Introducing LINQ

Introducing LINQ

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Objectives


Objectives
• Motivate the need for LINQ


Objectives
• Learn about the various LINQ providers


Objectives
• Learn about the various LINQ providers
• Investigate simple LINQ to Objects, LINQ to
SQL, and LINQ to XML samples


Agenda


Agenda
• Language Integrated Query


Agenda
• Structure of a LINQ Query


Agenda
• Structure of a LINQ Query
• Some LINQ Examples


Language Integrated Query


• Querying and manipulating data has always
been a fundamental part of our jobs as
developers


developers
• Data formats change, but core needs are the
same


developers
• Data formats change, but core needs are the
same
 Must create, retrieve, update, and delete data


Data Access
(DBASE, 1985 or so)


Data Access
(DBASE, 1985 or so)

USE empl

REPLACE ALL salary WITH (salary * 1.1)
FOR supervises > 0

LIST ALL fname, lname, salary FOR Supervises > 0


Data Access APIs (C#)
(late 1990s/early 2000s)


Data Access APIs (C#)
SqlCommand cmd = new SqlCommand(
@"SELECT fname, lname, salary
FROM Empl
WHERE supervises > @p0"
);

cmd.Parameters.AddWithValue("@po", 0);

SqlConnection c = new SqlConnection(…);
c.Open();
DataReader people = c.Execute(cmd);

while (people.Read()) {
string fname = (string) people["fname"];
string lname = (string) people["lname"];
double salary = (double) people["salary"];
}
people.Close();


Object/Relational Mapping (C#)
(last few years)


Object/Relational Mapping (C#)
(last few years)
public class Employee
{
public string FirstName;
public string LastName;
public double Salary;
}

IList employees =
session.CreateCriteria(typeof(Employee))
.Add(Expression.Gt("supervises", 0)
.List();
foreach(Employee employee in employees)
{
string fname = employee.FirstName;
string lname = employee.LastName;
double salary = employee.Salary;
}


Data Access APIs (VB)


Data Access APIs (VB)
Dim cmd As New SqlCommand( _
"SELECT fname, lname, salary" & _
" FROM Empl" & _
" WHERE supervises > @p0")

cmd.Parameters.AddWithValue("@p0", 0)
Dim cnn as New SqlConnection(…)
cnn.Open()

Dim people As DataReader = cnn.Execute(cmd)
While people.Read()
Dim fname As String = people("fname").ToString()
Dim lname As String = people("lname").ToString()
Dim salary As Double = CDbl(people("salary"))
End While
people.Close();


Object/Relational Mapping (VB)
(last few years)


Object/Relational Mapping (VB)
(last few years)

Public Class Employee
Public FirstName As String
Public LastName As String
Public Salary As Double
End Class

Dim employees As IList = _
session.CreateCriteria(GetType(Employee). _
Add(Expression.Gt("supervises", 0).List()

For Each employee As Employee In Employees
Dim fname As String = people("fname").ToString()
Dim lname As String = people("lname").ToString()
Dim salary As Double = CDbl(people("salary"))
Next employee


Object/Relational Mapping


• Provides:


• Provides:
 Mapping between relational data to/from objects


• Provides:
 Mapping between relational data to/from objects
 Cleaner integration of business rules and validation


LINQ to the Rescue


LINQ to the Rescue
• How do you retrieve non-relational data?


LINQ to the Rescue
 XML, RSS, Web Services, REST, AD, Files, and so on


LINQ to the Rescue
• How do you interact with plain old objects?


LINQ to the Rescue
 How do you interact and query custom domain
models?


LINQ to the Rescue
 How do you interact and query custom domain
models?
• How do you enable clean code in both a
strongly typed and dynamic language world?


LINQ


LINQ
• Query, Set, and Transform Operations for .NET


LINQ
• Makes querying data a core programming
concept


LINQ
concept
• Works with all types and shapes of data


LINQ
concept
 Relational databases


LINQ
concept
 XML


LINQ
concept
 XML
 Plain old objects


LINQ
• Works with all .NET languages


LINQ
 VB and C# have integrated language support


LINQ
• Works in any kind of project including Windows
and Web projects


LINQ
and Web projects
• Also can find third party LINQ providers


LINQ
and Web projects
• Also can find third party LINQ providers
 Amazon.com for example


LINQ Providers


LINQ Providers
• C# uses LINQ providers to map your LINQ
queries to the data source that you’re querying


LINQ Providers
• The LINQ provider takes the query that you
create in code, and converts it into commands
that the data source will be able to execute


LINQ Providers
• The LINQ provider takes the query that you
create in code, and converts it into commands
that the data source will be able to execute
• On return from executing the commands, the
provider also converts the data into objects that
create your query results


LINQ Overview


LINQ Overview

LINQ enabled data sources


LINQ Overview


LINQ
To Objects

Objects

LINQ Overview


LINQ enabled ADO.NET

LINQ LINQ LINQ LINQ
To Objects To Datasets To SQL To Entities

Objects Relational

LINQ Overview



LINQ LINQ LINQ LINQ LINQ
To Objects To Datasets To SQL To Entities To XML

<book>
<title/>

<author/>
<price/
>
</book>
Objects Relational XML

LINQ Overview
VB C# Others…

.NET Language-Integrated Query



LINQ LINQ LINQ LINQ LINQ
To Objects To Datasets To SQL To Entities To XML

<book>
<title/>

<author/>
<price/
>
</book>
Objects Relational XML

LINQ Providers
• LINQ to Objects


LINQ Providers
• LINQ to Objects
 Allows you to query in-memory of sets of objects


LINQ Providers
• LINQ to Objects
o Collections, arrays, and lists


LINQ Providers
• LINQ to Objects
o Collections, arrays, and lists
o If a class implements IEnumerable (or generic version), you
can use LINQ to query


LINQ Providers
• LINQ to SQL


LINQ Providers
• LINQ to SQL
 Allows you to query and modify data in SQL Server


LINQ Providers
• LINQ to SQL
 You provide mapping between a modeling class and
schema in data source


LINQ Providers
• LINQ to SQL
 Use System.Data.Linq.DataContext class to provide
the “plumbing”


LINQ Providers
• LINQ to SQL
the “plumbing”
 Can use Visual Studio’s O/R designer to make it easy


LINQ Providers
• LINQ to SQL
the “plumbing”
o For now, will mark up entity class manually


LINQ Providers
• LINQ to SQL
the “plumbing”
 DataContext manages communication


LINQ Providers
• LINQ to SQL
the “plumbing”
 DataContext manages communication
o Stores state for optimistic concurrency checks


LINQ Providers
• LINQ to XML


LINQ Providers
• LINQ to XML
 Easy to query, modify, and generate XML content


LINQ Providers
• LINQ to XML
 Can work with XML from file, or from stream


LINQ Providers
• LINQ to XML
 System.Linq.Xml.XObject, Xnode, XElement,
XAttribute


LINQ Providers
• LINQ to XML
XAttribute
o Make it easy to work with XML programmatically


LINQ Providers
• LINQ to XML
XAttribute
 LINQ to XML provider works with these classes to
create queryable collections of XML data


LINQ Providers
• LINQ to XML
XAttribute
 LINQ to XML provider works with these classes to
create queryable collections of XML data
 Visual Basic adds language-specific rich support for
LINQ to XML


LINQ Providers
• LINQ to DataSet


LINQ Providers
• LINQ to DataSet
 Allows you to query and update data in ADO.NET
dataset


LINQ Providers
• LINQ to DataSet
dataset
 Can use querying engine against data already cached
in client application


LINQ Providers
• LINQ to DataSet
dataset
 Strongly typed datasets easier to work with than
standard datasets


LINQ Providers
• LINQ to DataSet
dataset
standard datasets
o You already know the schema—it's just LINQ to Objects


LINQ Providers
• LINQ to DataSet
dataset
standard datasets
o You already know the schema—it's just LINQ to Objects
 Language features make it possible to query
standard datasets


LINQ Providers
• LINQ to Entities


LINQ Providers
 Allows developers to query data exposed using Entity
Data Model (EDM)


LINQ Providers
Data Model (EDM)
o EDM: conceptual model, allows applications to interact with
data as if it was a collection of objects, or entities


LINQ Providers
Data Model (EDM)
 Using EDM, ADO.NET exposes entities as objects
in .NET Framework, allows LINQ queries to operate


LINQ Providers
Data Model (EDM)
 Conceptually, same as LINQ to SQL, but works with
any data source


LINQ Providers
Data Model (EDM)
 Conceptually, same as LINQ to SQL, but works with
any data source
o Doesn't assume one-to-one mapping between classes and
schema


Structure of a LINQ Query


• LINQ query contains clauses that specify data
sources and iteration variables


• Query expressions can also include:


 Filtering


 Filtering
 Sorting


 Filtering
 Sorting
 Grouping


 Filtering
 Sorting
 Grouping
 Joining


 Filtering
 Sorting
 Grouping
 Joining
 Calculating


 Filtering
 Sorting
 Grouping
 Joining
 Calculating
• From required—indicates source of data


The Three Stages of a LINQ Query


• Each LINQ query requires three stages:


 Obtain the data source(s)


 Create the query


 Create the query
 Execute the query


The Data Source


The Data Source
• In the sample, data source is an array of
integers


The Data Source
integers
• Because array implements IEnumerable, can
query against it


The Data Source
integers
• Because array implements IEnumerable, can
query against it
 Treated as a "queryable" type


XML as Data Source (VB)

' VB
Dim items As XElement = XElement.Load("C:Grocery.xml")


XML as Data Source (VB)
• Could use LINQ to XML to load contents of an
XML file into a queryable collection of XElement
instances:

' VB
Dim items As XElement = XElement.Load("C:Grocery.xml")


XML as Data Source (C#)

// C#:
XElement items = XElement.Load(@"C:Grocery.xml");


XML as Data Source (C#)
• Could use LINQ to XML to load contents of an
XML file into a queryable collection of XElement
instances:

// C#:
XElement items = XElement.Load(@"C:Grocery.xml");


SQL as Data Source (VB)

' VB
Dim dc As New DataContext( _
My.Settings.NorthwindConnectionString)
Dim customers As Table(Of Customer) = _
dc.GetTable(Of Customer)


SQL as Data Source (VB)
• Can use LINQ to SQL once you have a class that
provides object-relational mapping:
' VB
Dim dc As New DataContext( _
My.Settings.NorthwindConnectionString)
Dim customers As Table(Of Customer) = _
dc.GetTable(Of Customer)


SQL as Data Source (C#)

// C#
DataContext dc = new DataContext(
Properties.Settings.Default.NorthwindConnectionString);
Table<Customer> customers = dc.GetTable<Customer>();


SQL as Data Source (C#)
• Can use LINQ to SQL once you have a class that
provides object-relational mapping:

// C#
DataContext dc = new DataContext(
Properties.Settings.Default.NorthwindConnectionString);
Table<Customer> customers = dc.GetTable<Customer>();


The Query


The Query
• Given the data source, can specify the query


The Query
 Defines the information you want to retrieve


The Query
 Optionally, can specify how to sort, filter, and group


The Query
 Optionally, can specify how to sort, filter, and group
• LINQ queries are generally the same, no matter
the data source


The Query Execution


The Query Execution
• Query definition doesn't execute the query


The Query Execution
• Instead, defines the query that will run when it's
time to retrieve the data


The Query Execution
• To execute, need to run code that forces query
reference to require the values


The Query Execution
• To execute, need to run code that forces query
reference to require the values
 In this course, For Each loop generally forces


Returning Multiple Values


• Want to return multiple values?


• In general, two options:


 Create class that exposes properties


o Specify new instance of class in Select


 Specify values in Select clause


 Specify values in Select clause
o Compiler creates a new anonymous type containing just the
properties you specify


Creating a Class for Return Values


Creating a Class for Return Values
• See MyFileInfo class and MultipleReturnValues1


Using an Anonymous Type


• In previous example, code created a simple type
(MyFileInfo)


(MyFileInfo)
 Type contained properties returned by query


(MyFileInfo)
 Why create a class simply to return values?


(MyFileInfo)
• Anonymous types generally simpler


(MyFileInfo)
• Compiler creates the type, based on values
specified in query Select clause


(MyFileInfo)
 You don't know the type's name


(MyFileInfo)
 You don't know the type's name
 Implicit type definition makes it possible to work with
the anonymous type


Deferred Execution


Deferred Execution
• Defining query doesn't cause query to execute


Deferred Execution
• Must actually force execution by requesting data


Deferred Execution
• In other words:


Deferred Execution
• In other words:
 Creating query doesn't return data


Deferred Execution
• In other words:
 Provides potential for returning data


Deferred Execution
• In other words:
• Can take advantage of deferred execution


Deferred Execution
• In other words:
 Break query into small steps without fear of multiple
trips to the server for data


Deferred Execution
• In other words:
 Break query into small steps without fear of multiple
trips to the server for data
 Makes debugging simpler, as well


Converting Results to Force Execution


• Sometimes, might want to immediately execute
query and cache its results


• Can use method like ToList or ToArray


• Can use method like ToList or ToArray
• Each executes query, and returns results


Some LINQ Examples


Some LINQ Examples
• Get a taste of the kinds of things you can do
with LINQ


Some LINQ Examples
with LINQ
• Introduce some of the new language features
for LINQ


Some LINQ Examples
with LINQ
for LINQ
• Examples of:


Some LINQ Examples
with LINQ
for LINQ
• Examples of:
 LINQ to Objects


Some LINQ Examples
with LINQ
for LINQ
• Examples of:
 LINQ to Objects
 LINQ to SQL


Some LINQ Examples
with LINQ
for LINQ
• Examples of:
 LINQ to Objects
 LINQ to SQL
 LINQ to XML


LINQ to Objects


LINQ to Objects
• Can use any collection that implements
IEnumerable (or generic version) as data source


LINQ to Objects
• Can use any collection that implements
IEnumerable (or generic version) as data source
• Note example uses multiple overloaded versions
of DisplayResults procedure


Query Syntax/Extension Methods
(C#)


(C#)
• How does language compiler turn keywords like
select, where, and orderby into executable
code?


(C#)
code?
 Keywords need to be converted into methods


(C#)
code?
• Each LINQ keyword corresponds to a single
method of the System.Linq.Enumerable


(VB)


(VB)
Select, Where, and Order By into executable
code?


(VB)
code?


(VB)
code?
• Each LINQ keyword corresponds to a single
method of the System.Linq.Enumerable


Continuing the Investigation


• So compiler converts keywords into methods


• But IEnumerable is an old interface


 It doesn't support Where, Select, and so on


 Where do those methods come from?


• LINQ requires ability to extend existing
interfaces/classes without changing their
definitions


definitions
 Available because of new extension methods


definitions
• Extension method exists in one class


definitions
• Extension method exists in one class
 Allows code to extend a different class

Extension Methods (VB)


• Add new methods to existing classes


• Not just for LINQ, but crucial for LINQ


• Extension methods must:


 Accept a parameter, the type of which defines the
class that you’re extending


 Exist within a module


 Exist within a module
 Include the <Extension()> attribute


Extension Methods (C#)


 Exist within a static class


 Be a static method


 Be a static method
 Include the this keyword in the parameter


Using LINQ to Group Output


• System.Linq.Enumerable class provides large
number of methods


• System.Linq.Enumerable class provides large
number of methods
• Look for grouping methods


LINQ to SQL


LINQ to SQL
• Allows standard LINQ queries to work with data
stored in relational databases


LINQ to SQL
 Extends ADO.NET


LINQ to SQL
 Extends ADO.NET
 Adds support for mapping tables and rows to classes and
properties


LINQ to SQL
 Extends ADO.NET
properties
• To support LINQ to SQL, must add custom .NET
attributes to class that represents the schema


LINQ to SQL
 Extends ADO.NET
properties
• Map a class to a table


LINQ to SQL
 Extends ADO.NET
properties
 Map properties within the class to columns


LINQ to SQL
 Extends ADO.NET
properties
 All by adding attributes


LINQ to SQL
 Extends ADO.NET
properties
 All by adding attributes
• Can represent relationships as well

Linking a Class to SQL Server


• Retrieve data from Northwind Customers table


• Retrieve CustomerID, CompanyName,
ContactName, Country, Region


• Retrieve CustomerID, CompanyName,
ContactName, Country, Region
• Indicate primary key, columns, and rename
column


Creating the Class


Creating the Class
• Start by creating a class with properties that
correspond to columns in the database


Creating the Class
• Start by creating a class with properties that
correspond to columns in the database
• See Customer1 class


Marking Up the Class


• Use attributes from the
System.Data.Linq.Mapping namespace


 Table


 Table
 Column


 Table
 Column
 Many others


 Table
 Column
 Many others
• See Customer class


Retrieving a Collection of Customers


• Need some mechanism to manage connection
between class and data


 Can't simply fill some huge data structure with all the
data


data
 Need some class that converts requests for data into
SQL strings, and executes/virtualizes


data
• System.Data.Linq.DataContext class


data
 Includes members that provide access to data


data
 Tracks changes to data for updates


data
 Tracks changes to data for updates
 O/R designer creates classes, and new class that
inherits from DataContext

Determining the Actual SQL String


• Set Log property of DataContext to stream


 Sends all information to the output stream


• To simply see the SQL string


• To simply see the SQL string
 Call ToString method of query variable


Working with Anonymous Types


• Anonymous types work well with LINQ to SQL


• Often need to retrieve just a subset of columns


 Or perform a calculation


 Or perform a calculation
• See DemoLinqToSqlWithAnonymousTypes


LINQ to XML


LINQ to XML
• Need to retrieve, create, query XML?


LINQ to XML
• .NET Framework provides rich support


LINQ to XML
 Steps can be complex


LINQ to XML
 Can require XSLT and XPath


LINQ to XML
• What if you could accomplish these using a
standard syntax?


LINQ to XML
standard syntax?
• That is, what about LINQ?


LINQ to XML
standard syntax?
• LINQ to XML makes it possible


LINQ to XML
standard syntax?
• Examples create XML content


LINQ to XML
standard syntax?
• Examples create XML content
 Examine Course class, and CreateCourseList

Life without LINQ to XML


• Without LINQ to XML, creating XML content
required multiple steps


 XmlDocument creates new elements


 Set properties


 Set properties
 Append as child of new parent


 Set properties
 Append as child of new parent
• See DemoNoLinqToXml


Using System.Xml.Linq Classes


• System.Xml.Linq namespace adds classes


 XElement, XAttribute, XComment


 All inherit from XObject


• Each provides overloaded constructors


 You pass as much information as necessary to
generate the XML


generate the XML
 More readable than XmlDocument code


generate the XML
 More readable than XmlDocument code
• Examine CreateXmlContent


Creating Data: From and Select


• From clause sets up range variable iterating
over data source


over data source
• Select clause specifies "shape" of resulting data


over data source
• Take advantage of these facts to use LINQ to
generate XML content


over data source
• Take advantage of these facts to use LINQ to
generate XML content
• Examine DemoLinqToXml


Visual Basic Only (VB)


• Only Visual Basic adds new features that make
it far easier to work with LINQ to XML


 XML literals


 XML literals
 XML replacement tokens


 XML literals
 XML replacement tokens
• See CreateXmlContentVBOnly


Learn More!


Learn More!
• This is an excerpt from a larger course. Visit
www.learnnowonline.com for the full details!


Introducing LINQ

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