Collections and LINQ

After this lesson, you will be able to: Use LINQ method syntax to filter, map, sort, group, and aggregate collections.

LINQ is C#'s killer feature. Once you're fluent, you'll write 5x less collection-manipulation code than in Java or pre-streams JS.

Prerequisites:C# Type System

LINQ method syntax basics

Functional pipelines on IEnumerable<T>.

tsx

using System.Linq;

var nums = new[] { 3, 1, 4, 1, 5, 9, 2, 6, 5, 3, 5 };

// Where: filter
var evens = nums.Where(n => n % 2 == 0);                 // 4, 2, 6

// Select: map
var squared = nums.Select(n => n * n);                   // 9, 1, 16, ...

// OrderBy: sort
var sorted = nums.OrderBy(n => n);                       // 1, 1, 2, 3, 3, ...
var descending = nums.OrderByDescending(n => n);

// Distinct, Take, Skip
var unique = nums.Distinct();                            // 3, 1, 4, 5, 9, 2, 6
var firstFive = nums.Take(5);
var afterFive = nums.Skip(5);

// Aggregations
int sum = nums.Sum();
int max = nums.Max();
double avg = nums.Average();
int count = nums.Count(n => n > 3);

// First, Single, Any, All
int first = nums.First(n => n > 4);                      // throws if no match
int? firstOrNull = nums.FirstOrDefault(n => n > 100);   // 0 (default) if no match
bool any = nums.Any(n => n > 8);
bool all = nums.All(n => n < 10);

Grouping + joining

GroupBy + Join, like SQL.

tsx

var users = new[] {
    new { Name = "Alex",   Dept = "Eng"   },
    new { Name = "Sam",    Dept = "Sales" },
    new { Name = "Jordan", Dept = "Eng"   },
};

// Group by department
var byDept = users.GroupBy(u => u.Dept)
    .Select(g => new { Dept = g.Key, Count = g.Count(), Names = g.Select(u => u.Name) });

foreach (var group in byDept) {
    Console.WriteLine($"{group.Dept}: {group.Count} ({string.Join(", ", group.Names)})");
}

// Join (like SQL inner join)
var depts = new[] {
    new { Code = "Eng",   Label = "Engineering" },
    new { Code = "Sales", Label = "Sales" },
};

var joined = users.Join(
    depts,
    user => user.Dept,
    dept => dept.Code,
    (user, dept) => new { user.Name, dept.Label });

Deferred execution

LINQ is LAZY by default. Queries don't run until enumerated.

tsx

var query = nums.Where(n => {
    Console.WriteLine($"checking {n}");
    return n > 3;
});
// Nothing printed yet!

foreach (var n in query) {     // NOW the lambda runs
    Console.WriteLine($"got {n}");
}

// Convert to list to force immediate execution + cache
var evaluated = query.ToList();        // runs once now
var evaluated2 = query.ToArray();      // runs once now

Common mistakes

Forgetting deferred execution → enumerating a query twice (runs DB queries twice in LINQ-to-SQL). ToList() too eagerly (loses laziness benefit). First() instead of FirstOrDefault() (throws when you didn't expect). Mixing query and method syntax in one expression (readable mess).

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