--- title: "Using LINQ" description: "In this article we will discuss the LINQ approach introduced in .NET 3.5 for querying." author: "Shankar M" published: 2013-03-21 updated: 2019-09-07 canonical: https://www.mindstick.com/articles/1152/using-linq category: "linq" tags: ["linq"] reading_time: 10 minutes --- # 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 difference between .NET 3.0 and .NET 3.5 framework - LINQ is designed to fill the gap that exists between tradition .NET, which offers strong typing and full Object-Oriented approach LINQ introduces an easy learn 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 Using LINQ offers with the following advantages: 1. LINQ offers common syntax for querying any type of data sources 2. Secondly, it binds the gap between relational and object-oriented approach 3. LINQ expedites development time by catching errors at compile time and includes IntelliSense & Debugging support 4. LINQ expressions are Strongly Typed. ##### What we mean by Strongly Typed? Strongly typed expressions ensure access to values as correct type at compile time & prevent type mismatch errors being caught when the code is compiled rather at run-time. ##### Core Assemblies in LINQ The core assemblies in LINQ are 1. using System.Linq Provides Classes & Interface to support LINQ Queries 2. using System.Collections.Generic Allows User to create Strongly typed collections that provide type safety and performance (LINQ to Objects) 3. using System.Data.Linq Provides functionality for access to relational databases(LINQ to SQL) 4. using System.Xml.Linq Provides functionality for accessing XML documents using LINQ(LINQ to XML) 5. 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 specific source 1. LINQ to Objects 2. LINQ to XML(or XLINQ) 3. LINQ to SQL(or DLINQ) Let us discuss each of them in brief, ##### LINQ to Objects - LINQ to Object is the basic of LINQ - It enables us to perform complex query operations against any enumerable object(object that supports IEnumerable interface) LINQ queries provides following advantages over traditional foreach loops 1. They are more concise & readable specially when filtering multiple conditions 2. 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: 1. Create a new Project and give a name UsingLinq. ![Using LINQ](https://www.mindstick.com/mindstickarticle/96331a94-3f8b-4e0c-b28a-7b52c6411344/images/0eeb7530-d9a7-4309-b6bc-d267b3fe9421.png) 2. 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 FilteredValues = from val in 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 \ ![Using LINQ](https://www.mindstick.com/mindstickarticle/96331a94-3f8b-4e0c-b28a-7b52c6411344/images/e3c064c3-ebda-47f5-8e3c-feddf12c3396.png) 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 and loop through the int array arr with foreach using the \ LINQ to Objects query syntax. ##### LINQ to XML LINQ to XML provides 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 XLINQ ##### Employee.xml ```       7369   SMITH   CLERK   7902   17-DEC-80   800       7499   ALLEN   SALESMAN   7698   20-FEB-81   1600   300       7521   WARD   SALESMAN   7698   22-FEB-81   1250   500       7566   JONES   MANAGER   7839   02-APR-81   2975       7654   MARTIN   SALESMAN   7698   28-SEP-81   1250   1400     ``` 1. Add a New Windows Form to the Project and name it FrmLoadXML. 2. Add ListView control and a button to the Form. 3. Change the buttons Name to btnLoadXML using Name property\ ##### Paste the code below in 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 ![Using LINQ](https://www.mindstick.com/mindstickarticle/96331a94-3f8b-4e0c-b28a-7b52c6411344/images/ae3d4433-c45f-44e0-b7b0-0d94f4f5d5a2.png) ##### LINQ to SQL LINQ to SQL is the last form of LINQ, which provides functionalities to query SQL-based data sources ##### \ Namespace Its functionalities are located in System.Data.Linq assembly. using System.Data.Linq; A Simple LINQ query requires 3 things 1. Entity Class 2. Data Context 3. LINQ query ##### Coding LINQ to SQL Query In this we will use LINQ to SQL to retrieve all employee details from EMP Table 1. Navigate to Solution Explorer and add a new form named FrmSQLLinq.cs 2. Add a DataGridView control ,button controls as shown ![Using LINQ](https://www.mindstick.com/mindstickarticle/96331a94-3f8b-4e0c-b28a-7b52c6411344/images/b4fcee28-d2e5-4778-92e1-fe5f1094ee71.png) 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 employees = db.GetTable();              // 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 ![Using LINQ](https://www.mindstick.com/mindstickarticle/96331a94-3f8b-4e0c-b28a-7b52c6411344/images/97391c0c-83b6-4636-a036-e2f33593f516.png) ##### Explanation As you know now that to work with SQL LINQ requires, 1. Entity Class 2. Data Context 3. LINQ query First, we define an entity class, ``` [Table]        public class EMP        {            [Column]            public int EMPNO;            [Column]            public string ENAME;            [Column]            public string JOB;            [Column]            public int SAL;         } ``` 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 give the name of a Table, which default to the \ class name. That 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 \ Table. You can declare fields in an entity class that don’t map to Table columns, \ but must be of 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 create objects from \ the result set. ```  // create typed table            Table employees = db.GetTable(); ``` A typed table is a collection usually of type System.Data.Linq.Table whose \ elements are of 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 typed local variable emp of type ```  from c in employees                      select c                      ; ``` select c, is like a SQL Select *, 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. --- Original Source: https://www.mindstick.com/articles/1152/using-linq Copyright © MindStick Software Pvt. Ltd. This Markdown version is provided for developers, AI systems, and offline reading.