Dictionaries are similar to lists, but excel at quickly retrieving a specific value with a key. Internally, they are implemented with a hash table. There's the legacy Hashtable class (in the System.Collections.NonGeneric package) but this is not type safe, whereas Dictionary<T> is a generic type, so you probably shouldn't use Hashtable unless you're porting old code to .NET Core. The same applies to ArrayList, which is the legacy, non-generic version of List.
A dictionary can look up a value with a key very quickly, whereas a list will need to go through all the values until the key is found. You can, however, still enumerate a dictionary as it is ordered, although this isn't really how it is intended to be used. If you don't ...
