Using LINQ in .NET

Shankar M
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UsingLinq.rar

Using LINQ

In this article we will discuss the LINQ approach introduced in .NET 3.5 for querying.

Introduction

LINQ is a new technology introduced in .NET 3.5
LINQ is acronym for Language Integrated Query
The LINQ feature is one of the major differences between the .NET 3.0 and .NET 3.5 frameworks
LINQ is designed to fill the gap that exists between traditional .NET, that offers strong typing, and a full Object-Oriented approach

LINQ introduces an easy-learning approach for querying and modifying data and can support querying various types of data sources including relational data, XML and in-memory data structures.

Advantages of Using LINQ

LINQ offers the following advantages:

LINQ offers a common syntax for querying any type of data sources
Secondly, it binds the gap between relational and object-oriented approachs
LINQ expedites development time by catching errors at compile time and includes IntelliSense & Debugging support
LINQ expressions are Strongly Typed.

What we mean by Strongly Typed?

Strongly typed expressions ensure access to values as the correct type at compile time & prevents type mismatch errors being caught when the code is compiled rather at run-time.

Core Assemblies in LINQ

The core assemblies in LINQ are:

using System.Linq
Provides Classes & Interface to support LINQ Queries
using System.Collections.Generic
Allows the user to create Strongly Typed collections that provide type safety and performance (LINQ to Objects)
using System.Data.Linq
Provides the functionality to access relational databases (LINQ to SQL)
using System.Xml.Linq
Provides the functionality for accessing XML documents using LINQ (LINQ to XML)
using System.Data.Linq.Mapping
Designates a class as an entity associated with a database.

Types of LINQ

LINQ provides three basic types of queries, each type offers specific functionality and is designed to query a specific source. The three basic types of queries are:

LINQ to Objects
LINQ to XML(or XLINQ)
LINQ to SQL(or DLINQ)

Let us briefly discuss each of them.

LINQ to Objects

LINQ to Object is the basics of LINQ
It enables us to perform complex query operations against any enumerable object (object that supports the IEnumerable interface)

LINQ queries provide the following advantages over traditional foreach loops:

They are more concise & readable, especially when filtering multiple conditions
Provides powerful filtering, grouping and ordering with little coding

Coding a Simple LINQ to Objects Query

Here, we will use LINQ to Objects to retrieve numbers greater than zero from an array of numbers.

Steps:

Create a new project and name it UsingLinq.
Add a ListView control to the Form1. In the Form's Load event add the following code:

int[] arr = { 20, -12, 97, 86, -12, 77, 12, 0, 17, -87, 78 };

           IEnumerable<int> FilteredValues = from valin arr

                                              where val > 0

                                              select val;

            listView1.View = View.List;

           foreach (var values in FilteredValues)

             listView1.Items.Add(values.ToString());

When running the code, we get the output as:

This displays the list of numbers greater than Zero.

Explanation

In this we declare an integer array called arr.

int[] arr = { 20, -12, 97, 86, -12, 77, 12, 0, 17, -87, 78 };

To retrive the numbers greater than Zero from the int array, we query the int arry using IEnumerable<int> and loop through the int array arr with foreach using the LINQ to Objects query syntax.

LINQ to XML

LINQ to XML provides an in-memory XML programming API that integrates XML querying capabilities.
LINQ to XML provides facilities to edit XML documents and elements trees in memory as well as streaming facilities.

Coding LINQ to XML Query

We will use LINQ to retrieve element values from an XML document. Here we will retrieve the element ENAME using LINQ.

Employee.xml

<?xml version="1.0" ?>

<EMP>

<ENAME>SMITH</ENAME>

<JOB>CLERK</JOB>

</EMP>

<EMP>

<ENAME>ALLEN</ENAME>

<JOB>SALESMAN</JOB>

</EMP>

<EMP>

<JOB>SALESMAN</JOB>

</EMP>

<EMP>

<ENAME>JONES</ENAME>

<JOB>MANAGER</JOB>

</EMP>

<EMP>

<ENAME>MARTIN</ENAME>

<JOB>SALESMAN</JOB>

</EMP>

</EMPLOYEES>

Add a New Windows Form to the Project and name it FrmLoadXML.
Add a ListView control and a button to the Form.
Change the button's name to btnLoadXML using the Name property.

Paste the code below in the button Click Event:

//Loading the Employee.xml in memory

XElement xdoc = XElement.Load(@"D:\Employee.xml");

//Query XML doc

var vExmployees = from EmployeeNames in xdoc.Descendants("ENAME")

select EmployeeNames.Value;

// Displaying details

listView1.View = View.List;

foreach (var EmpNames in vExmployees)

listView1.Items.Add(EmpNames);

When running the Form we get the output with the list of Employee names (ENAME column) from the Employee.xml file.

LINQ to SQL

LINQ to SQL is the last form of LINQ, that provides functionalities to query SQL-based data sources.

Namespace

Its functionalities are located in the System.Data.Linq assembly.

using System.Data.Linq;

A Simple LINQ query requires the following 3 things:

Entity Class
Data Context
LINQ query

Coding LINQ to SQL Query

In this we will use LINQ to SQL to retrieve all employee details from the EMP Table, as described in the following:

Navigate to Solution Explorer and add a new form named FrmSQLLinq.cs
Add a DataGridView control and button controls as in the following:

In the Button click event of "Load" we have used LINQ to SQL to retrieve Employee details from the EMP Table.

Program

using System;

using System.Collections.Generic;

using System.ComponentModel;

using System.Data;

using System.Drawing;

using System.Linq;

using System.Text;

using System.Windows.Forms;

using System.Data.Linq;

using System.Xml.Linq;

using System.Data.Linq.Mapping;

namespace UsingLinq

{

public partial class FrmSQLLinq : Form

{

[Table]

public class EMP

{

[Column]

public int EMPNO;

[Column]

public string ENAME;

[Column]

public string JOB;

[Column]

public int SAL;

}

public FrmSQLLinq()

{

InitializeComponent();

}

private void btClose_Click(object sender, EventArgs e)

{

this.Close();

}

private void btnLoad_Click(object sender, EventArgs e)

{

// Connection String to the Server

string connString = @"Data Source=yourServer;Initial Catalog=DatabaseName;user id=UserName;password=yourPwd;";

// create data context

DataContext db = new DataContext(connString);

// create typed table

Table<EMP> employees = db.GetTable<EMP>();

// Query Database

var emp = from c in employees select c;

dgEmployee.ColumnCount = 4;

dgEmployee.Columns[0].Name ="Emp No.";

dgEmployee.Columns[1].Name ="Emp Name";

dgEmployee.Columns[2].Name ="Job";

dgEmployee.Columns[3].Name ="Salary";

foreach (var c in emp)

dgEmployee.Rows.Add(c.EMPNO, c.ENAME, c.JOB, c.SAL);

}

When running the application we get the output as:

Explanation

You now know that working with SQL LINQ requires:

Entity Class
Data Context
LINQ query

First, we define an entity class, as in the following:

[Table]

public class EMP

{

[Column]

public int EMPNO;

[Column]

public string ENAME;

[Column]

public string JOB;

[Column]

public int SAL;

}

The Entity classes provide objects in which LINQ stores data from data sources. They are simply like other C# class.

[Table] – The Table attribute marks the class as an entity class and has an optional Name property that can be used to provide the name of a Table, that defaults to the class name. That is why we name the class EMP rather than Employee.

Here EMP is the table in the underlying database.

[Column] - The column Attribute sets the field as one that will hold data from the table. You can declare fields in an entity class that don't map to table columns, but must be of a type compatible with the table columns they map to.

// create data context

DataContext db = new DataContext(connString);

A Data Context manages the connection to the Data Source. It also translates LINQ requests into SQL, passes the SQL to the database server and creates objects from the result set.

// create typed table

Table<EMP> employees = db.GetTable<EMP>();

A typed table is a collection usually of type System.Data.Linq.Table<T> whose elements are of a specific type.

GetTable Method

The GetTable method of the DataContext tells the data context to access the results and indicates where to place them.
Here we get all the rows from the Employee Table, the data context creates an object for each row in the customers typed table.

// Query Database

var emp =

Here we declare implicitly a typed local variable emp of type var.

from c in employees
select c
;

select c, is like a SQL Select *, it gets all columns from the EMP class.

Article Sum-Up

In this article we have tried to learn about the technology called LINQ introduced in .NET 3.5 and the functionalities of each of the types of LINQ and how to use each of these types.

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