Builder Pattern

Contact.cs

public class Contact
{
    public Contact(string firstname, string surname, string address)
    {
        this.Firstname = firstname;
        this.Surname = surname;
        this.Address = address;
    }
    public string Firstname { get; private set; }
    public string Surname { get; private set; }
    public string Address { get; private set; }
}

ContactBuilder.cs

public class ContactBuilder
{
    private string _firstname = "Default Firstname";
    private string _surname = "Default Surname";
    private string _address = "Default address";

    public ContactBuilder WithFirstname(string firstname)
    {
        this._firstname = firstname;
        return this;
    }

    public ContactBuilder WithSurname(string surname)
    {
        this._surname = surname;
        return this;
    }

    public ContactBuilder WithAddress(string address)
    {
        this._address = address;
        return this;
    }

    public Contact Build()
    {
        return new Contact(_firstname,_surname,_address);
    }
}

SomeTests.cs

public class SomeTests
{
    [Fact]
    public void TestShouldStuff()
    {
        var newContactWithAllPropertiesSet = new ContactBuilder()
                            .WithFirstname("Andrew")
                            .WithSurname("Stewart")
                            .WithAddress("My Nice House")
                            .Build();

        var newContactWithOnePropertySetTheRestDefaults = new ContactBuilder()
                            .WithFirstname("Andrew")
                            .Build();
    }
}