LINQ advanced

Advanced LINQ combines grouping, joins, and aggregation—expressing data transforms declaratively instead of nested loops. Understanding IEnumerable<T> and deferred execution prevents subtle bugs in production pipelines.

GroupBy and aggregation

var words = new[] { "cat", "car", "bat" };
var byLetter = words.GroupBy(w => w[0]);
foreach (var g in byLetter) {
    Console.WriteLine($"{g.Key}: {g.Count()}");
}

First, Single, Any

nums.First(n => n > 10);   // throws if none
nums.FirstOrDefault(n => n > 10);
nums.Any(n => n < 0);

Choose FirstOrDefault when absence is valid—First throws InvalidOperationException.

Important interview questions and answers

1. Q: IEnumerable vs IQueryable?
   A: IEnumerable runs in memory; IQueryable can be translated to remote providers (SQL)—don't mix blindly.
2. Q: Materialize when?
   A: Call ToList or ToArray when you need stable snapshots or multiple passes.

Self-check

1. What does GroupBy return?
2. First vs FirstOrDefault?

Pitfall: IQueryable builds expression trees for providers like EF—do not mix arbitrary in-memory logic that cannot translate to SQL.