Iterator Design Pattern

This short article is to show you how to build Iterator Design Pattern in c#.

The pattern sequentially access the elements of a collection (

This pattern provides a way to access the elements of an aggregate object sequentially without exposing its underlying representation.

The logical diagram of Iterator Design Pattern as follow.

First, create an Iterator interface to define an interface for accessing and traversing elements.

interface Iterator


    object First();

    object Next();

    bool IsDone();

    object CurrentItem();



Then create a •  ConcreteIterator inherit from the Iterator interface. This class implements the Iterator interface and keeps track of the current position in the traversal of the aggregate.

class ConcreteIterator : Iterator


    private ConcreteAggregate _aggregate;

    private int _current = 0;


    // Constructor

    public ConcreteIterator(ConcreteAggregate aggregate)


        this._aggregate = aggregate;



    // Gets first iteration item

    public object First()


        return _aggregate[0];



    // Gets next iteration item

    public object Next()


        object ret = null;

        if (_current < _aggregate.Count – 1)


            ret = _aggregate[++_current];



        return ret;



    // Gets current iteration item

    public object CurrentItem()


        return _aggregate[_current];



    // Gets whether iterations are complete

    public bool IsDone()


        return _current >= _aggregate.Count;




Now, create a structure for storing data, the Aggregate interface that defines an interface for creating an Iterator object.

interface Aggregate


    Iterator CreateIterator();



Finally, create a concrete class derive from the class of Aggregate, ConcreteAggregate. This class implements the Iterator creation interface to return an instance of the proper ConcreteIterator.

class ConcreteAggregate : Aggregate


    private ArrayList _items = new ArrayList();


    public Iterator CreateIterator()


        return new ConcreteIterator(this);



    // Gets item count

    public int Count


        get { return _items.Count; }



    // Indexer

    public object this[int index]


        get { return _items[index]; }

        set { _items.Insert(index, value); }




The main class is like below.


static void Main()


    ConcreteAggregate a = new ConcreteAggregate();

    a[0] = “Item A”;

    a[1] = “Item B”;

    a[2] = “Item C”;

    a[3] = “Item D”;


    // Create Iterator and provide aggregate

    ConcreteIterator i = new ConcreteIterator(a);


    Console.WriteLine(“Iterating over collection:”);


    object item = i.First();

    while (item != null)



        item = i.Next();



    // Wait for user




You can also download a single solution from The sample file name is My MSN ID is

About chanmingman

Since March 2011 Microsoft Live Spaces migrated to Wordpress ( till now, I have is over 1 million viewers. This blog is about more than 50% telling you how to resolve error messages, especial for Microsoft products. The blog also has a lot of guidance teaching you how to get stated certain Microsoft technologies. The blog also uses as a help to keep my memory. The blog is never meant to give people consulting services or silver bullet solutions. It is a contribution to the community. Thanks for your support over the years. Ming Man is Microsoft MVP since year 2006. He is a software development manager for a multinational company. With 25 years of experience in the IT field, he has developed system using Clipper, COBOL, VB5, VB6, VB.NET, Java and C #. He has been using Visual Studio (.NET) since the Beta back in year 2000. He and the team have developed many projects using .NET platform such as SCM, and HR based applications. He is familiar with the N-Tier design of business application and is also an expert with database experience in MS SQL, Oracle and AS 400.
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