2010-08-27-Spot-the-bug.md

layout	status	title	slug
post	publish	Spot the bug	Spot-the-bug

What is wrong with the code below?

Safe Assumptions:

1. _dictionary is a valid non-null Dictionary<object,object>

2. _dictionary contains items that will match the passed expression

3. This code compiles with no warnings or errors

4. This code will throw an exception at runtime.

   public IList<TModel> GetAllByCriteria<TModel> ( Expression<Func<TModel, bool>> criteria ) { Func<TModel, bool> action = criteria.Compile(); return _dictionary.Where( pair => action( (TModel)pair.Value ) ).Cast<TModel>().ToList(); }

If you don’t see it right away then you’re not alone. I spent a while debugging to catch this. If you do see it right away then pat yourself on the back.