Easy immutable object derivation

gistfile1.cs

public class Immutable {

    public class Person {

        public readonly string Firstname;
        public readonly string Lastname;

        public Person(string firstname, string lastname) {
            Firstname = firstname;
            Lastname = lastname;
        }

        public Person With(string firstname = null, string lastname = null) {
            return new Person(firstname ?? this.Firstname, lastname ?? this.Lastname);
        }
    }

    public void Example() {
        var dad = new Person(
            firstname: "Joe",
            lastname: "Smith"
        );
        var son = dad.With(firstname: "Joe Jr.");
    }
}

Interesting, but sad you have to.