ModifiedFluentPerson.cs

public static class PersonFactory
    {
        public static FirstNameExpression CreatePerson()
        {
            return new FirstNameExpression(new Person());
        }
    }

    public class FirstNameExpression
    {
        private Person _person;
        public FirstNameExpression(Person person)
        {
            _person = person;
        }

        public Person WithFirstName(string first)
        {
            _person.FirstName = first;
            return _person;
        }
    }

ModifiedPerson.cs

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

        public Person WithMiddleName(string middle)
        {
            this.MiddleName = middle;
            return this;
        }

        public Person WithLastName(string last)
        {
            this.LastName = last;
            return this;
        }

        public override string ToString()
        {
            return string.Format("{0} {1} {2}", FirstName, MiddleName, LastName);
        }
    }