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.

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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
}

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Common LINQ Methods

Below is a table describing the most commonly used LINQ methods with examples and results:

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
All	Checks if all elements satisfy a condition	numbers.All(num => num > 0)	true
Any	Checks if any element satisfies a condition	numbers.Any(num => num > 3)	true
Contains	Checks if a sequence contains an element	numbers.Contains(3)	true
Distinct	Removes duplicate elements	numbers.Distinct()	{ 1, 2, 3, 4, 5 }
Take	Takes the first N elements	numbers.Take(3)	{ 1, 2, 3 }
Skip	Skips the first N elements	numbers.Skip(3)	{ 4, 5 }
TakeWhile	Takes elements while a condition is true	numbers.TakeWhile(num => num < 3)	{ 1, 2 }
SkipWhile	Skips elements while a condition is true	numbers.SkipWhile(num => num < 3)	{ 3, 4, 5 }
GroupBy	Groups elements by a key	students.GroupBy(s => s.Gender)	Groups by Male and Female
Join	Joins two collections	students.Join(products, s => s.Id, p => p.Id, (s, p) => new { s.Name, p.Name })	Joined results
Aggregate	Performs a custom aggregation	numbers.Aggregate((sum, next) => sum + next)	15 (sum of numbers)
DefaultIfEmpty	Returns default value if sequence is empty	new List<int>().DefaultIfEmpty(-1)	{-1}
Reverse	Reverses the order of elements	numbers.Reverse()	{ 5, 4, 3, 2, 1 }
Intersect	Returns common elements between sequences	numbers1.Intersect(numbers2)	Common elements in both collections
Union	Returns unique elements from both sequences	numbers1.Union(numbers2)	Unique elements combined from both
Except	Returns elements not in the second sequence	numbers1.Except(numbers2)	Elements in numbers1 but not in numbers2
ToDictionary	Converts a sequence into a dictionary	students.ToDictionary(s => s.Id, s => s.Name)	{ 1: "John", 2: "Jane" }

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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
}

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Advanced Example with Classes

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

public void AdvancedExample()
{
    var filteredStudents = students
        .Where(s => s.Age > 20)
        .OrderBy(s => s.Name)
        .Select(s => new { s.Name, s.Age })
        .ToList();

    var groupedByGender = students
        .GroupBy(s => s.Gender)
        .Select(group => new { Gender = group.Key, Count = group.Count() })
        .ToList();

    var availableProducts = products
        .Where(p => p.Stock > 0)
        .OrderByDescending(p => p.Price)
        .Take(3)
        .ToList();

    var studentExists = students.Any(s => s.Name == "Jane Smith");
    var allAdults = students.All(s => s.Age >= 18);
    var uniquePrices = products.Select(p => p.Price).Distinct().ToList();
}

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Summary

• LINQ Basics: Provides a way to query and manipulate collections using both query and method syntax.
• Comprehensive Methods: Methods like Where, Select, OrderBy, GroupBy, Join, Aggregate, and more offer flexibility in handling collections.
• Practical Use: Examples show how LINQ can simplify operations on both simple data types and objects.

LINQ in C# enhances code readability and efficiency, making data operations more intuitive and concise.