Notes

class MainApp
{
/// <summary>
/// Entry point into console application.
/// </summary>
static void Main()
{
// Create and item collection
var collection = new ItemCollection<Item>
{
new Item{ Name = "Item 0"},
new Item{ Name = "Item 1"},
new Item{ Name = "Item 2"},
new Item{ Name = "Item 3"},
new Item{ Name = "Item 4"},
new Item{ Name = "Item 5"},
new Item{ Name = "Item 6"},
new Item{ Name = "Item 7"},
new Item{ Name = "Item 8"}
};

Console.WriteLine("Iterate front to back");
foreach (var item in collection)
{
Console.WriteLine(item.Name);
}

Console.WriteLine("nIterate back to front");
foreach (var item in collection.BackToFront)
{
Console.WriteLine(item.Name);
}
Console.WriteLine();

// Iterate given range and step over even ones
Console.WriteLine("nIterate range (1-7) in steps of 2");
foreach (var item in collection.FromToStep(1, 7, 2))
{
Console.WriteLine(item.Name);
}
Console.WriteLine();

// Wait for user
Console.ReadKey();
}
}

/// <summary>
/// The 'ConcreteAggregate' class
/// </summary>
/// <typeparam name="T">Collection item type</typeparam>
class ItemCollection<T> : IEnumerable<T>
{
private List<T> _items = new List<T>();

public void Add(T t)
{
_items.Add(t);
}

// The 'ConcreteIterator'
public IEnumerator<T> GetEnumerator()
{
for (int i = 0; i < Count; i++)
{
yield return _items[i];
}
}

public IEnumerable<T> FrontToBack
{
get { return this; }
}

public IEnumerable<T> BackToFront
{
get
{
for (int i = Count - 1; i >= 0; i--)
{
yield return _items[i];
}
}
}

public IEnumerable<T> FromToStep(int from, int to, int step)
{
for (int i = from; i <= to; i = i + step)
{
yield return _items[i];
}
}

// Gets number of items
public int Count
{
get { return _items.Count; }
}

// System.Collections.IEnumerable member implementation
IEnumerator IEnumerable.GetEnumerator()
{
return GetEnumerator();
}
}

/// <summary>
/// The collection item
/// </summary>
class Item
{
// Gets or sets item name
public string Name { get; set; }
}
}