8.3 LINQ Queries

Let’s go through LINQ queries in C# — a powerful way to query collections in a readable, SQL-like style.

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📌 1. What is LINQ?

• LINQ (Language-Integrated Query) is a feature in C# that allows you to query collections (arrays, lists, XML, databases) using C# syntax.

• Provides type safety and intelliSense support in Visual Studio.

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📌 2. LINQ Syntaxes

There are two main syntaxes:

1. Query Syntax (SQL-like)

2. Method Syntax (Fluent API / Lambda expressions)

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📌 3. Query Syntax Example

using System;
using System.Linq;
using System.Collections.Generic;
 
int[] numbers = { 1, 2, 3, 4, 5, 6 };
 
// Get even numbers
var evenNumbers = from n in numbers
                  where n % 2 == 0
                  select n;
 
foreach (var n in evenNumbers)
    Console.WriteLine(n); // 2, 4, 6

• from → data source

• where → filter condition

• select → projection of data

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📌 4. Method Syntax Example

var evenNumbers = numbers.Where(n => n % 2 == 0).Select(n => n);
 
foreach (var n in evenNumbers)
    Console.WriteLine(n); // 2, 4, 6

• Uses lambda expressions for filtering and selecting

• Method chaining allows for concise and readable queries

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📌 5. Common LINQ Methods

Method	Description
Where(predicate)	Filters elements
Select(selector)	Projects elements (like map)
OrderBy(key)	Sort ascending
OrderByDescending(key)	Sort descending
First() / FirstOrDefault()	Get first element (or default)
Any() / All()	Check conditions
Count()	Number of elements
Distinct()	Remove duplicates
Sum(), Average()	Aggregation

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📌 6. Example with Objects

class Student
{
    public string Name { get; set; }
    public int Age { get; set; }
}
 
List<Student> students = new List<Student>
{
    new Student { Name = "Alice", Age = 20 },
    new Student { Name = "Bob", Age = 22 },
    new Student { Name = "Charlie", Age = 21 }
};
 
// Query students older than 20
var result = from s in students
             where s.Age > 20
             select s.Name;
 
foreach (var name in result)
    Console.WriteLine(name); // Bob, Charlie

• Works on any collection implementing IEnumerable<T>

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📌 7. LINQ Advantages

1. Readable → SQL-like syntax in C#

2. Consistent → Works on arrays, lists, XML, databases

3. Type-safe → Compile-time checking

4. Concise → No need for loops and manual filtering

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📌 8. Summary Table

Syntax	Description
Query Syntax	SQL-like: from ... in ... where ... select ...
Method Syntax	Lambda-based: .Where(...).Select(...)
Common Methods	Where, Select, OrderBy, First, Any, Count
Advantages	Readability, consistency, type safety, concise

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✅ Tip:

• Use query syntax for readability (especially with complex queries)

• Use method syntax for lambda-based chaining and inline functions

• LINQ works seamlessly with arrays, lists, dictionaries, XML, and databases (Entity Framework)