An example of the Builder Pattern, this file can be run in LinqPad. It is designed with a fluent interface, meaning that the methods return the builder and can be chained together. Objects can be created of different state to be used in unit test.

BuilderPattern.linq

void Main()
{
	Console.WriteLine(new PersonBuilder().WithValidDetails().Build());
	Console.WriteLine(new PersonBuilder().WithValidDetails().WithValidAddress().Build());
}

public class Person{
	public string FirstName {get;set;} 
	public string LastName {get;set;}
	public Address Address{get;set;}
}

public class Address{
	public string Address1{get;set;}
	public string PostCode{get;set;}
	public string Country{get;set;}
}

public class PersonBuilder
{
	private Person _person = new Person();

	public Person Build(){
		// Set standard default values for properties here if needed
		return _person;
	}

	public PersonBuilder WithValidDetails()
	{
		_person.FirstName="Joe";
		_person.LastName="Bloggs";
        return this;
    }

    public PersonBuilder WithValidAddress()
    {
		_person.Address=new Address{
			Address1="1 The Grove",
			PostCode="PembroVille",
			Country="United Kingdom"
		};
        return this;
    }
	
	 public PersonBuilder WithInValidAddress()
    {
		_person.Address=null;
        return this;
    }
}