Collection Type Guidence

When to use what

Use Arrays if you know the element types and have a fixed length and/or known-up-front collection size that will not change.
Use ArrayList if you have an unkown collection size with either unknown or mixed type elements.
Use a Generic List when know the type of the elements but not the size of the collection.
Use a HashTable if you are going to do key based lookups on a collection and don't know the object type of the elements.
Use a Dictionary<TKey, TValue> you are going to do key based lookups on a collection and you know the type of the elements.
Use a HashSet when you know the type of elements and just want unique values and quick lookups and assignmnets.