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Linq: Beware of the 'Access to modified closure' demon
If you're using Linq and Resharper, you've probably seen the warning Resharper shows when you use a foreach loop in which you use the loop variable in a Linq extension method (be it on IQueryable<T> or IEnumerable<T>). In case you don't know what it is or what damage it can do if you ignore the issue, I'll give you a database oriented query (so on IQueryable<T>, using LLBLGen Pro's Linq provider) which creates a dynamic Where clause based on input, the typical scenario you should be careful with when it comes to this particular problem.