如果IEnumerable.GetEnumerator()是不同的返回类型，那么GetEnumerator()的主要实现如何返回GetEnumerator()呢？

public class MyCollection<T> : IEnumerable<T>
{
    private List<T> items;

    public MyCollection()
    {
        items = new List<T>();
    }

    public void Add(T item)
    {
        items.Add(item);
    }

    public IEnumerator<T> GetEnumerator()
    {
        return new MyEnumerator<T>(items);
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        return GetEnumerator();
    }

    private class MyEnumerator<U> : IEnumerator<U>
    {
        private List<U> collection;
        private int currentIndex;

        public MyEnumerator(List<U> collection)
        {
            this.collection = collection;
            currentIndex = -1;
        }

        public U Current => collection[currentIndex];

        object IEnumerator.Current => Current;

        public void Dispose()
        {
            // 清理资源
        }

        public bool MoveNext()
        {
            currentIndex++;
            return currentIndex < collection.Count;
        }

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