Question

I have a code like:

        class T : IEnumerable, IEnumerator
        {
            private int position = -1;

            public T() { }

            public IEnumerator GetEnumerator() { return this; }

            public object Current { get { return position; } }

            public bool MoveNext()
            {
                position++;
                return (position < 5);
            }

            public void Reset() { position = -1; }
        }

//Using in code:
T t = new T();
foreach (int i in t)
 //to do something

In the code above all is working fine but when I use next:

foreach (int i in t)
   if (i == 2)
     foreach (int p in t)
       //print p
   else
       //print i

It prints (in brackets second loop): 0 1 (3 4) 2 instead of 0 1 (0 1 2 3 4) 2 3 4
I tested It on List and Collection and they do It right.
How can I to achive what I need?

Answer 1

You can’t because you have made your code surface a single enumerator, itself a mistake IMO. A better version would be, for me:

class T : IEnumerable<int> {
    public IEnumerator<int> GetEnumerator() {
        int i = 0;
        while(i < 5) {
            yield return i;
            i++;
        }
    }
    IEnumerator IEnumerable.GetEnumerator() { return GetEnumerator(); }
}

The compiler will create the right devices to achieve this with separate enumerators.

Unless you are writing for .NET 1.1, then if you find yourself manually writing an enumarator, there’s a very good chance that you are doing it the hard way, and getting it wrong as a bonus.

If you really must do it the hard way:

class T : IEnumerable<int>
{
    public T() { }

    public IEnumerator<int> GetEnumerator() { return new TEnumerator(); }
    IEnumerator IEnumerable.GetEnumerator() { return GetEnumerator(); }
    private class TEnumerator : IEnumerator<int>
    {
        private int position = -1;
        public int Current { get { return position; } }
        object IEnumerator.Current { get { return Current; } }
        void IDisposable.Dispose() {}
        public bool MoveNext()
        {
            position++;
            return (position < 5);
        }
        public void Reset() { position = -1; }
    } 
}

The significance here is that different instances of TEnumerator allow the same T instance to be iterated separately.