How to Use Where in LINQ - C#

2 May. 2021 | Updated 23 May. 2021 | 2 min read.

This is a quick guide on how to use the Where LINQ operator, using both method and query syntax.

The Where command allows us to filter a collection, based on a specified criteria.

Let's take a look at an example with a simple object collection:

class UsingLinq 
{
    public static void Main (string[] args) 
    {
        var people = new List<Person>() 
        { 
            new Person() { PersonId = 1, Name = "Belinda", Age = 70 },
            new Person() { PersonId = 2, Name = "Betty",  Age = 81 },
            new Person() { PersonId = 3, Name = "Will",  Age = 18 },
            new Person() { PersonId = 4, Name = "Andy", Age = 70 },
            new Person() { PersonId = 5, Name = "Rob", Age = 15 }
        };
	}
}

class Person
{
    public int PersonId { get; set; }
    public string Name { get; set; }
    public int Age { get; set; }

    public Person() {}

    public void Introduce() 
    {
        Console.WriteLine($"My name is {Name}, and I am {Age} years old.");
    }
}

Using Method Syntax

Using the method syntax, let's use Where to filter the people list to only those that are older than 20.

public static void Main (string[] args) 
{
	var people = new List<Person>() 
	{ 
		new Person() { PersonId = 1, Name = "Belinda", Age = 70 },
		new Person() { PersonId = 2, Name = "Betty",  Age = 81 },
		new Person() { PersonId = 3, Name = "Will",  Age = 18 },
		new Person() { PersonId = 4, Name = "Andy", Age = 70 },
		new Person() { PersonId = 5, Name = "Rob", Age = 15 }
	};

	var result = people		.Where(p => p.Age > 20); 	foreach(Person p in result)	{		p.Introduce();	}}

This will give us the following output:

My name is Belinda, and I am 70 years old.
My name is Betty, and I am 81 years old.
My name is Andy, and I am 70 years old.

Using Query Syntax

We can also use the query syntax, to achieve the same outcome:

public static void Main (string[] args) 
{
	var people = new List<Person>() 
	{ 
		new Person() { PersonId = 1, Name = "Belinda", Age = 70 },
		new Person() { PersonId = 2, Name = "Betty",  Age = 81 },
		new Person() { PersonId = 3, Name = "Will",  Age = 18 },
		new Person() { PersonId = 4, Name = "Andy", Age = 70 },
		new Person() { PersonId = 5, Name = "Rob", Age = 15 }
	};

	var result = from p in people 				 where p.Age > 20				 select p; 	foreach(Person p in result)	{		p.Introduce();	}}

This will give us the same output:

My name is Belinda, and I am 70 years old.
My name is Betty, and I am 81 years old.
My name is Andy, and I am 70 years old.
C SharpLINQBack-end