Private nested class

Why?
1. Framework, implements IEnumerable pattern as nested class. Of course these type of class has no other purpose other than parent class needs.

class MyUselessList : IEnumerable {
    // ...
    private List internalList;
    private class UselessListEnumerator : IEnumerator {
        private MyUselessList obj;
        public UselessListEnumerator(MyUselessList o) {
           obj = o;
        }
        private int currentIndex = -1;
        public int Current {
           get { return obj.internalList[currentIndex]; }
        }
        public bool MoveNext() {
           return ++currentIndex < obj.internalList.Count;
        }
    }
    public IEnumerator GetEnumerator() {
        return new UselessListEnumerator(this);
    }
}

2. A fully lazy implementation for Singleton pattern(http://suresh-anothernetprogrammer.blogspot.com/2011/11/singleton.html)

3. to continue:):):)