LINQ Queries in C# What Is LINQ and How to Use It

Language Integrated Query (LINQ) is one of the most powerful features in C#. It allows you to query, transform, and filter data directly in your C# code using a consistent, expressive syntax.

In this guide, we’ll cover what LINQ is, why it’s useful, how to use it, and provide plenty of practical examples for working with collections, databases, and beyond.

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What Is LINQ?

LINQ stands for Language Integrated Query. It brings SQL-like querying capabilities into C# and .NET languages, allowing you to work with in-memory collections, databases, XML, and more.

Example:

var numbers = new[] { 1, 2, 3, 4, 5 };
var evenNumbers = numbers.Where(n => n % 2 == 0);

You can write concise, readable queries over data — no matter where that data comes from.

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Why Use LINQ?

✅ Unified querying: Whether you work with arrays, lists, databases, or XML, the syntax stays the same.
✅ Type safety: Compile-time checking helps avoid mistakes.
✅ Readable code: More expressive and concise compared to loops or manual filtering.
✅ Composable: You can chain multiple queries together smoothly.

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LINQ Syntax Forms

There are two ways to write LINQ queries:

Query Syntax (SQL-like)

var result = from n in numbers
             where n % 2 == 0
             select n;

Method Syntax (Fluent)

var result = numbers.Where(n => n % 2 == 0);

Both are valid and can be mixed.

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Common LINQ Operations

Filtering with Where

var adults = people.Where(p => p.Age >= 18);

Projection with Select

var names = people.Select(p => p.Name);

Ordering with OrderBy

var sorted = people.OrderBy(p => p.LastName);

Aggregation

var totalAge = people.Sum(p => p.Age);
var averageAge = people.Average(p => p.Age);
var count = people.Count();

Grouping

var grouped = people.GroupBy(p => p.City);
foreach (var group in grouped)
{
    Console.WriteLine($"City: {group.Key}, Count: {group.Count()}");  
}

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LINQ with Collections

List<int> numbers = new() { 1, 2, 3, 4, 5, 6 };

var squaredEvens = numbers
    .Where(n => n % 2 == 0)
    .Select(n => n * n);

foreach (var num in squaredEvens)
{
    Console.WriteLine(num);
}

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LINQ with Entity Framework Core

var expensiveProducts = dbContext.Products
    .Where(p => p.Price > 1000)
    .OrderBy(p => p.Name)
    .ToList();

Important

• LINQ-to-Entities queries are translated to SQL and run on the database.
• Always test performance and understand generated queries.

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LINQ with Anonymous Types

var projected = people.Select(p => new { p.Name, p.Age });

You can project only the fields you need, which is efficient and reduces memory usage.

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Combining Multiple Queries

var query = products
    .Where(p => p.InStock)
    .OrderByDescending(p => p.Rating)
    .Take(10);

You can chain multiple operations for complex querying.

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Best Practices

✅ Use method syntax for flexibility.
✅ Avoid materializing queries too early (use ToList() only when needed).
✅ Be aware of deferred execution: queries don’t run until you enumerate them.
✅ Watch out for performance when querying large datasets.

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Summary

LINQ is a cornerstone of modern C# development, providing a consistent, expressive way to work with data. By mastering LINQ, you gain the ability to write cleaner, more maintainable, and efficient code for a wide range of data sources.