C# 5.0 Unleashed

Lazy Evaluation

Their piecemeal on-demand evaluation characteristic makes iterators an ideal construct to express queries. Because the underlying data source can potentially change after you have declared a query expression, delaying the execution is a much desirable feature:

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var nums = new List<int> { 2, 5, 7, 4, 1, 9 };var evens = from n in nums            where n % 2 == 0            select n;// Shows 2, 4foreach (var n in evens)    Console.WriteLine(n);// Somehow the collection changes, e.g. because the user edits the data.nums.Add(5);nums.Add(8);// Shows 2, 4, 8 - no need to redeclare the queryforeach (var n in evens)    Console.WriteLine(n);

The query expression used here is compiled into a call to the Where ...