C#窗体--下拉列表框和列表框的增删改

前言：完成如下图

一。下拉列表框

 public Form2()
        {
   
   
            InitializeComponent();//由系统生成的对于窗体界面的定义方法,初始化窗体组件的
        }
        public int index = -1;//设置
        public int index1 = -1;
        public int index2 = -1;
        private void btnProp_Click(object sender, EventArgs e)
        {
   
   
            //comboBox1.DropDownStyle = ComboBoxStyle.DropDownList;//文本部分不可编辑
            comboBox1.DropDownStyle = ComboBoxStyle.DropDown;
            string[] items = {
   
    "1", "2", "3", "4" };
            comboBox1.Items.AddRange(items);

            comboBox1.SelectedIndex = 2; //设置
           /* comboBox1.SelectedIndex = comboBox1.Items.Count - 1;//为什么要-1？以为0页计数*/
        }

        private void btnAdd1_Click(object sender, EventArgs e)
        {
   
   
            //判断输入内容是否重复
            //comboBox1.Text = "添加的内容";
            if (  string.IsNullOrEmpty(comboBox1.Text))
            {
   
   
                MessageBox.Show("没有输入内容","提示",MessageBoxButtons.OK);
                return;
            }
            if (comboBox1.Items.Contains(comboBox1.Text))
            {
   
   
                MessageBox.Show("该项已存在，重新输入");
                return;
            }
            comboBox1.Items.Add(comboBox1.Text);
            comboBox1.SelectedItem = comboBox1.Text;
        }

        private void btnSearch1_Click(object sender, EventArgs e)
        {
   
   
            if (string.