Giving Clarity to LINQ Queries by Extending Expressions

In this session we’ll learn about .Net Expression trees by discovering how they work and applying the knowledge to LINQ using the pipes and filters pattern. LINQ and Entity Framework are both commonly used in the .Net ecosystem, but even well-written applications can have LINQ queries that are difficult to understand. Because LINQ is so flexible, it can be written in ways that fail to communicate the developer’s intent. Well-written LINQ should be so clear as to be self-documenting. To write clear LINQ, it helps to understand the details of a few LINQ components that improve LINQ’s readability.

We’ll be showing how to use a pipe and filter pattern to make LINQ queries easier to comprehend. We will take a deep dive into expression trees to understand how they work, and how to manipulate them for maximum re-usability.

Transcript

1. Giving Clarity to LINQ Queries by Extending Expressions
2. Are Stirtrek 2018 @EdCharbeneau #LINQ
3. Are Do you use expressions? @EdCharbeneau #LINQ
4. Entity Framework & LINQ Expression< Func <TModel , bool >> People.Where(p => p.Title == “Developer”)
5. Automapper Expression< Func <TSource , object> > AutoMapper.Mapper.CreateMap<Post, BlogPostView>() .ForMember(dest => dest.Heading, opts => opts.MapFrom(src => src.Title));
6. HTML Helpers Expression< Func <TModel , TResult >> @Html.LabelFor(obj => obj.Prop)
7. Telerik Data Grid @(Html.Kendo().Grid<Post> .Columns(columns => { columns.Bound(p => p.Title); columns.Bound(p => p.Author); columns.Bound(p => p.Pubished); }) ) @EdCharbeneau #DataGrid
8. Expressions in C# • Representation of code as data • Meta -programming – Analyze , rewrite , and translate code at runtime
9. Homo- iconicity Same syntax for executable code as data representation 1 //Executable representation 2 Func<int, int> plusOne = n => n + 1; 3 4 //Data representation 5 Expression<Func<int, int>> plusOne = n => n + 1;
10. Homo- iconicity Same syntax for executable code as data representation 1 //Executable representation 2 Func<int, int> plusOne = n => n + 1; 3 4 //Data representation 5 Expression<Func<int, int>> plusOne = n => n + 1;
11. Expression Factory 1 // new Expression() invalid! 2 3 var x = Expression.Constant(1); 1 constant @EdCharbeneau #ExpressionFactory
12. Expression Factory 1 // new Expression() invalid! 2 3 var x = Expression.Constant(1); 4 var y = Expression.Constant(1); 1 constant 1 constant @EdCharbeneau #ExpressionFactory
13. Expression Factory 1 // new Expression() invalid! 2 3 var x = Expression.Constant(1); 4 var y = Expression.Constant(1); 5 var sum = Expression.Add(x,y); + 1 1 Add constant constant @EdCharbeneau #ExpressionFactory
14. Expression Factory 1 // new Expression() invalid! 2 3 var x = Expression.Constant(1); 4 var y = Expression.Constant(1); 5 var sum = Expression.Add(x,y); 6 7 Console.WriteLine(sum); 8 // (1 + 1) 9 10 Console.WriteLine(sum.GetType()); 11 // SimpleBinaryExpression + 1 1 Add constant constant SimpleBinaryExpression @EdCharbeneau #ExpressionFactory
15. Compile 1 Expression<Func<int, int>> plusOne = n => n + 1; 2 Console.WriteLine(plusOne.ToString()); 3 // n => (n + 1)
16. Compile 1 Expression<Func<int, int>> plusOne = n => n + 1; 2 Console.WriteLine(plusOne.ToString()); 3 // n => (n + 1) 4 5 var x = plusOne.Compile(); 6 Console.WriteLine(x(1)); 7 // 2
17. Runtime Modification ExpressionVisitor • used to traverse or rewriteexpression 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
18. Are OMG He ’s showing docs in a presentation @EdCharbeneau #ExpressionVisit
19. Expression Visitor + 1 n Add => post Lambda parameter parameter constant Body
20. Are How is this useful? @EdCharbeneau #ExpressionVisit
21. Pipeline Pipes and filters Filter Rule Rule Data Data Data Consumer Results Database Filter Rule Rule dbContext.Where (rule).Where(rule)
22. Maintainability & Readability
23. Refactoring What ’s going on here? 1 postRepository.GetAll() 2 .Where(post => post.IsPublished && 3 post.PostedOn <= today && 4 (post.PostedOn >= cutoffDate || 5 post.Author == featuredAuthor && 6 post.PostedOn >= featuredAuthorCutoffDate));
24. Let’s start simple What ’s going on here? 1 postRepository.GetAll() 2 .Where(post => post.IsPublished && post.PostedOn <= today);
25. Distinction The tale of two extension methods IEnumerable.Where (q => q.Value == true) Function delegate Func <T, bool > IQueryable.Where (q => q.Value == true) Expression Expression< Func <T, bool >
26. LINQ Where Chaining 1 //Both statements produce the same result 2 3 //Concise 4 var query = query.Where(x => x.Value == 1 && x.Name == ”foo”); 5 6 //Configurable 7 var query = query.Where(x => x.Value == 1) 8 .Where(x => x.Name == ”foo”); SELECT x FROM table WHERE (value = 1 AND name = ‘foo’) @EdCharbeneau #LINQ
27. LINQ Where Chaining 1 var query = query.Where(x => x.Value == 1); SELECT x FROM table WHERE (value = 1) => x == value 1
28. LINQ Where Chaining 1 var query = query.Where(x => x.Value == 1) 2 .Where(x => x.Name == ”foo”); SELECT x FROM table WHERE (value = 1 AND name = ‘foo’) && => x == name foo == value 1
29. LINQ Where Chaining 1 var query = query.Where(x => x.Value == 1); 2 query.Where(x => x.Name == ”foo”); 3 query.Where(x => x.IsDeleted == ”true”); SELECT x FROM table WHERE (value = 1 AND name = ‘foo’ AND IsDeleted = 1) && => x == name foo == value 1 && == isDeleted 1
30. Custom Filters 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 ); }
31. 1 postRepository.GetAll() 2 .Where(post => post.IsPublished && 3 post.PostedOn <= today); Readability? Before After What ’s the intent? 1 postRepository.GetAll() 2 .ArePublished() 3 .PostedOnOrBefore(today);
32. Now what? What ’s the intent? 1 IQueryable<Post> posts = postRepository.GetAll() 2 .Where(post => post.IsPublished && post.PostedOn <= today && 3 (post.PostedOn >= cutoffDate || post.Author == featuredAuthor && 4 post.PostedOn >= featuredAuthorCutoffDate));
33. Lambda as an Expression tree post=>post.PostedOn >= cutoffDate >= cutoffDate PostedOn GreaterThanOrEqual => post Lambda parameter parameter constant Body
34. Lambda as an Expression tree post=>post.PostedOn >= cutoffDate >= cutoffDate PostedOn GreaterThanOrEqual => post Lambda parameter parameter constant Body
35. Lambda as an Expression tree post=>post.PostedOn >=cutoffDate >= cutoffDate PostedOn GreaterThanOrEqual => post Lambda parameter parameter constant Body
36. Combine Like Expressions? >= => post Body == => post Body AND / OR? @EdCharbeneau #PredicateExtensions
37. CombineLambdas >= constant Post.Property => post Body == constant Post.Property => post Body AND / OR? CombineLambdas(left, right, ExpressionType.AndAlso)
38. Get Paramters >= constant Post.Property => post Body == constant Post.Property => post Body SubstituteParameterVisitor(left.Parameters[0], right.Parameters[0])
39. Replace Parameters >= constant Post.Property Body == constant Post.Property post Body post Post.Property SubstitutionVisitor.VisitParameter
40. Combined Expressions => post Lambda parameter Body >= && == Expression.MakeBinary Expression.Lambda<Func<T, bool>>
41. Readability? Before After 1 postRepository.GetAll() 2 .Where(post => post.IsPublished && post.PostedOn <= today && 3 (post.PostedOn >= cutoffDate || post.Author == featuredAuthor && 4 post.PostedOn >= featuredAuthorCutoffDate)); 1 postRepository.GetAll() 2 .ArePublished() 3 .PostedOnOrBefore(today) 4 .Where(PostedOnOrAfter(cutoffDate) 5 .Or(FeaturedAuthorPostedrOnOrAfter(featuredAuthor,featuredAuthorCutoffDate)));
42. New Requirements! Now we must support multiple featured authors Dynamic LINQ query? No problem, now we ’re equipped for it!
43. Dynamic Queries Try this without expressions. 1 var rule = PredicateExtensions.PredicateExtensions.Begin<Post>(); 2 foreach (var authorName in featuredAuthorNames) 3 { 4 rule = rule.Or(FeaturedAuthorPostedOnOrAfter(authorName, featuredAuthorCutoffDate)); 5 } 6 return rule;
44. Dynamic Expression Left Body.NodeType == ExpressionType.Constant Right if (IsExpressionBodyConstant(left)) return (right); >= constant Post.Property => post Body bool true
45. Are Questions? #2018 -- yellow @EdCharbeneau #LINQ
46. Ed Charbeneau Developer Advocate for Progress <Telerik > Author <T> Podcast => “Eat Sleep Code the Official Telerik Podcast ” Twitter .Where (user == @EdCharbeneau )
47. Resources • Article – https://goo.gl/kAM05P • GitHub – https://github.com/EdCharbeneau/PredicateExtensions • Telerik – www.Telerik.com • Twitter – @EdCharbeneau