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How do GroupBy() and its resulting IGrouping<TKey, TElement> work? 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? Why can LINQ cause the N+1 query problem and how do you fix it? 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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How does GroupBy translate and what performance pitfalls exist?

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Answer

GroupBy partitions by a key into IGrouping<TKey,T>. In LINQ-to-Objects it buffers; in EF Core only certain shapes translate to SQL GROUP BY, otherwise it fails or evaluates client-side.
var byCat = products.GroupBy(p => p.Category)
   .Select(g => new { g.Key, Total = g.Sum(p => p.Price) });
Real-world example Summing sales per region for a dashboard — ensure it translates to SQL for large tables.
All 18 LINQ questions