LINQ | Grouping Operator | ToLooKUp

In LINQ, grouping operators pick the elements of the sequence or collection which contains common attributes and serve them in a group. Or in other words, we can say that the grouping operator returns the group of elements based on the given key. This group is held in a special type of collection, which implements the IGrouping<TKey, TElement> interface, where TKey is the key through which the group is created and TElement is the collection of elements which peers with the grouping key value. For example, a sequence contains 6 elements, i.e, Abc, cvd, Abc, Abc, ert, Por, now we want a group in which contain all the Abc present in that sequence. So Abc is the key through which we create another group that contains 3 Abc. As shown in the below image:

The Standard Query Operator contains two different types of grouping operators:

1. GroupBy
2. ToLookup

ToLookup Operator

The ToLookup operator is used to put the element in a LookUp<TKey, TElement> based on a key selector function. Here, LookUp<TKey, TElement> is a one-to-many dictionary. The working of the ToLookup operator is quite similar to the GroupBy operator.

Important Points:

• It does not support query syntax in C# and VB.Net languages.
• It support method syntax in both C# and VB.Net languages.
• It only present in Enumerable class.
• It is overloaded in four different types.
• It throw ArgumentNullException if the source or keySelector is null.
• It is implemented by using immediate execution.

Example:

// C# program to divide the employees in
// groups according to their gender
using System;
using System.Linq;
using System.Collections.Generic;

// Employee details
public class Employee {

    public int emp_id
    {
        get;
        set;
    }

    public string emp_name
    {
        get;
        set;
    }

    public string emp_gender
    {
        get;
        set;
    }

    public string emp_hire_date
    {
        get;
        set;
    }

    public int emp_salary
    {
        get;
        set;
    }
}

class GFG {

    // Main method
    static public void Main()
    {
        List<Employee> emp = new List<Employee>() {

            new Employee() {emp_id = 209, emp_name = "Anjita", emp_gender = "Female",
                                    emp_hire_date = "12/3/2017", emp_salary = 20000},

            new Employee() {emp_id = 210, emp_name = "Soniya", emp_gender = "Female",
                                    emp_hire_date = "22/4/2018", emp_salary = 30000},

            new Employee() {emp_id = 211, emp_name = "Rohit", emp_gender = "Male",
                                  emp_hire_date = "3/5/2016", emp_salary = 40000},

            new Employee() {emp_id = 212, emp_name = "Supriya", emp_gender = "Female",
                                      emp_hire_date = "4/8/2017", emp_salary = 40000},

            new Employee() {emp_id = 213, emp_name = "Anil", emp_gender = "Male",
                                emp_hire_date = "12/1/2016", emp_salary = 40000},

            new Employee() {emp_id = 214, emp_name = "Anju", emp_gender = "Female",
                                  emp_hire_date = "17/6/2015", emp_salary = 50000},
        };

        // Query to divide the employees
        // in the groups according to
        // their gender Using the
        // ToLookup method
        var res = emp.ToLookup(e => e.emp_gender);

        foreach(var val in res)
        {
            // Here salary is the key value
            Console.WriteLine("Group By Gender: {0}", val.Key);

            // Display name of the employees
            foreach(Employee e in val)
            {
                Console.WriteLine("Employee Name: {0}", e.emp_name);
            }
        }
    }
}

Output:

Group By Gender: Female
Employee Name: Anjita
Employee Name: Soniya
Employee Name: Supriya
Employee Name: Anju
Group By Gender: Male
Employee Name: Rohit
Employee Name: Anil