ECMA-334: 26. Iterators

Microsoft .NET Framework, ASP.NET, Visual C# (CSharp, C Sharp, C-Sharp) Developer Training, Visual Studio

26. Iterators

An iterator is a means of implementing a function member whose return type is an enumerator interface or enumerable interface. The function member returns the ordered sequence of values as yielded by the iterator.

[Example: The following Stack<T> class implements its GetEnumerator method using an iterator. The iterator enumerates the elements of the stack in top to bottom order.

using System.Collections.Generic;
public class Stack<T>: IEnumerable<T>
{
   T[] items;
   int count;
   public void Push(T data) {…}
   public T Pop() {…}
   public IEnumerator<T> GetEnumerator() {
      for (int i = count – 1; i >= 0; --i) {
         yield return items[i];
      }
   }
}

Iterators are implemented using yield statements, which can only be used in methods whose return type is an enumerator interface. The presence of the GetEnumerator method makes Stack<T> an enumerable type, allowing instances of Stack<T> to be used in a foreach statement. The following example pushes the values 0 through 9 onto an integer stack and then uses a foreach loop to display the values in top to bottom order.

using System;
class Test
{
   static void Main() {
      Stack<int> s = new Stack<int>();
      for (int i = 0; i < 10; i++) s.Push(i);
      foreach (int i in s) Console.Write("{0} ", i);
      Console.WriteLine();
   }
}

The output of the example is:

9 8 7 6 5 4 3 2 1 0

end example]