Código de ejemplo para el post http://blog.koalite.com/2014/01/aplicando-el-patron-builder-para-escribir-tests-unitarios

Builders.cs

public static class Build
{
	public static OrderBuilder Order(Cutomer customer)
	{
		return new OrderBuilder(customer);
	}

	public static CustomerBuilder Customer(string name)
	{
		return new CustomerBuilder(name);
	}
	
	// TODO: Builders para product
}

public static class CustomerBuilder
{
	private readonly Customer customer;

	public CustomerBuilder(string name)
	{
		var address = new Address("C/ Molino, 12", "Alcorcón", "Madrid", "28056");
		customer = new Customer(Guid.New(), name, address);
	}

	public CustomerBuilder Preferred()
	{
		customer.MakePreferred();
	}

	public static implicit operator Customer(CustomerBuilder builder)
	{
		return builder.customer;
	}
}

public static class OrderBuilder
{
	private readonly Order order;

	public OrderBuilder(Customer customer)
	{
		order = new Order(1000, customer, DateTime.Today);
	}

	public OrderBuilder AddOne(string productName, decimal price)
	{
		Product product = Build.Product(productName);
		order.AddLine(product, 1, price);
		return this;
	}

	public static implicit operator Order(OrderBuilder builder)
	{
		return builder.order;
	}
}

Model.cs

public class Address
{
	public Address(string street, string city, string region, string zipCode)
	{
		Check.Require(!string.IsNullOrWhiteSpace(street), "Street cannot be empty");
		Check.Require(!string.IsNullOrWhiteSpace(city), "City cannot be empty");
		Check.Require(!string.IsNullOrWhiteSpace(region), "Region cannot be empty");
		Check.Require(!Regex.Matches(zipCode ?? "", @"\d{5}"), "ZipCode must be a valid Spanish ZipCode");

		Street = street;
		City = city;
		Region = region;
		ZipCode = zipCode;
	}

	public readonly string Street;
	public readonly string City;
	public readonly string Region;
	public readonly string ZipCode;

	// TODO: Override Equals & GetHashCode
}

public class Customer
{
	private readonly Guid code;
	private string name;
	private Address address;
	private bool isPreferred;

	public Customer(Guid code, string name, Address address)
	{
		Check.Require(code != null, "Customer code cannot be null");
		Check.Require(!string.IsNullOrWhiteSpace(name), "Customer name cannot be empty");
		Check.Require(address != null, "Customer must have a valid address");

		this.code = code;
		this.name = name;
		this.address = address;
	}

	public void MakePreferred() 
	{
		isPreferred = true;
	}

	// Etc...
}

public class Product
{
	private readonly string reference
	private string name;

	public Product(string reference, string name)
	{
		Check.Require(!string.IsNullOrWhiteSpace(referece), "Reference cannot be empty");
		Check.Require(!string.IsNullOrWhiteSpace(name), "Name cannot be empty");

		this.reference = referece;
		this.name = name;
	}

}


public class Order
{
	private readonly int number;
	private readonly DateTime date;
	private readonly Customer customer;
	private IList<OrderLine> lines;

	public Order(int number, Customer customer, DateTime date)
	{
		Check.Require(number >= 0, "Order number must be greater or equal than zero");
		Check.Require(customer != null, "Order must have a valid customer");

		this.number = number;
		this.customer = customer;
		this.date = date;
		this.lines = new List<OrderLine>();
	}

	public void AddLine(Product product, decimal quantity, decimal price)
	{
		Check.Require(product != null, "Cannot add order line without product");

		var orderLine = new OrderLine(product.Reference, product.Name, quantity, price);
		lines.Add(orderLine);
	}

	// Etc...

}


public class OrderLine
{
	// TODO
}