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();
asp.net输出 png 32位 图像，带透明alpha。
// pngtest.htm
<html>
<head></head>
<body bgColor="gray">
<img src="png.ashx" />
</body>
</html>
// png.ashx
<%@ WebHandler Language="C#" Class="Png" %>
using System.Web;
public class Png : IHtt ......
