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What is the difference between First(), FirstOrDefault(), Single(), and SingleOrDefault()? What is LINQ, and how does it simplify querying collections compared to writing manual loops? What is the difference between IEnumerable<T>, IQueryable<T>, and List<T>, and how does choosing the wrong one hurt performance against a database? How does GroupBy translate and what performance pitfalls exist? How does LINQ to Objects differ fundamentally from LINQ to Entities (Entity Framework), particularly regarding how the query is actually executed? How would you write a custom LINQ extension method that integrates seamlessly with the standard method-chaining style, like a 'DistinctBy' before it existed natively? How does LINQ's Aggregate() method work, and how would you use it to implement a custom reduction not covered by Sum()/Average()/Count()? How would you write an efficient LINQ query combining Join() with GroupBy() to replicate a SQL-style LEFT OUTER JOIN with aggregation? How would you optimize a LINQ-to-Entities query to avoid the classic 'N+1 query' performance problem when accessing related navigation properties?
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Why can LINQ cause the N+1 query problem and how do you fix it?

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Answer

Accessing a navigation property inside a loop triggers a query per item. Fix it by eager-loading with Include or projecting the needed data in one query.
// N+1: foreach (var o in orders) use(o.Customer.Name);
var data = db.Orders.Include(o => o.Customer).ToList();
Real-world example A report that reads each order's customer separately hammers the DB; Include batches it.
All 18 LINQ questions