1. Exploring C# DSLs:
LINQ, Fluent Interfaces and
Expression Trees
Rasmus Kromann-Larsen

2. Me
• Freelance .NET Developer
• Core Group Member
– Copenhagen .NET User Group
– Aarhus .NET User Group
• JetBrains Academy Expert

3. Agenda
• Domain Specific Languages
• Static Reflection
• Code

4. Domain Specific Languages
• Language targeted at a specific problem
• Goals
– Expressiveness / Productivity
– Declarative over Imperative
– Communication
• External vs Internal

5. SELECT ID, LastName
FROM Employees e
WHERE EXISTS (
SELECT *
FROM Orders o
WHERE e.ID = o.EmployeeID
AND OrderDate = '9/5/11')

6. (?:(?:rn)?[ t])*(?:(?:(?:[^()<>@,;:".[] 000-
031]+(?:(?:(?:rn)?[ t]
)+|Z|(?=[["()<>@,;:".[]]))|"(?:[^"r]|.|(?:(?:rn)?[
t]))*"(?:(?: rn)?[ t])*)(?:.(?:(?:rn)?[
t])*(?:[^()<>@,;:".[] 000-031]+(?:(?:(
?:rn)?[t])+|Z|(?=[["()<>@,;:".[]]))|"(?:[^"r]|.|(?
:(?:rn)?[ t]))*"(?:(?:rn)?[ t])*))*@(?:(?:rn)?[
t])*(?:[^()<>@,;:".[] 000-
... snip ...
"()<>@,;:".[]]))|"(?:[^"r]|.|(?:(?:rn)?[
t]))*"(?:(?:rn)?[ t]) *))*@(?:(?:rn)?[
t])*(?:[^()<>@,;:".[] 000-031]+(?:(?:(?:rn)?[ t])
+|Z|(?=[["()<>@,;:".[]]))|[([^[]r]|.)*](?:(?:rn)
?[ t])*)(?: .(?:(?:rn)?[ t])*(?:[^()<>@,;:".[] 000-
031]+(?:(?:(?:rn)?[ t])+|Z
|(?=[["()<>@,;:".[]]))|[([^[]r]|.)*](?:(?:rn)?[
t])*))*>(?:( ?:rn)?[ t])*))*)?;s*)

7. Talk Focus
• Internal C# DSLs
– Less ramp-up
– Automatic tooling support
– Emergent in many new libraries
• Developer benefits > non-dev communication

8. [Subject("Making a customer preferred")]
public class when_a_regular_customer_is_made_preferred
{
static Customer _customer;
Establish context = () =>
_customer = new Customer();
Because of = () =>
_customer.MakePreferred();
It should_mark_the_customer_as_preferred = () =>
_customer.IsPreferred.ShouldBeTrue();
}

9. people.Where(x => x.Age < 50)
from x in people
where x.Age < 50
select x

10. LINQ
• Most(?) successful C# DSL
• Depending on C# 3.0 features
– Anonymous Types
– Extension Methods
– Expression Trees
• Great DSL building blocks

11. LINQ
people.Where(x => x.Age < 50)
from x in people
where x.Age < 50
select x

12. Expression Trees

13. Expression Trees
• Extracting AST-like structure from lambda
expressions
• Expression<TDelegate>

14. Static Reflection
• Statically typed reference to code element
• No magic strings
• Tool support / Refactorability
• typeof(Person).GetProperty(”Name”);
vs
• GetProperty<Person>(x => x.Name);

15. GetProperty<Person>(x => x.Name)

16. GetPropertyName
public string GetPropertyName(Expression body)
{
// Unwrap boxing
if (body is UnaryExpression)
{
body = ((UnaryExpression) body).Operand;
}
var memberExpression = body as MemberExpression;
if (memberExpression == null)
{
throw new Exception("Descriptive exception message.");
}
return memberExpression.Member.Name;
}

17. GetProperty<Person>(x => x.Friend.Name)

18. Object Printing

19. Recap
• We use DSLs almost every day
• Expression Trees are not (that) dangerous
• It doesn’t have to be hard to write a DSL!

20. More Information?
• Domain Specific Languages by Martin Fowler
• Open Source Projects
– Fubu
– Most IoC containers
– AutoMapper
– AutoFixture
– Fluent Nhibernate
... and many more.

21. Questions?
Rasmus Kromann-Larsen
@rasmuskl
rasmus@kromann-larsen.dk
www.rasmuskl.dk