LINQ Basics

Language Integrated Query: Consistent querying over in-memory objects, XML, EF contexts, etc.

Namespaces

using System.Linq;

Query Styles

Method Syntax (fluent) vs Query Expression (SQL-like).

var scores = new[] { 78, 92, 66, 88 };
var high = scores.Where(s => s >= 80)
                 .OrderByDescending(s => s)
                 .Select(s => new { Score = s, Pass = s >= 70 });

Query Expression:

var high2 = from s in scores
            where s >= 80
            orderby s descending
            select new { Score = s, Pass = s >= 70 };

Key Operators

Category	Examples
Filtering	Where
Projection	Select, SelectMany
Ordering	OrderBy, ThenBy, Reverse
Grouping	GroupBy
Joining	Join, GroupJoin
Quantifiers	Any, All, Contains
Aggregation	Count, Sum, Average, Min, Max, Aggregate
Element	First, FirstOrDefault, Single, ElementAt
Set	Distinct, Union, Intersect, Except
Generation	Range, Repeat, Empty

Deferred vs Immediate

Most operators are deferred (executed upon enumeration). Terminal operations: ToList(), ToArray(), Count(), First(), etc.

Execution Pipeline

Source -> Where -> Select -> ToList ENUMERATES.

Custom Projection Example

var people = new List<Person>{ new("Ana", 22), new("Bill", 30) };
var summary = people.Select(p => new { p.Name, AgeGroup = p.Age < 25 ? "Junior" : "Adult" });

Grouping Example

var grouped = people.GroupBy(p => p.Age < 25 ? "Junior" : "Adult");
foreach (var g in grouped)
{
    Console.WriteLine($"{g.Key}: {g.Count()} persons");
}

Pitfalls

Multiple enumeration of IEnumerable (cache with ToList)
Using First() when sequence may be empty
OrderBy cost on large datasets

When to Use

Transform collections succinctly
Filter / aggregate without manual loops
Prepare data for UI or reports

Next Steps

Explore LINQ to Objects vs LINQ to Entities (EF Core) and differences (deferred translation, supported method set).