Okay, so I'm having a carstarter installed in my car and obviously have nothing better to do than keep tinkering with this DSL stuff.

So I'm thinking... If we create a DSL for the person writing the code, then the debug experience should also be DSL-like. Now I'm not about to write my own debugger, but instead of showing a plain DateTime (what kind of datatype is that anyway?!?!?) we're talking animals, persons and real 'talk' date of birth stuff here.

So I create a special DateOfBirth class which overrides the ToString() method and this creates a 'pretty' experience in my debug window. Notice the values for DateOfBirth. This could ofcourse be extended to person, which is still displayed as the same old school type information: 'DSL.Person'.

The complete code comes at the end of this post, but lets have a look at te code complexity when going from

Person p = new Person();
p.DateOfBirth = new DateTime(1972,9,25);

to our DSL

I guess the code is not too bad. Visual Studio still scores it green on the maintainability scale, but it is clear that quite a large amount of code is needed to create a DSL. And DSL's like this get more context sensitive. Imagine if I would also like to support birds with my DSL? Birds get hatched not born. I'd have to expand my extensions class, or maybe create a whole new set of classes and methods, having a class 'HatchedOn'.

Anyway, they just called, my car is done.

Here is the code for the final sample:

namespace DSL
{
    class Program
    {
        static void Main(string[] args)
        {
            Person mark = Person.Named("Mark").BornOn(Day.the25th).Of(Month.September).InTheYear(1972);
            Animal watson = Animal.Named("Watson").BornOn(Day.the2nd).Of(Month.January).InTheYear(2001).AndItIsA(Animal.KindOf.Dog);
        }
    }

    public class DateOfBirth
    {
        public int Year;
        public Month Month;
        public Day Day;

        public static implicit operator DateTime(DateOfBirth dob)
        {
            return new DateTime(dob.Year, (int)dob.Month, (int)dob.Year);
        }

        public override string ToString()
        {
            return "Born on " + Day.ToString() + " of " + Month.ToString() + " in the year " + Year.ToString() + ".";
        }
    }

    public enum Day : int
    {
        the1st = 1,
        the2nd = 2,
        the25th = 25
    }

    public enum Month : int
    {
        January = 1,
        February = 2,
        September = 9
    }

    public interface IDateOfBirth
    {
        DateOfBirth DateOfBirth { get; set; }
    }

    public class Person : IDateOfBirth
    {
        public DateOfBirth DateOfBirth { get; set; }
        public string Name { get; set; }

        public static Person Named(string name)
        {
            Person p = new Person(name);
            p.DateOfBirth = new DateOfBirth();
            return p;
        }

        protected Person(string name)
        {
            Name = name;
        }
    }

    public class Animal : IDateOfBirth
    {
        public enum KindOf
        {
            Dog,
            Cat
        }

        public DateOfBirth DateOfBirth { get; set; }
        public string Name { get; set; }
        public KindOf KindOfAnimal { get; set; }

        public static Animal Named(string name)
        {
            Animal animal = new Animal();
            animal.Name = name;
            animal.DateOfBirth = new DateOfBirth();
            return animal;
        }
        public Animal AndItIsA(KindOf kind)
        {
            KindOfAnimal = kind;
            return this;
        }
    }

    public static class DateOfBirthExtensions
    {
        public static T BornOn<T>(this T p, Day day) where T : IDateOfBirth
        {
            p.DateOfBirth.Day = day;
            return p;
        }

        public static T Of<T>(this T p, Month month)  where T : IDateOfBirth
        {
            p.DateOfBirth.Month = month;
            return p;
        }

        public static T InTheYear<T>(this T p, int year)  where T : IDateOfBirth
        {
            p.DateOfBirth.Year = year;
            return p;
        }
    }

}