LINQ (Language Integrated Query) lets you query collections in C# using readable, concise, and SQL-like syntax. It works on arrays, lists, databases, XML, and more. With LINQ, you can filter, sort, group, and transform data using built-in functions.

LINQ Syntax Styles

C# offers two main styles for writing LINQ:

Common LINQ Functions

Here’s a list of LINQ functions you’ll use most often, with short examples:

1. Where – Filter items by condition

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

2. Select – Transform each item

var squares = numbers.Select(n => n * n);

3. OrderBy / OrderByDescending – Sort items

var sorted = numbers.OrderBy(n => n);             // Ascending
var desc = numbers.OrderByDescending(n => n);     // Descending

4. First / FirstOrDefault – Get the first item

var firstEven = numbers.First(n => n % 2 == 0);
var orDefault = numbers.FirstOrDefault(n => n > 10); // Returns default (0 for int) if not found

5. Any / All – Boolean checks

bool hasEven = numbers.Any(n => n % 2 == 0);
bool allPositive = numbers.All(n => n > 0);

6. Count / Sum / Max / Min / Average

int count = numbers.Count();
int sum = numbers.Sum();
int max = numbers.Max();
double avg = numbers.Average();

7. Distinct – Remove duplicates

var unique = numbers.Distinct();

8. Take / Skip

var first3 = numbers.Take(3); // First 3 items
var skip2 = numbers.Skip(2);  // Skip first 2

9. GroupBy – Group items by key

var grouped = names.GroupBy(n => n[0]);
foreach (var group in grouped)
{
    Console.WriteLine($"Group: {group.Key}");
    foreach (var name in group)
        Console.WriteLine(name);
}

10. Join – Join two collections

var result = students.Join(
    grades,
    student => student.Id,
    grade => grade.StudentId,
    (student, grade) => new { student.Name, grade.Score }
);

Practical Example

using System;
using System.Linq;

class Program
{
    static void Main()
    {
        int[] numbers = { 5, 2, 9, 1, 5, 6 };

        var query = numbers
                    .Where(n => n > 3)
                    .Distinct()
                    .OrderBy(n => n)
                    .Select(n => $"Number: {n}");

        foreach (var item in query)
        {
            Console.WriteLine(item);
        }
    }
}

Output:

Number: 5  
Number: 6  
Number: 9

LINQ Method Cheat Sheet

Summary

Next Steps

1. Learn about deferred execution
2. Explore LINQ to SQL or Entity Framework
3. Try writing LINQ queries with complex data types