What is the where operator in LINQ

The where operator in LINQ (Language-Integrated Query) filters a series of data depending on the given criteria. It enables us to pick only those components from a collection that meet specific criteria. In LINQ queries, the where operator is frequently used to limit the results to those that satisfy particular requirements.

We can use the where operator in two ways:

  • Using where with select

  • Using the extension method

Using the where operator with the select operator

The simple way to use the where operator is to use it with the combination of the select operator, which is explained below:

Syntax

Here’s the basic syntax to use the where operator:

var result = from item in collection
             where condition
             select item;
Basic syntax to use the where operator with the select operator

Here, collection is the source collection (e.g., a list, array, or any IEnumerable). A boolean statement called condition defines the filtering requirements. The result will only be comprised of items for which this condition evaluates to true.

Code example

Let’s demonstrate the use of where in combination with the select operator with the following example:

using System;
using System.Linq;
using System.Collections.Generic;
class Test
{
    static void Main(string[] args)
    {
        List<int> numbers = new List<int> { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
        var evenNumbers = from num in numbers
                          where num % 2 == 0
                          select num;
        Console.WriteLine("Even values are");
        foreach (var n in evenNumbers)
        {
            Console.WriteLine(n);
        }
    }
}

Explanation

  • Lines 1–3: It imports the necessary namespaces for using system-related classes, LINQ, and generic collections.

  • Line 9: It creates a list of integers called numbers and initializes it with 10 numbers.

  • Lines 11–13: These create a new collection called evenNumbers by selecting and storing only the even numbers from the numbers list using a LINQ query. The where clause filters numbers based on the condition num % 2 == 0, ensuring only even values are included.

  • Lines 16–19: These iterate through the evenNumbers collection and prints each even number to the console.

Using the extension method

We can also use the Where operator using the extension methodWith the help of an extension method, we can expand the functionality of preexisting types in C# by adding new methods without changing their source code., which is explained below:

Syntax

Here’s the basic syntax to use Where operator using the extension method:

var result = collection.Where(item => condition);
Basic syntax to use the Where operator using the extension method

Here, collection is the source collection (e.g., a list, array, or any IEnumerable). A boolean statement called condition defines the filtering requirements. The result will only be comprised of items for which this condition evaluates to true.

Code example

Let’s demonstrate its usage:

using System;
using System.Linq;
using System.Collections.Generic;
class Test
{
    static void Main(string[] args)
    {
        List<int> numbers = new List<int> { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
        var evenNumbers = numbers.Where(num => num % 2 == 0);
        Console.WriteLine("Even values are");
        foreach (var n in evenNumbers)
        {
            Console.WriteLine(n);
        }
    }
}

Explanation

  • Line 11: It creates a new collection called evenNumbers by selecting and storing only the even numbers from the numbers list using a LINQ extension method. The lambda expression num => num % 2 == 0 is the condition that filters for even values.

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