 LINQ (pronounced link) stands for “Language Integrated Query”.
 This concept is introduced in .NET Framework 3.5.
 This is a “query writing technique”, developed based on SQL.
 This is most useful while working large amount of data in the projects.
Introduction:
 This relational database system, data is organized in the form of tables, on which you can write SQL queries to retrieve the required data according to the requirement in the application.
 But you can’t write a query to the non-database data, which in the form of objects in the application. There, you can write the queries using the new concept called “LINQ”.
 You can write queries on arrays, objects, XML and data sets using LINQ.

The following example shows a small demo on LINQ:

namespace LINQtoArrayDemo
{
class Program
{
static void Main(string[] args)
{
//data source
int[] numbers = { 2, 12, 5, 15 };
//ling query with where clause
var lowNums = from n in numbers where n < 10 select n;
//output
foreach (var x in lowNums)
Console.Write(x + ",");
Console.Read();
}
}
}

In the above application, the array contains few numbers. After executing the query, you got only the numbers, which are less than 10. In this manner, you can execute the queries on XML and data sets (in ADO.NET) also.

LINQ Syntax:
from … in … let … where … orderby … select … group …

 The above syntax consists of 7 clauses.
1. from clause
2. in clause
3. let clause
4. where clause
5. orderby clause
6. select clause
7. group clause

 Mandatory clauses:

1. from clause
2. in clause
3. select clause

1. from clause: This is used to specify the iteration variable name. This acts as alias name for the data source.
2. in clause: This is used to specify the main data source for the query.
3. let clause (optional): This is used to declare any identifier with a value, that is to be used during the query execution.
4. where clause (optional): This is most frequently used optional clause, using which you can specify the condition in the query.
5. orderby clause (optional): This is used to specify the sorting expression if required.
6. select clause: This is used to specify the object, which is required in the query results.
7. group (optional): This is similar to “group by” clause in SQL. This retrieves grouped data.

Note: The result of a LINQ query, should be assigned into a variant type variable. (“var” type).

LINQ to Objects

namespace LINQtoObjectsDemo
{
class Student
{
public int StudentID;
public string Name;
public string Course;
public int Marks;
public Student(int StudentID, string Name, string Course, int Marks)
{
this.StudentID = StudentID;
this.Name = Name;
this.Course = Course;
this.Marks = Marks;
}
}
class Program
{
static void Main(string[] args)
{
//data source
Student[] stu = { new Student(101, "Prakash", "MBA", 765),
new Student(102, "Pradeep", "MBA", 471),
new Student(103, "Pushpa", "Msc", 590),
new Student(104, "Purna", "MCA", 223),
new Student(105, "Purnima", "MCA", 450)};
//linq query with where clause
var query1 = from s in stu where s.Course == "MCA" select s;
Console.WriteLine("MCA Students:");
foreach (var q in query1)
Console.WriteLine(q.StudentID + ", " + q.Name + ", " + q.Course + ", " + q.Marks);
//linq query with let and where clauses
var query2 = from s in stu let avg=s.Marks/10 where avg<35 select s;
Console.WriteLine("\nFailed Students:");
foreach (var q in query2)
Console.WriteLine(q.StudentID + ", " + q.Name + ", " + q.Course + ", " + q.Marks);
//linq query with orderby clause
var query3 = from s in stu orderby s.Marks select s;
Console.WriteLine("\nStudents (sort on marks):");
foreach (var q in query3)
Console.WriteLine(q.StudentID + ", " + q.Name + ", " + q.Course + ", " + q.Marks);
//linq query with orderby clause (descending)
var query4 = from s in stu orderby s.Marks descending select s;
Console.WriteLine("\nStudents (sort on marks - descending):");
foreach (var q in query4)
Console.WriteLine(q.StudentID + ", " + q.Name + ", " + q.Course + ", " + q.Marks);
//linq query with group clause
var query5 = from s in stu group s by s.Course;
Console.WriteLine("\nStudents with grouping:");
foreach (var p in query5)
{
Console.WriteLine(p.Key + ":");
foreach (var q in p)
Console.WriteLine(" " + q.StudentID + ", " + q.Name + ", " + q.Course + ", " + q.Marks);
}
Console.Read();
}
}
}