Be sure to choose your System.Collections class carefully. Using the wrong type can restrict your use of the collection.

Consider the following questions:

• Do you need a sequential list where the element is typically discarded after its value is retrieved?

  • If yes, consider using the Queue class or the Queue<(Of <(T>)>) generic class if you need first-in-first-out (FIFO) behavior. Consider using the Stack class or the Stack<(Of <(T>)>) generic class if you need last-in-first-out (LIFO) behavior.

  • If not, consider using the other collections.

• Do you need to access the elements in a certain order, such as FIFO, LIFO, or random?

  • The Queue class and the Queue<(Of <(T>)>) generic class offer FIFO access.

  • The Stack class and the Stack<(Of <(T>)>) generic class offer LIFO access.

  • The LinkedList<(Of <(T>)>) generic class allows sequential access either from the head to the tail or from the tail to the head.

  • The rest of the collections offer random access.

• Do you need to access each element by index?

  • The ArrayList and StringCollection classes and the List<(Of <(T>)>) generic class offer access to their elements by the zero-based index of the element.

  • The Hashtable, SortedList, ListDictionary, and StringDictionary classes, and the Dictionary<(Of <(TKey, TValue>)>) and SortedDictionary<(Of <(TKey, TValue>)>) generic classes offer access to their elements by the key of the element.

  • The NameObjectCollectionBase and NameValueCollection classes, and the KeyedCollection<(Of <(TKey, TItem>)>) and SortedList<(Of <(TKey, TValue>)>) generic classes offer access to their elements by either the zero-based index or the key of the element.

• Will each element contain one value, a combination of one key and one value, or a combination of one key and multiple values?

  • One value: Use any of the collections based on the IList interface or the IList<(Of <(T>)>) generic interface.

  • One key and one value: Use any of the collections based on the IDictionary interface or the IDictionary<(Of <(TKey, TValue>)>) generic interface.

  • One value with embedded key: Use the KeyedCollection<(Of <(TKey, TItem>)>) generic class.

  • One key and multiple values: Use the NameValueCollection class.

• Do you need to sort the elements differently from how they were entered?

  • The Hashtable class sorts its elements by their hash codes.

  • The SortedList class and the SortedDictionary<(Of <(TKey, TValue>)>) and SortedList<(Of <(TKey, TValue>)>) generic classes sort their elements by the key, based on implementations of the IComparer interface and the IComparer<(Of <(T>)>) generic interface.

  • ArrayList provides a Sort method that takes an IComparer implementation as a parameter. Its generic counterpart, the List<(Of <(T>)>) generic class, provides a Sort method that takes an implementation of the IComparer<(Of <(T>)>) generic interface as a parameter.

• Do you need fast searches and retrieval of information?

  • ListDictionary is faster than Hashtable for small collections (10 items or fewer). The Dictionary<(Of <(TKey, TValue>)>) generic class provides faster lookup than the SortedDictionary<(Of <(TKey, TValue>)>) generic class.

• Do you need collections that accept only strings?

  • StringCollection (based on IList) and StringDictionary (based on IDictionary) are in the System.Collections.Specialized namespace.

  • In addition, you can use any of the generic collection classes in the System.Collections.Generic namespace as strongly typed string collections by specifying the String class for their generic type arguments.