using System;

using System.Linq;

using System.Collections.Generic;

​

​

// Filtering Operators - Where

// http://www.tutorialsteacher.com/linq/linq-filtering-operators-where

public class Program

{

    public static void Main()

    {

        /*The Where operator (Linq extension method) filters the collection based on a given criteria 

        expression and returns a new collection. The criteria can be specified as lambda expression or 

        Func delegate type. 

        Example: Where clause - LINQ query syntax C#*/

        IList<Student> studentList = new List<Student>() { 

                new Student() { StudentID = 1, StudentName = "John", Age = 13} ,

                new Student() { StudentID = 2, StudentName = "Moin",  Age = 21 } ,

                new Student() { StudentID = 3, StudentName = "Bill",  Age = 18 } ,

                new Student() { StudentID = 4, StudentName = "Ram" , Age = 20} ,

                new Student() { StudentID = 5, StudentName = "Ron" , Age = 15 } 

            };

​

        // LINQ Query Syntax to find out teenager students

        var teenAgerStudent = from s in studentList

            where s.Age > 12 && s.Age < 20

            select s;

        Console.WriteLine("Teen age Students:");

        foreach(Student std in teenAgerStudent){            

            Console.WriteLine(std.StudentName);

        } /*

        -----------------------------------------------------------------

        -----------------------------------------------------------------

        Alternatively, you can also use a Func type delegate with an anonymous method to 

        pass as a predicate function as below (output would be the same):       */

        // Student collection

        Func<Student,bool> isTeenAger = delegate(Student s) { 

            return s.Age > 12 && s.Age < 20; 

        };

​

        var filteredResult = from s in studentList

                             where isTeenAger(s)

                             select s;

        foreach (var std in filteredResult)

            Console.WriteLine(std.StudentName);     /*

        -----------------------------------------------------------------

​

​

        -----------------------------------------------------------------

        You can also call any method that matches with Func parameter with one of Where() method overloads. */

        var filteredResult2 = from s in studentList

                         where IsTeenAger2(s)

                         select s;

​

        foreach (var std in filteredResult2)

            Console.WriteLine(std.StudentName);             /*

        ----------------------------------------------------------------

        ----------------------------------------------------------------

        Where extension method in Method Syntax:

​

        Unlike the query syntax, you need to pass whole lambda expression as a predicate function instead of just body expression in LINQ method syntax.

        Example: Where in method syntax in C# */

        var filteredResult3 = studentList.Where(s => s.Age > 12 && s.Age < 20);

        Console.WriteLine("Teen age Students:");

        foreach(Student std in teenAgerStudent){            

            Console.WriteLine(std.StudentName);

        } /*

        ----------------------------------------------------------------

​

​

​

        ----------------------------------------------------------------

        As mentioned above, the Where extension method also has a second overload 

        that includes index of current element in the collection. 

        You can use that index in your logic if you need.

        The following example uses the Where clause to filter out odd elements 

        in the collection and return only even elements. Please remember that index starts from zero.

        Example: Linq - Where extension method in C#        */

​

        var filteredResult4 = studentList.Where((s, i) => { 

                    if(i % 2 ==  0) // if it is even element

                        return true;

                return false;

            });

        foreach (var std in filteredResult)

                Console.WriteLine(std.StudentName); /*

        ----------------------------------------------------------------

​

​

​

        ----------------------------------------------------------------

        Multiple Where clause:

        You can call the Where() extension method more than one time in a single LINQ query.

        Example: Multiple where clause in Query Syntax C# */

        var filteredResult5 = from s in studentList

            where s.Age > 12

            where s.Age < 20

            select s;

        /* Example: Multiple where clause in Method Syntax C# */

        var filteredResult6 = studentList.Where(s => s.Age > 12).Where(s => s.Age < 20);

​

    }   

    public static bool IsTeenAger2(Student stud)

    {

        return stud.Age > 12 && stud.Age < 20;  

    }

}

​

public class Student{

​

    public int StudentID { get; set; }

    public string StudentName { get; set; }

    public int Age { get; set; }

}