LINQ Deep Dive

A Simple Analogy

Imagine organizing a library catalog. Instead of manually walking through shelves, LINQ is like using a powerful search system that lets you filter books by author, sort by date, and transform the results—all with a single query.

What Is LINQ?

LINQ (Language Integrated Query) is a C# feature that provides a unified way to query different data sources (collections, databases, XML) using a SQL-like syntax.

Query Syntax vs. Method Syntax

Query Syntax (SQL-like)

var adults = from person in people
             where person.Age >= 18
             orderby person.Name
             select person;

Method Syntax (Fluent)

var adults = people
    .Where(p => p.Age >= 18)
    .OrderBy(p => p.Name)
    .ToList();

Common LINQ Operators

Filtering

var expensiveProducts = products
    .Where(p => p.Price > 100)
    .ToList();

Ordering

var sorted = people
    .OrderBy(p => p.Age)
    .ThenByDescending(p => p.Name)
    .ToList();

Projection (Select)

var names = people.Select(p => p.Name).ToList();
var details = people.Select(p => new { p.Name, p.Age }).ToList();

Aggregation

int count = numbers.Count();
decimal sum = prices.Sum();
double average = scores.Average();
decimal max = prices.Max();

Grouping

var grouped = people
    .GroupBy(p => p.Department)
    .Select(g => new { Department = g.Key, Count = g.Count() })
    .ToList();

Joins

var result = people
    .Join(orders,
        p => p.Id,
        o => o.PersonId,
        (p, o) => new { p.Name, o.OrderDate })
    .ToList();

Practical Examples

E-commerce Product Filtering

var expensiveInStock = products
    .Where(p => p.Price > 50 && p.Stock > 0)
    .OrderByDescending(p => p.Rating)
    .Select(p => new { p.Name, p.Price, p.Rating })
    .Take(10)
    .ToList();

Data Transformation

var userSummary = users
    .GroupBy(u => u.Country)
    .Select(g => new 
    { 
        Country = g.Key, 
        Total = g.Count(), 
        AgeAverage = g.Average(u => u.Age) 
    })
    .OrderByDescending(x => x.Total)
    .ToList();

Performance Considerations

Deferred execution: LINQ is lazy; nothing executes until you call ToList() or iterate
Avoid multiple enumerations: Call ToList() once if used multiple times
LINQ to Objects vs. LINQ to EF Core: Database queries execute on the server

Related Concepts to Explore

LINQ to SQL and Entity Framework
Async LINQ operations
Expression trees
Custom LINQ operators
Parallel LINQ (PLINQ)

Summary

LINQ transforms data querying from imperative loops to declarative, readable expressions. Master filtering, ordering, grouping, and joins, and you'll write cleaner, more maintainable C# code.

Isaac.