如何检查组合框c#未选择相同的索引

using System;
using System.Windows.Forms;

namespace ComboBoxExample
{
    public partial class Form1 : Form
    {
        private int previousIndex; // 保存之前的索引值

        public Form1()
        {
            InitializeComponent();
            previousIndex = comboBox1.SelectedIndex; // 初始化保存的索引值
            comboBox1.SelectedIndexChanged += ComboBox1_SelectedIndexChanged; // 绑定事件处理程序
        }

        private void ComboBox1_SelectedIndexChanged(object sender, EventArgs e)
        {
            ComboBox comboBox = (ComboBox)sender;
            int selectedIndex = comboBox.SelectedIndex;

            if (selectedIndex == previousIndex)
            {
                // 用户未选择新的索引
                MessageBox.Show("请选择不同的索引！");
            }
            else
            {
                // 用户选择了新的索引
                MessageBox.Show("选中的索引为：" + selectedIndex);
                previousIndex = selectedIndex; // 更新保存的索引值
            }
        }
    }
}