我们知道，无论是ASP.NET1.x,2.0，甚至是以后的版本，ASP.NET最终Render到Client端通过浏览器浏览的都是一样：一个单纯的HTML。Client通过Submit Form的方式将填入Form的数据提交给Server进行处理。我们现在来看看ASP.NET整个Postback程序处理的过程。
首先我们通过一个Sample来看ASP.NET如何处理一个通过Click一个Button引起的Postback。下面是Web Page的HTML：
复制代码 代码如下:
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default.aspx.cs" Inherits="_Default" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title>Test Page</title>
</head>
<body>
<form id="form1" runat="server">
<div>
<asp:Label runat="server" ID="LabelMessage" ForeColor="red"></asp:Label>
</div>
<div>
<asp:Button runat="server" ID="Button1" Text="Button1" OnClick="Button1_Click" OnCommand="Button_Command" CommandArgument="Button1" />
<asp:Button runat="server" ID="Button2" Text="Button2" OnClick="Button2_Click" OnCommand="Button_Command" CommandArgument="Button2" UseSubmitBehavior="false" />
<asp:Button runat="server" ID="Button3" Text="Button3" OnClick="Button3_Click" OnCommand="Button_Command" CommandArgument="Button3" UseSubmitBehavior="false" />
</div>
</form>
</body>
</html>
很简单，定义了3个Button，分别注册了他们的两个Event：Click和Command。3个Button的Command Event Hander是一样的：Button_Command，通过指定的CommandArgument来让Event Handler判断到底是哪个Button触发了Command Event。
下面是Code Behind：
复制代码 代码如下:
using System;
using System.Data;
using System.Configuration;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
public partial class _Default : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
}
protected void Button1_Click(object sender, EventArgs e)
{
string message = string.Format("The {0} event of {1} is fired", "Click", "Button1");
this.LabelMessage.Text = message;
}
protected void Button2_Click(object sender, EventArgs e)
{
string message = string.Format("The {0} event of {1} is fired", "Click", "Button2");
this.LabelMessage.Text = message;
}
protected void Button3_Click(object sender, EventArgs e)
{
string message = string.Format("The {0} event of {1} is fired", "Click", "Button3");
this.LabelMessage.Text = message;
}
protected void Button_Command(object sender, CommandEventArgs e)
{
string message = string.Format("The {0} event of {1} is fired", "Command", e.CommandArgument);
this.LabelMessage.Text += "; " + message;
}
}
我们来运行这个Page，并Click某个按钮（比如Button2）： 
我们通过最上方的Message可以看出，Button2的Click Event和Command先后触发。
这篇Blog的主旨就是从方法调用的角度讲述整个程序运行的过程：从HTML 被Render到Client端，到用户Click某个按钮，输入被Postback到Server端，并触发两个Event，执行Event Handler打印出相关的Message。
首先我们来看看ASP.NET设计的Page Render到Client端的HTML是什么样子：
复制代码 代码如下:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title>
Test Page
</title>
</head>
<body>
<form name="form1" method="post" action="Default.aspx" id="form1">
<div>
<input type="hidden" name="__EVENTTARGET" id="__EVENTTARGET" value="" />
<input type="hidden" name="__EVENTARGUMENT" id="__EVENTARGUMENT" value="" />
<input type="hidden" name="__VIEWSTATE" id="__VIEWSTATE" value="/wEPDwUKMTA0NDQ2OTE5OWRk281L4eAk7iZT10hzg+BeOyoUWBQ=" />
</div>
<script type="text/javascript">
<!--
var theForm = document.forms['form1'];
if (!theForm) {
theForm = document.form1;
}
function __doPostBack(eventTarget, eventArgument) {
if (!theForm.onsubmit || (theForm.onsubmit() != false)) {
theForm.__EVENTTARGET.value = eventTarget;
theForm.__EVENTARGUMENT.value = eventArgument;
theForm.submit();
}
}
// -->
</script>
<div>
<span id="LabelMessage" style="color:Red;"></span>
</div>
<div>
<input type="submit" name="Button1" value="Button1" id="Button1" />
<input type="button" name="Button2" value="Button2" onclick="javascript:__doPostBack('Button2','')" id="Button2" />
<input type="button" name="Button3" value="Button3" onclick="javascript:__doPostBack('Button3','')" id="Button3" />
</div>
</form>
</body>
</html>
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