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How does GroupBy translate and what performance pitfalls exist? 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 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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C# › LINQ

How does LINQ's Aggregate() method work, and how would you use it to implement a custom reduction not covered by Sum()/Average()/Count()?

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Answer

Aggregate() applies an accumulator function CUMULATIVELY across a sequence, carrying forward a running result from each step to the next — useful for custom reductions (like building a concatenated string, computing a running maximum with extra logic, or a custom fold) that don't match any of LINQ's specific built-in aggregate methods.
var words = new[] { "Hello", "World", "LINQ" };
string sentence = words.Aggregate((acc, word) => acc + " " + word);
// "Hello World LINQ"

int product = new[] { 1, 2, 3, 4 }.Aggregate(1, (acc, n) => acc * n); // 24, with a seed value
Real-world example Implementing a custom reduction, like computing a running maximum-so-far or concatenating a list into a single formatted string.

Common follow-ups: What's the difference between the overload of Aggregate() with a seed value versus the one without?

Functional Programming
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