combobox 添加"请选择"

  /// <summary>
        /// 绑定combobox的数据
        /// </summary>
        public void bind()
        {
            //从数据库获取要绑定到下拉框的数据
            DataTable dt =BLL.GetData().Tables[0];
            if (dtList.Rows.Count == 0)
            {
                this.ComboBox.Items.Add(new System.Collections.DictionaryEntry("--无数据--", 0));
                //new System.Collections.DictionaryEntry("--无数据--", 0) --------->创建键值对
            }
            else//
            {
                this.ComboBox.Items.Add(new System.Collections.DictionaryEntry("--请选择--", 0));
                foreach (DataRow dr in dtList.Rows)
                {
                    this.ComboBox.Items.Add(new System.Collections.DictionaryEntry(dr["Title"].ToString(), int.Parse(dr["ID"].ToString())));//用取出数据创建值对
                }
            }
           
            this.ComboBox.DisplayMember = "key";//显示值
            this.ComboBox.ValueMember = "value";//数据值
            this.ComboBox.SelectedIndex = 0;//默认显示
        }
        /// <summary>
        /// 选择事件
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void ComboBox_SelectedIndexChanged(object sender, EventArgs e)
        {
            if (this.ComboBox.SelectedIndex != 0)//不是默认选项
            {
                int vaule = int.Parse(((System.Collections.DictionaryEntry)ComboBox.SelectedItem).Value.ToString());//取得选择项的值
                MessageBox.Show(vaule.ToString());
            }
            else
            {
                //默认选项则不做任何操作
            }
        }