listbox的索引问题

1、UI

2、代码

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace WindowsFormsApplication8
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }


        /// <summary>
        /// 显示当前的listbox的索引值
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void button3_Click(object sender, EventArgs e)
        {
            MessageBox.Show(listBox1.SelectedIndex.ToString());
        }

        /// <summary>
        /// 让蓝条向上，循环移动
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void buttonShang_Click(object sender, EventArgs e)
        {
            if(listBox1.SelectedIndex==0)
            {
                listBox1.SelectedIndex = listBox1.Items.Count-1;
            }
            else
            {
                listBox1.SelectedIndex--;
            }
        }

        /// <summary>
        /// 有了这个默认值，就好像有了参考系一样。简化了很多代码
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void Form1_Load(object sender, EventArgs e)
        {
            //在窗体加载的时候就给Listbox一个默认值吧，这样可以简化代码
            listBox1.SelectedIndex = 0;
        }

        /// <summary>
        /// 让蓝条向下循环移动
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void buttonXia_Click(object sender, EventArgs e)
        {
            if (listBox1.SelectedIndex == listBox1.Items.Count - 1)
            {
                listBox1.SelectedIndex = 0;
            }
            else
            {
                listBox1.SelectedIndex++;
            }
        }
    }
}

3、效果图