Iterator Pattern

Introduction

The iterator pattern’s role is to provide a way to access aggregate objects sequentially without the knowledge of the structure of the aggregate.
The pattern is widely used in C# and in .NET Framework. We have the IEnumerator and IEnumerable interfaces to help us to implement iterators for aggregates.
When you implement your own aggregate object, you should implement these interfaces to expose a way to traverse your aggregate.

Use Cases for the Iterator Pattern

You should use the pattern in the following cases:

• You need a uniform interface to traverse different aggregate structures.
• You have various ways to traverse an aggregate structure.  
• You don't won't to expose the aggregate object's internal representation.

UML Diagram

Example in C# 

    #region Aggregate Item
    class AggregateItem
    {
        #region Properties
        /// <summary>
        /// The AggregateItem's data
        /// </summary>
        public string Data { get; set; }
        #endregion

        #region Ctor
        /// <summary>
        /// Construct a new AggregateItem with the given data
        /// </summary>
        /// <param name="data">The given data</param>
        public AggregateItem(string data)
        {
            Data = data;
        }
        #endregion
    }
    #endregion

    #region Aggregate Object
    interface Aggregate
    {
        Iterator GetIterator();
    }

    class AggregateImpl : Aggregate
    {
        #region Members
        private readonly List<AggregateItem> _aggregate;
        #endregion

        #region Properties
        /// <summary>
        /// The number of items in the aggregate
        /// </summary>
        public int Count
        {
            get
            {
                return _aggregate.Count;
            }
        }

        /// <summary>
        /// The indexer for the aggregate
        /// </summary>
        public AggregateItem this[int index]
        {
            get
            {
                return _aggregate[index];
            }
            set
            {
                _aggregate[index] = value;
            }
        }
        #endregion

        #region Ctor
        /// <summary>
        /// Construct a new AggregateImpl
        /// </summary>
        public AggregateImpl()
        {
            _aggregate = new List<AggregateItem>();
        }
        #endregion

        #region Aggregate Members
        /// <summary>
        /// Returns the Iterator for this aggregate object.
        /// </summary>
        /// <returns>Iterator</returns>
        public Iterator GetIterator()
        {
            return new IteratorImpl(this);
        }
        #endregion
    }
    #endregion

    #region Iterator
    interface Iterator
    {
        object First();
        object Next();
        bool IsDone();
        object Current();
    }

    class IteratorImpl : Iterator
    {
        #region Members
        private readonly AggregateImpl _aggregate;
        private int _nCurrentIndex;
        #endregion

        #region Iterator Members
        /// <summary>
        /// Return the first object of the iterator.
        /// </summary>
        /// <returns>First object of the iterator</returns>
        public object First()
        {
            return _aggregate[0];
        }

        /// <summary>
        /// Return the current object in the iterator and
        /// advance to the next one.
        /// </summary>
        /// <returns>The next object in the iterator</returns>
        public object Next()
        {
            object result = null;
            if (_nCurrentIndex < _aggregate.Count - 1)
            {
                result = _aggregate[_nCurrentIndex];

                _nCurrentIndex++;

            }

            return result;
        }

        /// <summary>
        /// Returns true if the iteration is done.
        /// </summary>
        /// <returns>True if the iteration is done</returns>
        public bool IsDone()
        {
            return _nCurrentIndex >= _aggregate.Count;
        }

        /// <summary>
        /// Return the current object in the iterator.
        /// </summary>
        /// <returns></returns>
        public object Current()
        {
            return _aggregate[_nCurrentIndex];
        }
        #endregion

        #region Ctor
        /// <summary>
        /// Construct a new IteratorImpl with the given aggregate.
        /// </summary>
        /// <param name="aggregate">The given aggregate</param>
        public IteratorImpl(AggregateImpl aggregate)
        {
            _nCurrentIndex = 0;
            _aggregate = aggregate;
        }
        #endregion
    }
    #endregion

There are 5 players in the example. The first player is an aggregate item which is a simple data structure.
We also have an aggregate interface which has a GetIterator method that returns the iterator. There is an Iterator interface that gives the guidelines of the iterator behavior. I used the two interfaces to implement an aggregate and an iterator.

The IEnumerator and IEnumerable Interfaces

The IEnumerator and the IEnumerable are the ways to implement the iterator pattern in C#.
The IEnumerable interface exposes the enumerator, which supports a simple iteration over a non-generic or generic collection. It is used in the collection itself to expose the functionality of enumerator. The IEnumerable is widely used in LINQ and it is the building block to expose LINQ functionality.
The IEnumerator interface supports a simple iteration over a non-generic or generic collection.
The enumerators are a read only way to traverse a collection. You should use these interfaces in order to implement the iterator pattern in C#. The way to implement them is close to the implementation that
I provided earlier for the iterator pattern. 

Simple Traverse Example 

Even though it is more preferable to use a foreach loop, you can traverse collections with the IEnumerator interface as shown in the following example:

    // build a new string list 
    var strList = new List<string>
                      {
                          "str1",
                          "str2",
                          "str3"
                      };

    // get list enumerator
    IEnumerator<string> enumerator = strList.GetEnumerator();

    // use the enumerator to traverse the list
    // and output the list's items to console
    string str;

    while (enumerator.MoveNext())
    {
        str = enumerator.Current;

        if (!string.IsNullOrEmpty(str))
        {
            Console.WriteLine("{0}", str);
        }
    } 

Summary

To sum up, we are widely using the iterator pattern even if we don’t know it. Whenever you run a foreach loop, the iterator pattern is used underneath the hood.

The LINQ extensions are built upon the IEnumerable interface which is a part of the iterator pattern implementation in .NET Framework.