Iterator Pattern

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 Memebrs

    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
like 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.