LINQ pocket reference

168 Pages·2008·0.543 MB·English

by  Joseph Albahari, Ben Albahari

LINQ
Pocket Reference
Joseph Albahari and Ben Albahari
Beijing • Cambridge • Farnham • Köln • Paris • Sebastopol • Taipei • Tokyo
LINQ Pocket Reference
by Joseph Albahari and Ben Albahari
Copyright © 2008 Joseph Albahari and Ben Albahari. All rights reserved.
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February 2008: First Edition.
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ISBN: 978-0-596-51924-7
[TM]
Contents
Getting Started 1
Lambda Queries 4
Chaining Query Operators 4
Composing Lambda Expressions 6
Natural Ordering 9
Other Operators 9
Comprehension Queries 10
Iteration Variables 12
Query Syntax Versus SQL Syntax 13
Query Syntax Versus Lambda Syntax 13
Mixed Syntax Queries 14
Deferred Execution 15
Reevaluation 16
Outer Variables 17
How Deferred Execution Works 17
Chaining Decorators 19
How Queries Are Executed 20
Subqueries 22
Subqueries and Deferred Execution 25
Composition Strategies 25
Progressive Query Building 25
The into Keyword 27
Wrapping Queries 28
Projection Strategies 30
Object Initializers 30
Anonymous Types 30
The let Keyword 32
Interpreted Queries 33
How Interpreted Queries Work 35
AsEnumerable 38
LINQ to SQL 40
LINQ to SQL Entity Classes 40
DataContext 42
Automatic Entity Generation 45
Associations 45
Deferred Execution with LINQ to SQL 47
DataLoadOptions 48
Updates 50
Building Query Expressions 52
Delegates Versus Expression Trees 53
Expression Trees 55
Query Operator Overview 59
Filtering 62
Where 63
Take and Skip 65
TakeWhile and SkipWhile 65
Distinct 66
Projecting 66
Select 67
SelectMany 72
Joining 82
Join and GroupJoin 83
Ordering 92
OrderBy, OrderByDescending, ThenBy, ThenByDescending 92
Grouping 95
GroupBy 96
Set Operators 100
Concat and Union 100
Intersect and Except 100
Conversion Methods 101
OfType and Cast 101
ToArray, ToList, ToDictionary, ToLookup 103
AsEnumerable and AsQueryable 104
Element Operators 104
First, Last, Single 105
ElementAt 106
DefaultIfEmpty 107
Aggregation Methods 107
Count and LongCount 107
Min and Max 108
Sum and Average 109
Aggregate 110
Quantifiers 111
Contains and Any 111
All and SequenceEqual 112
Generation Methods 112
Empty 112
Range and Repeat 113
LINQ to XML 113
Architectural Overview 114
X-DOM Overview 115
Loading and Parsing 117
Saving and Serializing 118
Instantiating an X-DOM 118
Functional Construction 119
Specifying Content 120
Automatic Deep Cloning 121
Navigating/Querying an X-DOM 122
Child Node Navigation 122
Parent Navigation 126
Peer Node Navigation 127
Attribute Navigation 128
Updating an X-DOM 128
Simple Value Updates 128
Updating Child Nodes and Attributes 129
Updating Through the Parent 130
Working with Values 133
Setting Values 133
Getting Values 133
Values and Mixed Content Nodes 135
Automatic XText Concatenation 136
Documents and Declarations 136
XDocument 136
XML Declarations 139
Names and Namespaces 140
Specifying Namespaces in the X-DOM 142
The X-DOM and Default Namespaces 143
Prefixes 145
Projecting into an X-DOM 147
Eliminating Empty Elements 149
Streaming a Projection 150
Transforming an X-DOM 151
Index 153
LINQ Pocket Reference
LINQ, or Language Integrated Query, allows you to write
structuredtype-safequeriesoverlocalobjectcollectionsand
remotedatasources.ItisanewfeatureofC#3.0and.NET
Framework 3.5.
LINQ lets you query any collection implementing
IEnumerable<>,whetheranarray,list,XMLDOM,orremote
datasource(suchasatableinSQLServer).LINQoffersthe
benefits of both compile-time type checking and dynamic
query composition.
ThecoretypesthatsupportLINQaredefinedintheSystem.
LinqandSystem.Linq.ExpressionsnamespacesintheSystem.
Core assembly.
NOTE
TheexamplesinthisbookmirrortheexamplesinChap-
ters 8–10 of C# 3.0 in a Nutshell (O’Reilly) and are pre-
loaded into an interactive querying tool called LINQPad.
You can download LINQPad fromhttp://www.linqpad.net/.
Getting Started
ThebasicunitsofdatainLINQaresequencesandelements.
A sequence is any object that implements the generic
IEnumerable interface, and an element is each item in the
sequence.Inthefollowingexample,namesisasequence,and
Tom,Dick, andHarry are elements:
1
string[] names = { "Tom", "Dick", "Harry" };
We call such a sequence a local sequence because it repre-
sents a local collection of objects in memory.
A query operator is a method that transforms a sequence. A
typicalqueryoperatoracceptsaninputsequenceandemitsa
transformed output sequence. In the Enumerable class in
System.Linq,therearearound40queryoperators,allimple-
mented as static extension methods, called standard query
operators.
NOTE
LINQalsosupportssequencesthatcanbedynamicallyfed
fromaremotedatasourcesuchasaSQLServer.Thesese-
quences additionally implement the IQueryable<> inter-
faceandaresupportedthroughamatchingsetofstandard
queryoperatorsintheQueryableclass.Formoreinforma-
tion, see the upcoming “Interpreted Queries” section.
A query is an expression that transforms sequences with
query operators. The simplest query comprises one input
sequence and one operator. For instance, we can apply the
Where operator on a simple array to extract those whose
length is at least four characters as follows:
string[] names = { "Tom", "Dick", "Harry" };
IEnumerable<string> filteredNames =
System.Linq.Enumerable.Where (
names, n => n.Length >= 4);
foreach (string n in filteredNames)
Console.Write (n + "|"); // Dick|Harry|
Because the standard query operators are implemented as
extension methods, we can call Where directly on names—as
though it were an instance method:
IEnumerable<string> filteredNames =
names.Where (n => n.Length >= 4);
2 | LINQ Pocket Reference
For this to compile, you must import the System.Linq
namespace. Here’s a complete example:
using System;
using System.Linq;
class LinqDemo
{
static void Main()
{
string[] names = { "Tom", "Dick", "Harry" };
IEnumerable<string> filteredNames =
names.Where (n => n.Length >= 4);
foreach (string name in filteredNames)
Console.Write (name + "|");
}
}
// RESULT: Dick|Harry|
NOTE
If you are unfamiliar with C#’s lambda expressions, ex-
tension methods, or implicit typing, visit www.albahari.
com/cs3primer.
We can further shorten our query by implicitly typing
filteredNames:
var filteredNames = names.Where (n => n.Length >= 4);
Mostqueryoperatorsacceptalambdaexpressionasanargu-
ment. The lambda expression helps guide and shape the
query. In our example, the lambda expression is as follows:
n => n.Length >= 4
Theinputargumentcorrespondstoaninputelement.Inthis
case,theinputargumentnrepresentseachnameinthearray
and is of type string. The Where operator requires that the
lambda expression return a bool value, which if true, indi-
cates that the element should be included in the output
sequence.
Getting Started | 3