How to handle an “infinite” IEnumerable?

Question

A trivial example of an \"infinite\" IEnumerable would be

IEnumerable Numbers() {
  int i=0;
  while(true) {
    yield return unchecked(i++);
  }
}
         


        

Answer 1

Yes, you are guaranteed that the code above will be executed lazily. While it looks (in your code) like you'd loop forever, your code actually produces something like this:

IEnumerable Numbers()
{
    return new PrivateNumbersEnumerable();
}

private class PrivateNumbersEnumerable : IEnumerable
{
    public IEnumerator GetEnumerator() 
    { 
        return new PrivateNumbersEnumerator(); 
    }
}

private class PrivateNumbersEnumerator : IEnumerator
{
    private int i;

    public bool MoveNext() { i++; return true; }   

    public int Current
    {
        get { return i; }
    }
}

(This obviously isn't exactly what will be generated, since this is pretty specific to your code, but it's nonetheless similar and should show you why it's going to be lazily evaluated).