LINQ in C#

# CHAPTER 21

LINQ in C# #

1. Introduction #

LINQ (Language-Integrated Query) revolutionizes this. Introduced in C# 3.0, it allows you to query collections (Lists, Arrays, Databases) using a clean, declarative syntax similar to SQL.

2. Learning Objectives #

• Include the System.Linq namespace.

• Understand Lambda Expressions (=>).

• Filter data using .Where().

• Sort data using .OrderBy().

• Project data using .Select().

3. Lambda Expressions (The Heart of LINQ) #

Syntax: (parameters) => expression *(Read the => symbol as "goes to").*

// "Given an employee 'e', return true if their age is greater than 18"
e => e.Age > 18

4. Filtering Data with .Where() #

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

class Program
{
    static void Main()
    {
        List<int> numbers = new List<int> { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

        // The LINQ Way (Method Syntax)
        // "Filter where n goes to n modulus 2 equals 0"
        var evenNumbers = numbers.Where(n => n % 2 == 0).ToList();

        foreach (int num in evenNumbers)
        {
            Console.WriteLine(num); // Prints 2, 4, 6, 8, 10
        }
    }
}

*Note: LINQ queries return IEnumerable<T>. We use .ToList() at the end to convert the result back into a standard List.*

5. Sorting Data with .OrderBy() #

List<string> names = new List<string> { "Zack", "Alice", "Bob" };

// Sort alphabetically
var sortedNames = names.OrderBy(name => name).ToList(); 

// Sort reverse-alphabetically
var reverseSorted = names.OrderByDescending(name => name).ToList();

6. Projecting Data with .Select() #

class User
{
    public string Name { get; set; }
    public string Email { get; set; }
}

// Assume we have a list of users
List<User> users = GetUsers(); 

// Extract just the emails into a new List<string>
List<string> emails = users.Select(u => u.Email).ToList();

7. Chaining LINQ Methods #

// Find all users named "John", extract their emails, and sort them alphabetically
var johnEmails = users.Where(u => u.Name == "John")
                      .Select(u => u.Email)
                      .OrderBy(e => e)
                      .ToList();

8. Common Mistakes #

• Deferred Execution: A LINQ query is NOT actually executed when you define it. It only executes when you iterate over it (e.g., in a foreach loop) or call a materialization method like .ToList(). If you modify the original list before calling .ToList(), the LINQ query will reflect the modified data! Always append .ToList() if you want to snapshot the data immediately.

• Forgetting using System.Linq;: If you try to call .Where() and get an error saying the method doesn't exist, you forgot the using directive at the top of the file.

9. Best Practices #

• Prefer LINQ over standard for loops when filtering and sorting data. It makes the code instantly readable.

• Be careful with .ToList() inside loops. Calling .ToList() allocates new memory on the Heap. Doing it too often can cause performance issues.

10. Exercises #

1. 1. Create a List<int> with numbers 1 to 20. Use LINQ to find all numbers greater than 10, then sort them descending.

1. 2. Create a List<string> of 5 animal names. Use .Where() to find all names that contain the letter "a".

11. MCQs with Answers #

12. Interview Questions #

• Q: Explain Deferred Execution in LINQ. Why is it useful?

• Q: What is the difference between .FirstOrDefault() and .SingleOrDefault()? (Answer: FirstOrDefault returns the first match or null; SingleOrDefault returns the match, but throws an exception if *more than one* match exists).

13. Summary #

14. Next Chapter Recommendation #