Array is not a member of System.Collections

ArrayList
A dynamic, contiguous collection of objects.
Hashtable
Associative, unordered collection of key-value pairs of objects.
Queue
First-in-first-out (FIFO) collection of objects.
SortedList
Associative, ordered collection of key-value pairs of objects.
Stack
Last-in-first-out (LIFO) collection of objects.