在C#中实现listbox的项上下移动（winform）

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

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();

        }

        private void button1_Click(object sender, EventArgs e)
        {
            try
            {
                string item = listBox1.SelectedItem.ToString();
                int i = listBox1.SelectedIndex;
                if (i == 0)
                    return;
                listBox1.Items.Remove(listBox1.SelectedItem.ToString());
                listBox1.Items.Insert(i - 1, item);
                listBox1.SelectedIndex = i - 1;   
            }
            catch (Exception)
            {
                MessageBox.Show("未选择项！");
            }

        }

        private void button2_Click(object sender, EventArgs e)
        {
            try
            {
                string item = listBox1.SelectedItem.ToString();
                int i = listBox1.SelectedIndex;
                if (i == listBox1.Items.Count - 1)
                    return;
                listBox1.Items.Remove(listBox1.SelectedItem.ToString());
                listBox1.Items.Insert(i + 1, item);
                listBox1.SelectedIndex = i + 1;  
            }
            catch (Exception)
            {

                MessageBox.Show("未选择项！");
            }
    
        }
    }
}