When does a dictionary throw an IndexOutOfRangeException on Add or ContainsKey?

Question

On a busy ASP .NET website, I have a Dictionary, which acts as a cache, basically storing key/value pairs for later retrieval.

On high load, the Dictionary some time

Answer 1

Here's a thread-safe dictionary http://devplanet.com/blogs/brianr/archive/2008/09/26/thread-safe-dictionary-in-net.aspx also check out this SO question.