// Act
		_context.Students.Insert(student);
		_context.SaveChanges();

		var studentFromDatabase = _context.Students.First(x => x.Id == student.Id);

		// Assert
		Assert.AreEqual(student.LastName, studentFromDatabase.LastName);
	}

	[Test]
	public void ShouldSaveCNP()
	{
		// Arrange
		var student = new Student("TEST", "TESTER");
		student.ChangeCNP("TEST");

		// Act
		_context.Students.Insert(student);
		_context.SaveChanges();

		var studentFromDatabase = _context.Students.First(x => x.Id == student.Id);

		// Assert
		Assert.AreEqual(student.CNP, studentFromDatabase.CNP);
	}

	[Test]
	public void ShouldSaveEnrolledOn()
	{
		// Arrange
		var student = new Student("TEST", "TESTER");

		// Act
		_context.Students.Insert(student);
		_context.SaveChanges();

		var studentFromDatabase = _context.Students.First(x => x.Id == student.Id);

		// Assert
		Assert.AreEqual(student.EnrolledOn, studentFromDatabase.EnrolledOn);
	}

	[Test]
	public void ShouldSaveAddress()
	{
		// Arrange
		var student = new Student("TEST", "TESTER");

		var address = new Address("A", "B", "C", "D", "E");

		// Act
		_context.Students.Insert(student);
		_context.SaveChanges();

		var studentFromDatabase = _context.Students.First(x => x.Id == student.Id);

		// Assert
		Assert.AreEqual(student.Address, studentFromDatabase.Address);
	}
}