LINQ in C#

1. Why LINQ? Stop Writing Foreach Hell

Kunal has 1000 pizza orders. He wants veg pizzas over ₹500. Without LINQ: create list, foreach, if checks, add to result. With LINQ: one line.

var expensiveVeg = orders.Where(o => o.IsVeg && o.Price > 500).ToList();

2. Two Flavors: Method vs Query Syntax

// Method syntax
var names = orders.Where(o => o.IsVeg).Select(o => o.CustomerName);

// Query syntax - compiles to same thing
var names = from o in orders
            where o.IsVeg
            select o.CustomerName;

Pick one. Industry uses Method syntax 90% of the time.

3. Deferred Execution: The Big Idea

LINQ doesn't run until you enumerate. Prerna builds a query but doesn't call ToList(). No DB hit yet. Only when Kaushal does foreach or ToList() does it execute once.

var query = orders.Where(o => o.IsVeg); // Nothing runs
var result = query.ToList(); // NOW it runs

1. Why Do We Need LINQ?

Before LINQ, you wrote different code for arrays, lists, XML, databases. LINQ unifies it. One pattern to query anything implementing IEnumerable<T> or IQueryable<T>.

2. Core Operators: Filter, Project, Order

var summary = orders
  .Where(o => o.Date > DateTime.Today.AddDays(-7)) // Last 7 days
  .OrderByDescending(o => o.Price) // Expensive first
  .Select(o => new { o.Id, o.CustomerName, o.Price }) // Anonymous type
  .Take(10); // Top 10

3. IQueryable vs IEnumerable: Remote vs Local

Karan calls .Where() on DbSet<Order>. EF builds SQL: SELECT * FROM Orders WHERE IsVeg = 1. Only matching rows come over network.

4. Join & GroupBy: Relating Data

var byTopping = orders.GroupBy(o => o.Topping)
                     .Select(g => new { Topping = g.Key, Count = g.Count() });

5. Performance & Memory: What Actually Runs

LINQ to Objects creates iterators. No extra lists until you materialize. Where yields items one by one. Memory usage = O(1) until ToList(). But OrderBy must buffer everything to sort. If Kashvee runs OrderBy on 10M items, heap explodes. Do it in SQL with IQueryable instead.