Collections Tutorial
Implement collections with C#

Date: 2002-April
Author: (luKa)
Environent: .NET Framework v1.0

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Why implement a new collection?

The .Net Framework offers many colletions for all the uses, so why should you need to implement a new collection?

• To implement a business object that's also a container
  In example an Orders class with a 1 to many aggregation relationship with Order class, an Order class with a 1 to many composition relationship with OrderItem class or a Customer class with a simple 1 to many relationship with PaymentOption class.
• To implement an efficent collection of Value-Type elements
  Collectons in the Framework are made for Object type generic elements. To avoid continuous Boxing and Unboxing for Value-Type elements and performance consequences you need a collection with elements of the specific Value-Type in the interface parameters but also in the inner implementation.
  

How to implement a collection with inheritance

In System.Collections and System.Collections.Specialized namespaces there are abstract base classes to inherit from. They have virtual methods you can override, they have weak-typed methods with explicit interface implementation so you can define strong-type methods in your class, they have protected virtual methods you can code to customize the behaviour (validation, etc.) and at the end you can always hide the other methods of the base class with no parameters and only a return value. Here is the list of the abstract base classes you can inherit from:

• CollectionBase
  Use it to implement an IList collection. It implements also ICollection and IEnumerator interfaces. See:
  MS .NET Framework SDK Doc - .NET Framework Class Library - CollectionBase Class     (local)
  CollectionBase is based on an ArrayList so it get the same computational complexity. See:
  MS .NET Framework SDK Doc - .NET Framework Class Library - ArrayList Class     (local)
  
• ReadOnlyCollectionBase
  It's just like CollectionBase without modifiers methods. so interface IList is not exposed. See:
  MS .NET Framework SD1K Doc - .NET Framework Class Library - ReadOnlyCollectionBase Class     (local)
  
• DictionaryBase
  It's a (key, value) pairs collection, so it's an IDictionary collection. It also implement ICollection and IEnumerator interfaces. See:
  MS .NET Framework SDK Doc - .NET Framework Class Library - DictionaryBase Class     (local)
  DictionaryBase is based on an HashTable so it get the same computational complexity. See:
  MS .NET Framework SDK Doc - .NET Framework Class Library - HashTable Class     (local)
  
• NameObjectCollectionBase
  It's a (key, value) pairs collection that permit many values associated to a single key. It just implemens ICollection and IEnumerator interfaces. See:
  MS .NET Framework SDK Doc - .NET Framework Class Library - NameObjectCollectionBase Class     (local)
  NameObjectCollectionBase is based on an HashTable so it get the same computational complexity. See:
  MS .NET Framework SDK Doc - .NET Framework Class Library - HashTable Class     (local)
  
  

See examples:
- COLLECTIONBASE.CS inherits from CollectionBase.
   An utility is available to automagically generate the source code:  CollGen.zip for Beta2 and CollGen.zip for Release 1 (local)
- READONLYCB.CS inherits from ReadOnlyCollectionBase.
- DICTIONARYBASE.CS inherits from DictionaryBase.

How to implement a collection with containment

Write your own implementation of the collection from scratch, just implement the interfaces needed by a collection: all the collections must implement IEnumerable interface that's extended by ICollection. Then you can choose from two interfaces derived from ICollection: IDictionary and IList. IList interface is for collections of values. The interface IDictionary is for collections of (Key, Value) pairs.



The VS.NET Add-In ".NET CollectionGen" is available to automagically generate source code. See:
    http://www.sellsbrothers.com/tools/default.aspx  
Download Add-In from:
    http://www.sellsbrothers.com/tools/collectiongen.zip   (local)

See examples:
- INNERARRAY.CS use Add-In generated code for an IList collection with an inner strong-typed Array.
- INNERHASH.CS use Add-In generated code for an IDictionary collection with an inner HashTable.

How to choose between collection implementation with inheritance or containment

Inheritance is better suited to model structural facts that are immutable, it's an Is-A relationship. Containment is suited to model non-structural facts, it's an Has-A relationship.

• With inheritance you need to write less code
• Containment let you improve performance avoiding Boxing/Unboxing
• Containment let you full control and pose no limitation, i.e. you can code a class that implemnt 2 different collections
  
• Containment protect you from future changes that MS could (even if sholdn't...) made to interfaces of base classes for collections.

Here is the decision tree:

1. If you need a Value-Type collection (structure included) with excellent performances, use containment
2. otherwise... if you need to implements multiple collections in a class, use containments
3. otherwise... use inheritance.

If you are in doubt... implement th collection with inheritance.