When to use a HashTable

Question

In C#, I find myself using a List, IList or IEnumerable 99% of the time. Is there a case when it would be b

Answer 1

Use a hashtable, when you need to be able to (quickly) look up items by key.

Of course, you can search through an IList or IEnumerable etc for a matching key but that will take O(n) time rather than O(1) for Hashtable or Dictionary.