Road to Dynamic LINQ

Vedran Maršić AKA Fosna

Warning: Expression Trees Ahead

About me Vedran Maršić AKA Fosna

dev at http://www.axilis.com/author/vmarsic/

organizing programming workshops for kids Logo, Scratch, HTML, CSS, JavaScript, Python, C++

We’ll cover Part 1

What is expression tree in C# How is it different from similar looking lambda delegates How to build expression trees How to parse expression trees Where are you already using them

Part 2 Check out few expression tree exploits Point to brilliant, not easy to understand, ideas or projects from some cool

developer maniacs

We won’t cover Implementing LINQ providers Implementing deep copy Implementing faster static dictionary Implementing IoC container

Introduction to Expression Trees

Expression Tree NodesPlain old C#• c = a + b

Lambda expression• (a, b) => a + b

Gists: AddStatement.linqSimpleBinaryExpression.linq

Definition represent code in a tree-like data structure

not in MSIL (Microsoft Intermediate Language) each node is an expression

has compiler smell all over it can compile to get functions!

get some good ol’ MSIL MSIL ex:

IL_0007: ldloc.0 // a IL_0008: ldloc.1 // b IL_0009: add IL_000A: stloc.2 // c

History Came with .NET 3.5 Built some muscle in .NET 4.0

‘nuff said

Expression Tree Dissection

x => x < 5

Gist: SimpleLogicalExpression.linq

Why Expression Tree?

analyze expression treediscover what you want done

decide how to do it

Expression Trees Vs Lambda DelegatesOne lambda expression

(a, b) => a + b

Expression Trees Vs Lambda DelegatesOne lambda expression

(a, b) => a + b

Two type signatures

Expression Trees Vs Lambda Delegates

...Gist: TreeVsDelegate.linq

Applied Expression Trees Vs Lambda Delegates Which technology exploits expression trees and which one

doesn’t? LINQ to Objects LINQ to Entities (EF) LINQ to XML Parallel LINQ

How to Create an Expression Tree? From lambda expression at compile time

Method invocation flavor

Query syntax Using Expression API at run time

Why would anyone do that?

Gist: BuildExp.linq

Famous Extension Methods...attached to Clever Interfaces

Lazy: AsEnumerable()

If code can be executed in process then the job can be done with the simple type called IEnumerable<T>

remember yield return

Lazy: AsQueryable()

If you need to translate a query expression into a string that will be passed to another process, then use IQueryable<T> and expression trees. 

Eager Extension Methods• ToList()• ToArray()• ToDictionary()• ToLookup()

• First()• FirstOrDefault()• Single()• SingleOrDefault()

Applied Expression TreesNext time...

Thanks. Bye!No questions, right?

Resources Start with

http://stackoverflow.com/a/403233/953338 Expression Tree Basics, Charlie Calvert 31 Jan 2008 3:49 AM

http://blogs.msdn.com/b/charlie/archive/2008/01/31/expression-tree-basics.aspx

Published by Olexander Ivanitskyi on Tuesday, June 11, 2013 LINQ: Func<T> vs. Expression<Func<T>> http://ivanitskyi.blogspot.hr/2013/06/linq-func-vs-expression.html

 asked Apr 27 '09 at 13:50, Richard Nagle Why would you use Expression<Func<T>> rather than Func<T>? http://

stackoverflow.com/questions/793571/why-would-you-use-expressionfunct-rather-than-funct

Resources C# 6.0 Features

https://github.com/dotnet/roslyn/wiki/New-Language-Features-in-C%23-6 Canonical Functions

https://msdn.microsoft.com/en-us/library/vstudio/bb738626(v=vs.100).aspx

The .NET Programmer’s Playground - LinqPad http://www.linqpad.net/