MotherFactory pattern

Domain.cs

public class PostCode
{
    public PostCode() {}
    public PostCode(string postcode) {}
}

public class Address
{
    public Address(string street, string city, PostCode postCode) {}
}

MotherFactory.cs

// An anchor for extension methods.
public abstract class MotherFactory {}

public static class AddressMotherFactory
{
    public static Address Address(
        this MotherFactory a,
        string street = "",
        string city = "",
        PostCode postCode = null)
    {
        postCode = postCode ?? a.PostCode();
        return new Address(street, city, postCode);
    }
}

public static class PostCodeMotherFactory
{
    public static PostCode PostCode(this MotherFactory a, string postCode = "NL-1000")
    {
        return new PostCode(postCode);
    }

    public static PostCode EmptyPostCode(this MotherFactory a)
    {
        return new PostCode();
    }
}

UsageInTests.cs

public class AddressTests
{
    public static MotherFactory an;

    [Fact]
    public void Should_not_fail()
    {
        var address = an.Address(postCode: an.EmptyPostCode());
    }
}