How to Use LINQ with Objects in C# - Simple Guide

You can use LINQ in C# to query collections of objects by writing queries with from, where, and select keywords or using method syntax like .Where() and .Select(). LINQ lets you filter, sort, and transform object lists in a readable and concise way.

Syntax

LINQ queries can be written in two main styles: query syntax and method syntax.

• Query syntax: Uses keywords like from, where, and select to form readable queries.
• Method syntax: Uses extension methods like .Where(), .Select(), and .OrderBy() on collections.

Both styles work on collections of objects that implement IEnumerable<T>.

csharp

var result = from obj in objects
             where obj.Property == value
             select obj;

Example

This example shows how to use LINQ to filter and select objects from a list of Person objects by age.

csharp

using System;
using System.Collections.Generic;
using System.Linq;

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

public class Program
{
    public static void Main()
    {
        List<Person> people = new List<Person>
        {
            new Person { Name = "Alice", Age = 30 },
            new Person { Name = "Bob", Age = 20 },
            new Person { Name = "Charlie", Age = 25 }
        };

        // Query syntax
        var adultsQuery = from person in people
                          where person.Age >= 21
                          select person.Name;

        // Method syntax
        var adultsMethod = people.Where(p => p.Age >= 21).Select(p => p.Name);

        Console.WriteLine("Adults using query syntax:");
        foreach (var name in adultsQuery)
        {
            Console.WriteLine(name);
        }

        Console.WriteLine("Adults using method syntax:");
        foreach (var name in adultsMethod)
        {
            Console.WriteLine(name);
        }
    }
}

Output

Adults using query syntax:
Alice
Charlie
Adults using method syntax:
Alice
Charlie

Common Pitfalls

Some common mistakes when using LINQ with objects include:

• Forgetting to include using System.Linq; which enables LINQ methods.
• Trying to query on null collections, which causes exceptions.
• Confusing query syntax and method syntax mixing without proper parentheses.
• Not understanding deferred execution, so queries run only when enumerated.

csharp

/* Wrong: Missing using directive */
// var adults = people.Where(p => p.Age >= 21);

/* Right: Include using System.Linq; */
using System.Linq;
var adults = people.Where(p => p.Age >= 21);

Quick Reference

LINQ Keyword/Method	Purpose	Example
from	Start a query over a collection	from p in people
where	Filter items by condition	where p.Age > 20
select	Choose what to return	select p.Name
.Where()	Filter items using a lambda	people.Where(p => p.Age > 20)
.Select()	Project items to new form	people.Select(p => p.Name)
.OrderBy()	Sort items ascending	people.OrderBy(p => p.Name)

Key Takeaways

Use LINQ query or method syntax to filter and select objects from collections.

Always include 'using System.Linq;' to access LINQ methods.

LINQ queries run when you enumerate the results, not when you write them.

Be careful to avoid null collections to prevent runtime errors.

LINQ makes object collection queries readable and concise.