giving clarity to linq queries by extending expressions
TRANSCRIPT
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