LINQ in depth

In plain English

LINQ, or Language Integrated Query, builds pipelines that filter, project and group collections. Deferred execution means a query runs only when you enumerate it, not when you write it. SelectMany flattens nested collections into one, GroupBy collects items sharing a key, and Join combines two sequences on a matching key. Call ToList or ToArray when you need to materialise the results and reuse them.

Step by step

1. 1Build a query and observe nothing runs until enumeration.
2. 2Flatten nested sequences with SelectMany.
3. 3Group lessons by course with GroupBy.
4. 4Combine courses and lessons with Join.
5. 5Materialise with ToList when the result must be reused.

record Course(int Id, string Name);
record Lesson(int CourseId, string Title, int Minutes);

var courses = new[]
{
    new Course(1, "C#"),
    new Course(2, "C"),
};

var lessons = new[]
{
    new Lesson(1, "Variables", 12),
    new Lesson(1, "Methods", 11),
    new Lesson(2, "Pointers", 15),
    new Lesson(2, "Structs", 14),
};

var query = lessons.Where(lesson => lesson.Minutes > 11);

var flat = courses
    .SelectMany(course => lessons.Where(lesson => lesson.CourseId == course.Id))
    .ToList();

var grouped = lessons
    .GroupBy(lesson => lesson.CourseId)
    .Select(group => new { CourseId = group.Key, Count = group.Count() })
    .ToList();

var joined = courses
    .Join(lessons,
        course => course.Id,
        lesson => lesson.CourseId,
        (course, lesson) => course.Name + ": " + lesson.Title)
    .ToList();

var materialised = query.ToList();

Common mistakes

• Re-enumerating a query re-runs it against the current source data.
• Capturing a loop variable in a query can close over a value you did not expect.
• Materialising too early duplicates work and memory you may not need.
• Mixing side effects into a query makes its behaviour depend on when it runs.