λGiving Clarity to LINQ Queries

by Extending Expressions

Ed CharbeneauDeveloper Advocate, TelerikCode PaLOUsa Co-ChairmanAuthor: TDN & Simple-TalkPodcast: Eat Sleep Code the Official Telerik PodcastTwitter: @EdCharbeneau

Are

Do you use

expressions?

Common uses

Entity Framewo

rkLINQ HTML

Helpers

Expression<Func<TModel, TResult>>

@Html.LabelFor(obj => obj.Prop)

Expression<Func<TModel, bool>>

People.Where(p => p.Title == “Developer”)

Expressions in C#

• Representation of code as data• Meta-programming– Analyze, rewrite, and translate code at

runtime

Expression Factory

+

11

Add

constant constant

SimpleBinaryExpression

Homo-iconicity

Same syntax for executable code as data representation

Compile

Runtime Modification

ExpressionVisitor• used to traverse or rewrite

expression trees• Abstract class– Inherit and override

• .Visit(expression)– Recursively walks the tree– Returns an Expression

https://msdn.microsoft.com/en-us/library/bb882521(v=vs.90).aspx

Are

How is this

useful?

Pipeline

Pipes and filters

Filter

Rule

Rule

Data

Data

Data

Consumer

Results

Database Filter

Rule

Rule

dbContext.Where(rule).Where(rule)

Maintainability & Readability

Refactoring

What’s

going on

here?

Let’s start simple

What’s

going on

here?

Distinction

The tale of two extension methods

IEnumerable.Where(q => q.Value == true)Func (delegate)Func<T, bool>

IQueryable.Where(q => q.Value == true)Expression

Expression<Func<T, bool>

LINQ Where Chaining

Custom Filters

(Expression<Func<TSource, bool>> predicate)

ParameterMethod

.Where<TSource>

Extends type

IQueryable<TSource>

Return type

IQueryable<TSource>

Generic type

Extends type

IQueryable<MyType>

Return type

IQueryable<MyType>

Method

.CustomMethodName()

public static IQueryable<Post> ArePublished(this IQueryable<Post> posts){ return posts.Where(post => post.IsPublished);}

Readability?

Before After

What’s the intent

?

Now what?

What’s

the intent

?

Lambda as an Expression tree

post => post.PostedOn >= cutoffDate

>=

cutoffDatePostedOn

GreaterThanOrEqual

=>

post

Lambda

parameter

parameter constant

Body

Combine Like Expressions?

>=

=>

post

Body

==

=>

post

Body

AND / OR?

Combined Expressions

=>

post

Lambda

parameter

Body

>=

&&

==

Readability?

Before

After

New Requirements!

Now we must support multiple featured authors

Dynamic LINQ

query?

No problem, now we’re equipped for it!

Dynamic Queries

Try this without expressions.

AreQuestion

s?

Resources

• Article– https://goo.gl/kAM05P

• GitHub– https://

github.com/EdCharbeneau/PredicateExtensions

• Nuget– https://

www.nuget.org/packages?q=predicate+extensions

• Twitter– @EdCharbeneau