In LINQ, the GroupBy operator is used to grouping the list/collection items based on the specified key-value, it returns a collection of IGrouping<Key, Values>. The Groupby method in LINQ is the same as the SQL group by clause. 

Syntax

IEnumerable<IGrouping<key, value>> group = collection.GroupBy( condition)

Example 

using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

 public static void Main()

 {

  List<Student> students = new List<Student>(){

  new Student() { StudentId = 1, Name = 'Ashu', Marks = 500 },

  new Student() { StudentId = 2, Name = 'Shyam', Marks = 500 },

  new Student() { StudentId = 3, Name = 'Shriyam', Marks = 400 },

  new Student() { StudentId = 4, Name = 'Sunny', Marks = 550 },

  new Student() { StudentId = 5, Name = 'Ram', Marks = 600 },

  new Student() { StudentId = 6, Name = 'Krishna', Marks = 550 },

  new Student() { StudentId = 7, Name = 'Anupam', Marks = 550 }

  } ;

  List<IGrouping<int, Student>> list = students.GroupBy(stu => stu.Marks).ToList();

  list.ForEach( evt => {

   Console.WriteLine('\nkey : ' + evt.Key+' No of times : ' + evt.Count() + '\n');

   Console.WriteLine('{0,-10} {1,4} {2,5}','Name','ID','Marks');

   foreach(Student student in evt){

    Console.WriteLine('{0,-10} {1,4} {2,5}',student.Name, student.StudentId,student.Marks);

   }	

  }) ;

  Console.ReadLine();

 }

}

class Student

{

public int StudentId { get; set; }

public string Name { get; set; }

public int Marks { get; set; }

}

Output

Key : 500 No of times : 2

Name         ID Marks

Ashu          1   500

Shyam         2   500

Key : 400 No of times : 1

Name         ID Marks

Shriyam       3   400

Key : 550 No of times : 3

Name         ID Marks

Sunny         4   550

Krishna       6   550

Anupam        7   550

Key : 600 No of times : 1
Name         ID Marks
Ram           5   600