Chapter 15. Introduction
to LINQ
Hoang Anh Viet

Hanoi University of Technology
1
Microsoft
Objectives
“LINQ allows you to build strongly typed query expressions,
which can be applied to a number of LINQ targets to
manipulate “data” in the broadest sense of the word.
Here, you will learn about LINQ to Objects, which allows
you to apply LINQ expressions to containers of data
(arrays, collections, custom types). This information will
serve you well when we examine how to apply LINQ
expressions to relational databases (via LINQ to ADO)
and XML documents.”
2
Microsoft
Roadmap
15.1 Understanding the Role of LINQ
15.2 A First Look at LINQ Query Expressions
15.3 LINQ and Generic Collections
15.4 LINQ and Nongeneric Collections
15.5 The Internal Representation of LINQ Query Operators
15.6 Investigating the C# LINQ Query Operators
3
Microsoft
15.1 The Role of LINQ

The LINQ API is an attempt to provide a consistent,

symmetrical manner in which programmers can obtain
and manipulate “data” in the broad sense of the term

LINQ Expressions Are Strongly Typed and Extendable

The Core LINQ Assemblies

System.Core.dll

System.Data.Linq.dll

System.Data.DataSetExtensions.dll

System.Xml.Linq.dll
4
Microsoft
The Role of LINQ

Advantages of LINQ

For now:

Dynamically-created string queries

Doesn't provide compile-time checking

Native code and non-native code is mixed

With LINQ


Static typing

Compile-time syntax checking

IntelliSense

Native code
5
Microsoft
Roadmap
15.1 Understanding the Role of LINQ
15.2 A First Look at LINQ Query Expressions
15.3 LINQ and Generic Collections
15.4 LINQ and Nongeneric Collections
15.5 The Internal Representation of LINQ Query Operators
15.6 Investigating the C# LINQ Query Operators
6
Microsoft
15.2 A First Look at LINQ Query
Expressions

LINQ and Implicitly Typed Local Variables

Type of the variable induced from expression

Must include initializer

Can’t be null. Why not?

What happens if “var” is a class in scope?


Works in for loops
7
Microsoft
LINQ Query Expressions

LINQ and Extension Methods

The Problem: How to add a method to a type you don’t control
and can’t subtype?

Example: How to add Standard Query Operators to
IEnumerable<T>, without deriving a new interface?

Solution: Extension methods aren’t really members of the target
type, but the syntax makes them look that way

Compile-time type safety is preserved
8
Microsoft
LINQ Query Expressions

LINQ and Extension Methods

Not for properties, events, operators

Currently under consideration

Equivalent to calling the static method


Difference from actually extending the class?

How could we use this for Polynomials?

Disadvantages?

Implicitness

Security
9
Microsoft
LINQ Query Expressions

Creating Extension Methods

Define a Static Class

Define a static method whose first argument is an object of type
T

[System.Runtime.CompilerServices.Extension] attribute on first
parameter tells CLR the method is an Extension

C# binds the attribute to keyword “this”

Create a T object, and call the method as if it were a member of
the object

When the compiler fails to find the method among T’s members,
it searches for an extension method in scope with matching

name and signature
10
Microsoft
Replacing LINQ Extension Methods

Extension Methods have lower priority of resolution than type
members

So if a type has a member with same signature as a LINQ
extension method, the type’s member will be called, and there is
no ambiguity

This is how DLINQ and XLINQ replace the default LINQ
implementations of the Standard Query Operators
11
Microsoft
Lambda Expressions

Lambda Expressions

Generalized function syntax

λ x . x + 1

in C# 3.0, have x => x + 1

From anonymous delegate syntax:

delegate(int x) { return x + 1;}


Can have implicitly typed variables

Can have more than one variable

Can have expression or statement body

NB: no statement bodies in preview compiler
12
Microsoft
13

Lambda expressions

Can be converted to delegate type

if parameters and body match

Participate in type inference

If expression body, get expression trees
Lambda Expression
Microsoft
Lambda Expression

Lambda expressions

Type inference

If call Select(customers, c => c.Name);


T, S mapped to appropriate types
14
Microsoft
The Role of Differed Execution

LINQ query expressions are not actually evaluated
until you iterate over their contents

Apply the same LINQ query multiple times to the
same container and obtain the latest and greatest
results
15
Microsoft
The Role of Immediate Execution

To evaluate a LINQ expression from outside the confines
of foreach logic, you are able to call any number of
extension methods defined by the Enumerable type

Extension methods:ToArray<T>,
ToDictionary<TSource,TKey>(), and ToList<T>()

Capture a LINQ query result set in a strongly typed
container
16
Microsoft
Roadmap
15.1 Understanding the Role of LINQ
15.2 A First Look at LINQ Query Expressions
15.3 LINQ and Generic Collections

15.4 LINQ and Nongeneric Collections
15.5 The Internal Representation of LINQ Query Operators
15.6 Investigating the C# LINQ Query Operators
17
Microsoft
15.3 LINQ and Generic Collections

LINQ query expressions can also manipulate data within
members of the System.Collections.Generic namespace,
such as the List<T> type
18
Microsoft
Classes

System.Collections.Generic classes

List<ItemType>

Dictionary<K,V>

Stack<ItemType>

Queue<ItemType>

System.Collections.Generic interfaces

IList<ItemType>

IDictionary<K,V>


ICollection<ItemType>

IEnumerable<ItemType>

IEnumerator<ItemType>

IComparable<OperandType>

IComparer<OperandType>
19
Microsoft
Applying a LINQ Expression

grabbing items from the List<T> where the Speed
property is greater than 55
20
Microsoft
Roadmap
15.1 Understanding the Role of LINQ
15.2 A First Look at LINQ Query Expressions
15.3 LINQ and Generic Collections
15.4 LINQ and Nongeneric Collections
15.5 The Internal Representation of LINQ Query Operators
15.6 Investigating the C# LINQ Query Operators
21
Microsoft
15.4 LINQ and Nongeneric Collections

It is possible to iterate over data contained within
nongeneric collections using the generic

Enumerable.OfType<T>() method
22
Microsoft
Nongeneric Collections

Use the previously defined Car type and import the
System.Collections namespace
23
Microsoft
Roadmap
15.1 Understanding the Role of LINQ
15.2 A First Look at LINQ Query Expressions
15.3 LINQ and Generic Collections
15.4 LINQ and Nongeneric Collections
15.5 The Internal Representation of LINQ Query Operators
15.6 Investigating the C# LINQ Query Operators
24
Microsoft
15.5 The Internal Representation of LINQ
Query Operators

Introduced to the process of building query expressions
using various C# query operators (such as from, in,
where, orderby, and select)

C# compiler actually translates these tokens into calls on
various methods of the System.Linq.Enumerable type

Many methods require a generic delegate of type
Func<>

25