将IEnumerator/IEnumerable实现到自定义队列类中，而不是使用队列

using System;
using System.Collections;

public class CustomQueue<T> : IEnumerable
{
    private T[] items;
    private int front;
    private int rear;

    public CustomQueue()
    {
        items = new T[10];
        front = 0;
        rear = -1;
    }

    public void Enqueue(T item)
    {
        if (rear == items.Length - 1)
        {
            Array.Resize(ref items, items.Length * 2);
        }
        items[++rear] = item;
    }

    public T Dequeue()
    {
        if (IsEmpty())
        {
            throw new InvalidOperationException("Queue is empty");
        }
        T item = items[front++];
        if (front == rear + 1)
        {
            front = 0;
            rear = -1;
        }
        return item;
    }

    public bool IsEmpty()
    {
        return front == rear + 1;
    }

    public int Size()
    {
        return rear - front + 1;
    }

    public IEnumerator GetEnumerator()
    {
        return new CustomQueueEnumerator<T>(items, front, rear);
    }
}

public class CustomQueueEnumerator<T> : IEnumerator
{
    private T[] items;
    private int front;
    private int rear;
    private int current;

    public CustomQueueEnumerator(T[] items, int front, int rear)
    {
        this.items = items;
        this.front = front;
        this.rear = rear;
        current = front - 1;
    }

    public bool MoveNext()
    {
        if (current < rear)
        {
            current++;
            return true;
        }
        return false;
    }

    public void Reset()
    {
        current = front - 1;
    }

    public object Current
    {
        get { return items[current]; }
    }
}

public class Program
{
    public static void Main(string[] args)
    {
        CustomQueue<int> queue = new CustomQueue<int>();
        queue.Enqueue(1);
        queue.Enqueue(2);
        queue.Enqueue(3);

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