When to use a HashTable

Question

In C#, I find myself using a List<T>, IList<T> or IEnumerable<T> 99% of the time. Is there a case when it would be better to use a HashTable (or Dictionary<T,T> in 2.0 and above) over these?

Edit:

As pointed out, what someone would like to do with the collection often dictates what one should be using, so when would you use a Hashtable/Dictonary<T,T> over a List<T>?

Answer 1

Maybe not directly related to the OPs question, but there's a useful blog post about which collection structure to use at: SortedSets

Basically, what you want to do with the collection determines what type of collection you should create.

To summarise in more detail:

• Use IList if you want to be able to enumerate and / or modify the collection (normally adding at end of list)
• Use IEnumeration if you just want to enumerate the collection (don't need to add / remove - usually used as a return type)
• Use IDictionary if you want to access elements by a key (adding / removing elements quickly using a key)

• Use SortedSet if you want to access a collection in a predefined order (most common usage being to access the collection in order)

• Overall, use Dictionary if you want to access / modify items by key in no particular order (preferred over list as that's generally done in order, preferred over enumeration as you can't modify an enumeration, preferred over hashtable as that's not strictly typed, preferred over sortedlist when you don't need keys sorted)