A more efficient iterator pattern implementation since C# 1.0

Yes, there is CLR 2.0 now, with C# 2.0 generics and iterators. Yet, the more design tools for your professional designer’s toolbox, the better.

Implementing global::System.Collections.IEnumerable and global::System.Collections.IEnumerator implies some necessary performance costs due boxing and unboxing operations since IEnumerator.Current property returns global::System.Object type, and "Even if the collection contains reference types, you still incur the penalty of the down-casting from Object".

There is an alternative since C# 1.0: design your classes with the same methods as the above interfaces but returning the case-specific type, the CLR will invoke yours as expected, here is a simple example:

 class T
{
  private string[] strs;
  public T(string[] s) { strs = s; }

  public S GetEnumerator() { return new S(this); }

  public class S
  {
    private T t;
    private int index;
    public S(T x) { t = x; index = -1; }
    public bool MoveNext() { ++index; return index < t.strs.Length; }
    public string Current { get { return t.strs[index]; } }
  }
}

class exe
{
  static T g()
  {
    return new T(new string[] { "zero", "one", "two" });
  }
  static void Main()
  {
    foreach (object x in g())
      System.Console.WriteLine(x);
  }
}