private bool state = false;                                         //定义全局变量标识

        private void cbox_Url_TextChanged(object sender, EventArgs e)

        {

            if (state)

            {

                string importText = cbox_Url.Text;                          //获得输入的文本

                int index = cbox_Url.FindString(importText);                //在ComboBox集合中查找匹配的文本

                if (index >= 0)                                             //有查找到结果时

                {

                    state = false;                                                 //关闭编辑状态

                    cbox_Url.SelectedIndex = index;                              //找到对应项

                    state = true;                                               //打开编辑状态

                    cbox_Url.Select(importText.Length, cbox_Url.Text.Length);   //设置文本选择长度

                }

            }

        }

        private void cbox_Url_KeyDown(object sender, KeyEventArgs e)

        {

            state = (e.KeyCode != Keys.Back && e.KeyCode != Keys.Delete);

            cbox_Url.DroppedDown = true;                                    //弹出下拉列表

        }