LINQ Basics

C# LINQ

LINQ (Language Integrated Query) in C# provides a streamlined and readable way to query collections of data. It integrates seamlessly with arrays, lists, XML, databases, and other collections, allowing developers to write expressive and concise code.

Key Components of LINQ

1. Query Syntax

Similar to SQL syntax.

Example:

int[] numbers = { 1, 2, 3, 4, 5 };
var evenNumbers = from num in numbers
                  where num % 2 == 0
                  select num;

foreach (var num in evenNumbers)
{
    Console.WriteLine(num); // Output: 2, 4
}

2. Method Syntax

Uses extension methods.

Example:

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

foreach (var num in evenNumbers)
{
    Console.WriteLine(num); // Output: 2, 4
}

Common LINQ Methods

Method	Description	Example	Result
Where	Filters a sequence	numbers.Where(num => num > 2)	{ 3, 4, 5 }
Select	Projects values	numbers.Select(num => num * 2)	{ 2, 4, 6, 8, 10 }
OrderBy	Sorts in ascending order	numbers.OrderBy(num => num)	{ 1, 2, 3, 4, 5 }
OrderByDescending	Sorts in descending order	numbers.OrderByDescending(num => num)	{ 5, 4, 3, 2, 1 }
First	Returns the first element	numbers.First()	1
FirstOrDefault	Returns first element or default	numbers.FirstOrDefault(num => num > 5)	0 (default for int)
Sum	Calculates the sum of elements	numbers.Sum()	Sum of numbers
Max	Finds the maximum element	numbers.Max()	Maximum value
Min	Finds the minimum element	numbers.Min()	Minimum value
Average	Calculates the average	numbers.Average()	Average value
Count	Counts the number of elements	numbers.Count()	Number of elements

Combining LINQ Methods

Example:

int[] numbers = { 1, 2, 3, 4, 5 };
var filteredNumbers = numbers.Where(num => num > 2)
                             .OrderByDescending(num => num)
                             .Select(num => num * 2);

foreach (var num in filteredNumbers)
{
    Console.WriteLine(num); // Output: 10, 8, 6
}

Advanced Example with Classes

Let's use the provided Student and Product classes to illustrate LINQ in action:

public class Test
{
    List<Student> students = new List<Student>
    {
        new Student
        {
            Id = 1,
            Name = "John Doe",
            FatherName = "Robert Doe",
            Age = 20,
            Address = "123 Elm Street, Springfield",
            DateOfBirth = new DateTime(2003, 5, 15),
            MobileNo = "123-456-7890",
            Gender = "Male"
        },
        new Student
        {
            Id = 2,
            Name = "Jane Smith",
            FatherName = "Michael Smith",
            Age = 22,
            Address = "456 Oak Avenue, Metropolis",
            DateOfBirth = new DateTime(2001, 3, 10),
            MobileNo = "987-654-3210",
            Gender = "Female"
        },
        new Student
        {
            Id = 3,
            Name = "Alice Johnson",
            FatherName = "William Johnson",
            Age = 19,
            Address = "789 Pine Road, Gotham",
            DateOfBirth = new DateTime(2004, 8, 20),
            MobileNo = "555-666-7777",
            Gender = "Female"
        },
        new Student
        {
            Id = 4,
            Name = "Mark Lee",
            FatherName = "Thomas Lee",
            Age = 21,
            Address = "101 Maple Drive, Star City",
            DateOfBirth = new DateTime(2002, 12, 5),
            MobileNo = "222-333-4444",
            Gender = "Male"
        }
    };

    List<Product> products = new List<Product>
    {
        new Product
        {
            Id = 1,
            Code = "P1001",
            Name = "Wireless Mouse",
            Price = 25.99,
            Stock = 120,
            Description = "Ergonomic wireless mouse with USB receiver.",
            Discount = 0.10m,
            ProductCategory = "Electronics"
        },
        new Product
        {
            Id = 2,
            Code = "P1002",
            Name = "Gaming Keyboard",
            Price = 79.99,
            Stock = 75,
            Description = "RGB mechanical gaming keyboard.",
            Discount = 0.15m,
            ProductCategory = "Electronics"
        },
        new Product
        {
            Id = 3,
            Code = "P1003",
            Name = "Coffee Mug",
            Price = 9.99,
            Stock = 200,
            Description = "Ceramic coffee mug with handle.",
            Discount = 0.05m,
            ProductCategory = "Home & Kitchen"
        },
        new Product
        {
            Id = 4,
            Code = "P1004",
            Name = "Notebook",
            Price = 2.49,
            Stock = 500,
            Description = "100-page spiral notebook for students.",
            Discount = 0.00m,
            ProductCategory = "Stationery"
        }
    };

    public void Example()
    {
        var lst = students.Where(x => x.Age > 20).ToList();
        var lstFemale = students.Where(x => x.Gender == "Female").ToList();
        var lstMale = students.Where(x => x.Gender == "Male").ToList();
        var lstOrder = students.OrderBy(x => x.Name).ToList();
        var max = students.Max(x => x.Age);
        var dob = students.Where(x => x.DateOfBirth.Year <= 2003).ToList();
        var stock = products.Where(x => x.Stock < 100).ToList();
        var price = products.Where(x => x.Price > 5).ToList();
        var order2 = products.OrderByDescending(x => x.Price).ToList();
        var order3 = products.OrderByDescending(x => x.Stock).ToList();
        var order4 = products.OrderBy(x => x.Stock).ToList();
        var totalAmount = products.Sum(x => Convert.ToDecimal(x.Price * 10));

        products.Add(new Product
        {
            Id = 5,
            Code = "P1009",
            Name = "Gaming Keyboard",
            Price = 800,
            Stock = 1,
            Description = "RGB mechanical gaming keyboard.",
            Discount = 0.15m,
            ProductCategory = "Electronics"
        });
    }
}

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Summary

• LINQ Basics: Provides a way to query and manipulate collections using both query and method syntax.

• Common Methods: Include Where, Select, OrderBy, First, Sum, Max, etc.

• Examples: Demonstrate how to filter, sort, and aggregate data using LINQ.

LINQ in C# enhances code readability and efficiency, making data operations more intuitive and concise. It's an essential tool for any C# developer.