Question

After moving to .NET 2.0+ is there ever a reason to still use the systems.Collections namespace (besides maintaining legacy code)? Should the generics namespace always be used instead?

Answer 1

For the most part, the generic collections will perform faster than the non-generic counterpart and give you the benefit of having a strongly-typed collection. Comparing the collections available in System.Collections and System.Collections.Generic, you get the following ‘migration’:

     Non-Generic               Generic Equivalent     ------------------------------------------------------------     ArrayList                 List<T>     BitArray                  N/A     CaseInsensitiveComparer   N/A     CollectionBase            Collection<T>     Comparer                  Comparer<T>     DictionaryBase            Dictionary<TKey,TValue>     Hashtable                 Dictionary<TKey,TValue>     Queue                     Queue<T>     ReadOnlyCollectionBase    ReadOnlyCollection<T>     SortedList                SortedList<TKey,TValue>     Stack                     Stack<T>      DictionaryEntry           KeyValuePair<TKey,TValue>      ICollection               N/A (use IEnumerable<T> or anything that extends it)     IComparer                 IComparer<T>     IDictionary               IDictionary<TKey,TValue>     IEnumerable               IEnumerable<T>     IEnumerator               IEnumerator<T>     IEqualityComparer         IEqualityComparer<T>     IList                     IList<T> 

ICollection is immutable (no members to change the contents of the collection) while ICollection<T> is mutable. This makes the interfaces similar in name only while ICollection and IEnumerable<T> differ by very little.

From this list, the only non-generic classes that don’t have a generic counterpart are BitArray and CaseInsensitiveComparer.