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LINQ

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Chuck Durfee
Chuck Durfee

This document discusses the evolution of LINQ (Language Integrated Query) in .NET from version 1.0 through 3.5, including the introduction of lambda expressions and extension methods. Key concepts covered include LINQ query syntax, Enumerable extension methods like Where, Select, OrderBy, and Joining queries. Examples demonstrate basic LINQ queries on collections and joining data from multiple sources.

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Chuck Durfee, 3 October 2012
LINQ
LINQ
BIG DATA
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Enumerable.Select( Enumerable.OrderBy( Enumerable.Where(names, s => s.Length == 5), s => s ), s => s.ToUpper() ); versus names.Where(s => s.Length == 5) .OrderBy(s => s) .Select(s => s.ToUpper())
LINQ
, my dear Watson
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1.0 delegate bool FilterMethod(string s); private bool IsFiveLetterName(string s) { return s.Length == 5; } public DotNet10Land() { FilterMethod filter = IsFiveLetterName; }
delegate bool FilterMethod(string s); public DotNet20Land() { FilterMethod filter = delegate(string s) { return s.Length == 5; }; } 2.0
C# 3 = .NET 3.5 delegate bool FilterMethod(string s); public DotNet35Land() { string chuck = "chuck"; FilterMethod filter = delegate(string s) { return s != chuck && s.Length == 5; }; }
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Func // delegate bool FilterMethod(string s); public DotNet35Land() { Func<string, bool> filter = delegate(string s) { return s.Length == 5; }; }
Func<string, bool> Func<string, int, DateTime> Func<List<string>, int, bool> but not Func<string, System.Void>
// delegate void ActionMethod(string s); public DotNet35Land() { Action<string> action = delegate(string s) { if (s.Length != 5) throw new ArgumentException("Length != 5"); }; } and Action
LAMBDA EXPRESSIONS Func<string, bool> filter = delegate(string s) {return s.Length == 5;}; var filter = (string s) => {return s.Length == 5;}; var filter = (string s) => s.Length == 5; var filter = (s) => s.Length == 5; var filter = s => s.Length == 5;
by Alonzo Church, 1930s
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string[] names = { "Tom", "Dick", "Harry" }; names.Select((s, i) => (i + 1) + "=" + s); 1=Tom 2=Dick 3=Harry
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int[] numbers = { 3, 5, 7 }; string[] words = { "three", "five", "seven", "ignored" }; IEnumerable<string> zip = numbers.Zip(words, (n, w) => n + "=" + w); 3=three 5=five 7=seven
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IEnumerable<Order> spanishOrders = customers .Where(c => c.Country == "Spain") .SelectMany(c => c.Orders);
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var slowQuery = from c in customers from p in purchases where c.ID == p.CustomerID select c.Name + " bought a " + p.Description; var fastQuery = from c in customers join p in purchases on c.ID equals p.CustomerID select c.Name + " bought a " + p.Description;
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var easyToRead = from c in customers join p in purchases on c.ID equals p.CustomerID select c.Name + " bought a " + p.Description; var harderToRead = customers.Join ( purchases, c => (int?)(c.ID), p => p.CustomerID, (c, p) => ((c.Name + " bought a ") + p.Description) );
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LINQ
public static class EnumerableExtensions { public static string ToCsv<T>( this IEnumerable<T> sequence) { const string delimiter = ", "; return sequence.Aggregate( new StringBuilder(), (sb, s) => sb.Append(s + delimiter), sb => sb.ToString() .TrimEnd(delimiter.ToArray())); } } new[] { 1, 2, 3, 5, 8, 13, 20 } => 1, 2, 3, 5, 8, 13, 20
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LINQ

  • 1. Chuck Durfee, 3 October 2012
  • 8. Enumerable.Select( Enumerable.OrderBy( Enumerable.Where(names, s => s.Length == 5), s => s ), s => s.ToUpper() ); versus names.Where(s => s.Length == 5) .OrderBy(s => s) .Select(s => s.ToUpper())
  • 10. , my dear Watson
  • 12. 1.0 delegate bool FilterMethod(string s); private bool IsFiveLetterName(string s) { return s.Length == 5; } public DotNet10Land() { FilterMethod filter = IsFiveLetterName; }
  • 13. delegate bool FilterMethod(string s); public DotNet20Land() { FilterMethod filter = delegate(string s) { return s.Length == 5; }; } 2.0
  • 14. C# 3 = .NET 3.5 delegate bool FilterMethod(string s); public DotNet35Land() { string chuck = "chuck"; FilterMethod filter = delegate(string s) { return s != chuck && s.Length == 5; }; }
  • 16. Func // delegate bool FilterMethod(string s); public DotNet35Land() { Func<string, bool> filter = delegate(string s) { return s.Length == 5; }; }
  • 17. Func<string, bool> Func<string, int, DateTime> Func<List<string>, int, bool> but not Func<string, System.Void>
  • 18. // delegate void ActionMethod(string s); public DotNet35Land() { Action<string> action = delegate(string s) { if (s.Length != 5) throw new ArgumentException("Length != 5"); }; } and Action
  • 19. LAMBDA EXPRESSIONS Func<string, bool> filter = delegate(string s) {return s.Length == 5;}; var filter = (string s) => {return s.Length == 5;}; var filter = (string s) => s.Length == 5; var filter = (s) => s.Length == 5; var filter = s => s.Length == 5;
  • 32. string[] names = { "Tom", "Dick", "Harry" }; names.Select((s, i) => (i + 1) + "=" + s); 1=Tom 2=Dick 3=Harry
  • 34. int[] numbers = { 3, 5, 7 }; string[] words = { "three", "five", "seven", "ignored" }; IEnumerable<string> zip = numbers.Zip(words, (n, w) => n + "=" + w); 3=three 5=five 7=seven
  • 36. IEnumerable<Order> spanishOrders = customers .Where(c => c.Country == "Spain") .SelectMany(c => c.Orders);
  • 38. var slowQuery = from c in customers from p in purchases where c.ID == p.CustomerID select c.Name + " bought a " + p.Description; var fastQuery = from c in customers join p in purchases on c.ID equals p.CustomerID select c.Name + " bought a " + p.Description;
  • 40. var easyToRead = from c in customers join p in purchases on c.ID equals p.CustomerID select c.Name + " bought a " + p.Description; var harderToRead = customers.Join ( purchases, c => (int?)(c.ID), p => p.CustomerID, (c, p) => ((c.Name + " bought a ") + p.Description) );
  • 43. public static class EnumerableExtensions { public static string ToCsv<T>( this IEnumerable<T> sequence) { const string delimiter = ", "; return sequence.Aggregate( new StringBuilder(), (sb, s) => sb.Append(s + delimiter), sb => sb.ToString() .TrimEnd(delimiter.ToArray())); } } new[] { 1, 2, 3, 5, 8, 13, 20 } => 1, 2, 3, 5, 8, 13, 20

Editor's Notes

  • #3: Lets talk about the history of LINQ
  • #4: C# 2 introduced generics, at that point, C# is feature complete from an OO perspective
  • #5: Next challenge to tackle after OO features is Big Data, how to reduce complexity of integrating information
  • #6: Have to juggle translation between database constructs and objects
  • #7: Easy way to query data source Extension methods, query comprehension syntax Way to return arbitrary results Anonymous types, var keyword Way to pass input data Closures, collection and object initializers
  • #11: May be too complicated to know at design time, but known at compile time. For more flexibility, you need dynamics and the DLR in .NET 4.
  • #12: For the last piece of the puzzle, lets talk about variable binding
  • #13: http://www.paintingsoncanvas.net/print-111552-4043120/stone-age-man-making-weapon-giclee-print/
  • #15: http://www.travelwalls.net/wallpapers/2012/08/Walt-Disney-Concert-Hall-Modern-City-Los-Angeles-California-United-States-300x420.jpg
  • #17: Dont need to declare a delegate
  • #18: System.Void isnt a true Type Image: http://www.efunda.com/math/bessel/images/BesselJPlot.gif
  • #19: Need a separate generic delegate for void return methods Image: http://portcityfilmfestival.com/clap-board.jpg
  • #20: Distill down to simpler forms, all equivalent Image: http://watermarked.cutcaster.com/cutcaster-photo-100783484-Lambda-symbol-in-glass-3d.jpg
  • #21: The term lambdas comes from lambda calculus. Instead of set-based theories like Venn diagrams, Church wanted to look at it from a functional design perspective.
  • #22: Take the number 5
  • #23: I could express that as a function that always returns 5
  • #24: This is more like Churchs notation
  • #25: In an operation like adding, I can substitute the functions for the numbers
  • #26: If I can do that, adding itself can be defined as a function. Adding has no side effects. 5 is still 5 after I use it while adding.
  • #27: If you take this to its logical conclusion, any group of operations can be expressed as a hierarchy of functions which C# calls Expression Trees.
  • #28: But if thats true, it makes sense that I can write some function-by-function transformations that turn a C# function into a SQL function
  • #29: With a provider of transformations, I can take queries in C# and turn them into queries against a bunch of different data sources. .NET ships with a few, but you can write others.
  • #30: Now to the practical side of things.
  • #31: Here, I use LINQPad (http://linqpad.com) to demonstrate Where and the like.
  • #32: Map or Project is a functional-language based description of the operation. We call it Select because of most C# programmers SQL heritage.
  • #33: Select can take a position argument. Many LINQ operators have overloads, check them out.
  • #35: Zip can replace simple Join statements.
  • #36: Like fibers in a thread, binding with SelectMany takes many sets and combines them into one.
  • #37: With Select, this would look like { {Andys orders}, {Beths orders}, {Charlies orders}} With SelectMany, this would look like { Andy1, Andy2, Beth1, Charlie1, Charlie2}
  • #39: SelectMany is like doing a cross join in SQL, whereas Join is an inner join
  • #41: Easy to Read is query comprehension syntax. It came from the Entity Framework folks that were also working on this data query problem. Hard to Read is extension methods.
  • #42: Skip and Take allow you to page through your data. Theres also SkipWhile and TakeWhile.
  • #43: Examples include Count(), Sum()
  • #44: Here, I show the Aggregate command.
  • #45: Included in .NET 4.5 or can be downloaded as an external library in .NET 4 Use ToParallel() to invoke PLINQ
  • #46: Image: http://cdn.abovethelaw.com/uploads/2012/02/thank-you.jpg