模拟IEnumerable <T>类型的GetEnumerator()方法

using System;
using System.Collections;
using System.Collections.Generic;

class CustomCollection<T> : IEnumerable<T>
{
    private List<T> _items = new List<T>();

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

    public IEnumerator<T> GetEnumerator()
    {
        foreach (T item in _items)
        {
            yield return item;
        }
    }

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

class Program
{
    static void Main(string[] args)
    {
        CustomCollection<int> collection = new CustomCollection<int>();
        collection.Add(1);
        collection.Add(2);
        collection.Add(3);

        foreach (int item in collection)
        {
            Console.WriteLine(item);
        }
    }
}