LINQ ToLookup() Method

ToLookup operator in LINQ is an extension method, and it is used to extract a set of key/value pairs from the source. Here, each element in the resultant collection is a generic Lookup object. Lookup object holds the Key and subsequence items that matched with the Key.

Syntax of LINQ ToLookup Operator

Here is the syntax of using the LINQ ToLookup operator to return the collection as a key/value pair.

C# Code

In the above syntax, we are converting the collection of "objEmployee" to key/value pair list using the ToLookup operator.

Example of LINQ ToLookup() Operator in method syntax

Here is the example of using the LINQ ToLookup() in method syntax to convert the input collection items as a key/value pair list.

C# Code

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApp1
{
    class Program
    {
        static void Main(string[] args)
        {
//create an objEmployee of the class Employee and create a list of Employee
            List<Employee> objEmployee = new List<Employee>()
            {
                new Employee(){ Name="Akshay Tyagi", Department="IT", Country="India"},
                new Employee(){ Name="Vaishali Tyagi", Department="Marketing", Country="Australia"},
                new Employee(){ Name="Arpita Rai", Department="HR", Country="China"},
                new Employee(){ Name="Shubham Ratogi", Department="Sales", Country="USA"},
                new Employee(){ Name="Himanshu Tyagi", Department="Operations", Country="Canada"}
            };
    //objEmployee.ToLookup() method is used to print the value of the data in the pair/collection of items.
                var Emp = objEmployee.ToLookup(x => x.Department);
                Console.WriteLine("Grouping Employees by Department");
                Console.WriteLine("---------------------------------");
                foreach (var KeyValurPair in Emp)
                {
                    Console.WriteLine(KeyValurPair.Key);
                // Lookup employees by Department
                foreach (var item in Emp[KeyValurPair.Key])
                {
                    Console.WriteLine("\t" + item.Name + "\t" + item.Department + "\t" + item.Country);
                }
          }
                Console.ReadLine();
    }
}
    class Employee
    {
        public string Name { get; set; }
        public string Department { get; set; }
        public string Country { get; set; }
    }
}

In the above example, we grouped the Employee by Department using the ToLookup method. Since ToLookup produces the key/value pair so that we used it in the foreach loop and the inner loop extracts the values based on the Key passed as input.

OUTPUT:

Example of LINQ ToLookup() in Query Syntax

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApp1
{
    class Program
    {
        static void Main(string[] args)
        {
            List<Employee> objEmployee = new List<Employee>()
            {
                new Employee(){ Name="Ashish ", Department="Marketing", Country="India"},
                new Employee(){ Name="John", Department="IT", Country="Australia"},
                new Employee(){ Name="Kim", Department="Sales", Country="China"},
                new Employee(){ Name="Marcia", Department="HR", Country="USA"},
                new Employee(){ Name="John", Department="Operations", Country="Canada"}
            };
                var emp = (from employee in objEmployee select employee).ToLookup(x => x.Department);
                 Console.WriteLine("Grouping Employees by Department");
                Console.WriteLine("---------------------------------");
                foreach (var KeyValurPair in emp)
                {
                    Console.WriteLine(KeyValurPair.Key);
                    // Lookup employees by Department
                    foreach (var item in emp[KeyValurPair.Key])
                    {
                        Console.WriteLine("\t" + item.Name + "\t" + item.Department + "\t" + item.Country);
                    }
                }
                    Console.ReadLine();
          }
    }
        class Employee
        {
            public string Name { get; set; }
            public string Department { get; set; }
            public string Country { get; set; }
         }
}

OUTPUT: