How is this Dictionary exception possible?

Question

Given the following stack trace:

MESSAGE: Value cannot be null.Parameter name: key  
SOURCE: mscorlib  
TARGETSITE:          


        

Answer 1

I've had this problem happen frequently because I made the mistake of allowing multiple threads to access the same dictionary. Make sure that this is not the case, because Dictionary is not thread-safe.

(Incidentally, your method can be greatly simplified. Dictionary is already an IEnumerable>. You should be able to just do ToArray on one.