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Domain Specific Language using C# 4.0 - Part 2

Domain Specific Language using C# 4.0 - Part 2

Sunday, 26 December 2010 23:52 | Author: M Sheik Uduman Ali | Print

In the first part of this article, we have seen the domain model and its usage for examination question paper as Level 0 in our step by step making of a DSL.

Level 1: Static Class and Method for Instantiation

Let us pick up the Exam constructor.

var dotnetExam = new Exam(".NET Fundamentals", Level.Beginner);

On what way we can make this as domain-friendly.

How the question preparer can know the first parameter is title of the exam, if he has provided with NotePad only? Forgot about API documentation et al. He is not a developer
Is it ok to have "var" and "new" keywords in this DSL?

To answer the first question, let us try to use C# "Named Argument" feature.

var dotExam = new Exam(title: ".NET Fundamentals", level: Level.Beginner);

Alternatively, when instantiating the Exam class, we can use the C# object initializer feature, if your domain allow this. When I'm designing the domain model for exam, I put constraints that Title and Level properties should be specified during instantiation. After that, these are only "get". And also, I hate to design domain objects with default constructor when business requires some constraints. For this case, If we relax this constraint, we can refactor the Exam as

//Exam.cs
public string Title { get;  set; }
public Level Level { get; set; }
public List Questions { get; private set; }
 
public Exam()
{
        Question.QNo = 1;
        Questions = new List();
}

and the instantiation would be

var dotnetExam = new Exam { Title = ".NET Fundamentals", Level = Level.Beginner };

If we relax the rule of Exam object instantiation, we can use C# optional parameter in the constructor as

//Exam.cs
public Exam(string title="Untitled", Level level = Level.Beginner)
{
        Title = title;
        Level = level;
        Question.QNo = 1;
        Questions = new List();
}

and the instantiation would be

var dotnetExam = new Exam(title: ".NET Fundamentals");

Since, the one I'm preparing here is for Beginners, I left the default option for Level.

Almost, we have answered the first question. However, how are we going to answer the second question. Avoid using "new", "var" C# keywords. A static factory method would be the option for this. Where can we create that method? In Exam class itself or on a separate static class. Exam.create(...) wouldn't fit into the DSL. So, let us create a static class ExamBuilder.

//ExamBuilder.cs
public static class ExamBuilder
{
        public static Exam exam(string title = "Untitled", Level level = Level.Beginner)
        {
                return new Exam() { Title = title, Level = level };
        }
}

You may wonder, why the method name violate the .NET method naming convention. Yes, I'm violated, after all, I'm going to create this DSL for a domain user, not for a .NET developer. Readability matters in DSL. See the usage of this:

var dotnetExam = ExamBuilder.exam(title: ".NET Fundamentals");

Still, "var" creates some noises in the DSL. But as of now, it is a required devil. Otherwise, how can we call Exam's AddQuestion() method.

But, Why do we require intermediatery variable "dotnetExam" here, because as a DSL, the user will add questions immediately after the "exam()" method. So, we can enforce:


ExamBuilder.exam(".NET Fundamentals").addQuestion(...);

But, how can we pass "question" objects to its "AddOption" method to add options and how can we create second, third, forth...n number of questions.

CodeProject

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