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Selecting a Collection Class 

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 generic class if you need first-in-first-out (FIFO) behavior. Consider using the Stack class or the Stack 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 generic class offer FIFO access.

    • The Stack class and the Stack generic class offer LIFO access.

    • The LinkedList 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?

  • 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 generic interface.

    • One key and one value: Use any of the collections based on the IDictionary interface or the IDictionary generic interface.

    • One value with embedded key: Use the KeyedCollection 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 and SortedList generic classes sort their elements by the key, based on implementations of the IComparer interface and the IComparer generic interface.

    • ArrayList provides a Sort method that takes an IComparer implementation as a parameter. Its generic counterpart, the List generic class, provides a Sort method that takes an implementation of the IComparer 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 SortedDictionary generic class provides faster lookup than the Dictionary 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.

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