It is difficult for me to imagine a program that doesn't deal with collections of some type – all our applications do proliferate with Arrays, Lists HashSets, DataTables and dozens of others. When writing code in C# the first tool I consider when faced with a collection is LINQ. Its capabilities combined with the fact that LINQ queries can be applied to nearly everything make it a very powerful tool saving a lot of time for any .NET developer. Still, no power comes for free and every piece of equipment must be used with some caution.
One of the paradigms widely used by LINQ is lazy evaluation. Most LINQ methods, although it seems that these just return results immediately, are actually deferred. What it means is that whenever you write something like var strings = myArray.Select(n => n.ToString()); the only thing done at the moment of execution of these lines is creation of a query object. Real work in this case is not approached until someone wants the results. This is a well-known fact about LINQ and generally doesn't do any harm. However, when the processing wrapped into LINQ queries is not pure in the functional sense of the word quite strange and difficult to track down problems may occur. Below I will try to demonstrate such a case.
