C# Iterator

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

Frequency of use:
high
C# Design Patterns

UML class diagram

A visualization of the classes and objects participating in this pattern.


Participants

The classes and objects participating in this pattern include:

  • Iterator  (AbstractIterator)
    • defines an interface for accessing and traversing elements.
  • ConcreteIterator  (Iterator)
    • implements the Iterator interface.
    • keeps track of the current position in the traversal of the aggregate.
  • Aggregate  (AbstractCollection)
    • defines an interface for creating an Iterator object
  • ConcreteAggregate  (Collection)
    • implements the Iterator creation interface to return an instance of the proper ConcreteIterator

Structural code in C#

This structural code demonstrates the Iterator pattern which provides for a way to traverse (iterate) over a collection of items without detailing the underlying structure of the collection.

using System;
using System.Collections.Generic;

namespace Iterator.Structural
{
    /// <summary>
    /// Iterator Design Pattern
    /// </summary>

    public class Program
    {
        public static void Main(string[] args)
        {
            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

            Iterator i = a.CreateIterator();

            Console.WriteLine("Iterating over collection:");

            object item = i.First();

            while (item != null)
            {
                Console.WriteLine(item);
                item = i.Next();
            }

            // Wait for user

            Console.ReadKey();
        }
    }

    /// <summary>
    /// The 'Aggregate' abstract class
    /// </summary>

    public abstract class Aggregate
    {
        public abstract Iterator CreateIterator();
    }

    /// <summary>
    /// The 'ConcreteAggregate' class
    /// </summary>

    public class ConcreteAggregate : Aggregate
    {
        List<object> items = new List<object>();

        public override Iterator CreateIterator()
        {
            return new ConcreteIterator(this);
        }

        // Get item count

        public int Count
        {
            get { return items.Count; }
        }

        // Indexer

        public object this[int index]
        {
            get { return items[index]; }
            set { items.Insert(index, value); }
        }
    }

    /// <summary>
    /// The 'Iterator' abstract class
    /// </summary>

    public abstract class Iterator
    {
        public abstract object First();
        public abstract object Next();
        public abstract bool IsDone();
        public abstract object CurrentItem();
    }

    /// <summary>
    /// The 'ConcreteIterator' class
    /// </summary>

    public class ConcreteIterator : Iterator
    {
        ConcreteAggregate aggregate;
        int current = 0;

        // Constructor

        public ConcreteIterator(ConcreteAggregate aggregate)
        {
            this.aggregate = aggregate;
        }

        // Gets first iteration item

        public override object First()
        {
            return aggregate[0];
        }

        // Gets next iteration item

        public override object Next()
        {
            object ret = null;
            if (current < aggregate.Count - 1)
            {
                ret = aggregate[++current];
            }

            return ret;
        }

        // Gets current iteration item

        public override object CurrentItem()
        {
            return aggregate[current];
        }

        // Gets whether iterations are complete

        public override bool IsDone()
        {
            return current >= aggregate.Count;
        }
    }
}
Output
Iterating over collection:
Item A
Item B
Item C
Item D

Real-world code in C#

This real-world code demonstrates the Iterator pattern which is used to iterate over a collection of items and skip a specific number of items each iteration.

using System;
using System.Collections.Generic;

namespace Iterator.RealWorld
{
    /// <summary>
    /// Iterator Design Pattern
    /// </summary>

    public class Program
    {
        public static void Main(string[] args)
        {
            // Build a collection

            Collection collection = new Collection();
            collection[0] = new Item("Item 0");
            collection[1] = new Item("Item 1");
            collection[2] = new Item("Item 2");
            collection[3] = new Item("Item 3");
            collection[4] = new Item("Item 4");
            collection[5] = new Item("Item 5");
            collection[6] = new Item("Item 6");
            collection[7] = new Item("Item 7");
            collection[8] = new Item("Item 8");

            // Create iterator

            Iterator iterator = collection.CreateIterator();

            // Skip every other item

            iterator.Step = 2;

            Console.WriteLine("Iterating over collection:");

            for (Item item = iterator.First();
                !iterator.IsDone; item = iterator.Next())
            {
                Console.WriteLine(item.Name);
            }

            // Wait for user

            Console.ReadKey();
        }
    }
    /// <summary>
    /// A collection item
    /// </summary>

    public class Item
    {
        string name;

        // Constructor

        public Item(string name)
        {
            this.name = name;
        }

        public string Name
        {
            get { return name; }
        }
    }

    /// <summary>
    /// The 'Aggregate' interface
    /// </summary>

    public interface IAbstractCollection
    {
        Iterator CreateIterator();
    }

    /// <summary>
    /// The 'ConcreteAggregate' class
    /// </summary>

    public class Collection : IAbstractCollection
    {
        List<Item> items = new List<Item>();

        public Iterator CreateIterator()
        {
            return new Iterator(this);
        }

        // Gets item count

        public int Count
        {
            get { return items.Count; }
        }

        // Indexer

        public Item this[int index]
        {
            get { return items[index]; }
            set { items.Add(value); }
        }
    }

    /// <summary>
    /// The 'Iterator' interface
    /// </summary>

    public interface IAbstractIterator
    {
        Item First();
        Item Next();
        bool IsDone { get; }
        Item CurrentItem { get; }
    }

    /// <summary>
    /// The 'ConcreteIterator' class
    /// </summary>

    public class Iterator : IAbstractIterator
    {
        Collection collection;
        int current = 0;
        int step = 1;

        // Constructor

        public Iterator(Collection collection)
        {
            this.collection = collection;
        }

        // Gets first item

        public Item First()
        {
            current = 0;
            return collection[current] as Item;
        }

        // Gets next item

        public Item Next()
        {
            current += step;
            if (!IsDone)
                return collection[current] as Item;
            else
                return null;
        }

        // Gets or sets stepsize

        public int Step
        {
            get { return step; }
            set { step = value; }
        }

        // Gets current iterator item

        public Item CurrentItem
        {
            get { return collection[current] as Item; }
        }

        // Gets whether iteration is complete

        public bool IsDone
        {
            get { return current >= collection.Count; }
        }
    }
}
Output
Iterating over collection:
Item 0
Item 2
Item 4
Item 6
Item 8

.NET Optimized code in C#

The .NET optimized code demonstrates the same real-world situation as above but uses modern, built-in .NET features, such as, generics, reflection, LINQ, lambda functions, etc. You can find an example on our Singleton pattern page.

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