asp.net 的ComboBox 可输入可选择下拉列表

文章出处：http://www.cnblogs.com/ejiyuan/archive/2007/11/09/954325.html
using System;
using System.ComponentModel;
using System.Web;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Collections;
using System.Web.UI.Design;
[assembly: TagPrefix("AtomNet.Web.UI.WebControls", "AtomNet")]
namespace AtomNet.Web.UI.WebControls
{
    /// <summary>
    /// Represents a control that allows the user to select a single item from a drop-down list,
    /// or type in a new value that is not in the list.
    /// </summary>
    /// <remarks>
    /// Since the ComboBox control inherits from <see cref="System.Web.UI.WebControls.ListBox"/>,
    /// you can use it in exactly the same manner (including DataBinding).
    /// For older browsers that do not support the functionality required to render this control,
    /// a normal dropdown box will be emitted instead.
    /// </remarks>
    [ToolboxData("<{0}:ComboBox runat=\"server\"></{0}:ComboBox>")]
    [ValidationPropertyAttribute("SelectedValue")]
    [Designer(typeof(AtomNet.Web.UI.WebControls.ComboBoxDesigner))]
    public class ComboBox : System.Web.UI.WebControls.ListBox
    {
        #region 私有属性
        /// <summary>
        /// The <see cref="System.Web.UI.WebControls.TextBox"/> used internally to allow the user
        /// to type in new values.
        /// </summary>
        TextBox textBox = new TextBox();