asp.net DoPostBack 引發回傳的原理

asp.net有兩種方法引發回傳

1、通過button,ImageButton直接引發回傳

2、其他控件通過__doPostBack 函數引發回傳

具體原理如下

一、Button和ImageButton直接引發回傳，當在這兩種控件上觸發事件時，控件的ID會作為參數傳給服務器，從而使服務器得知該映射到哪個后端事件
1、Default.aspx
<%@ Page Title="Home Page" Language="C#" MasterPageFile="~/Site.master" AutoEventWireup="true"
    CodeBehind="Default.aspx.cs" Inherits="dopostback._Default" %>

<asp:Content ID="HeaderContent" runat="server" ContentPlaceHolderID="HeadContent">
</asp:Content>
<asp:Content ID="BodyContent" runat="server" ContentPlaceHolderID="MainContent">
    <asp:Button ID="btnQuery1" runat="server" Text="query1"
    onclick="btnQuery_Click" />
    <br />
    <asp:Button ID="btnQuery2" runat="server" Text="query2"
    onclick="btnQuery2_Click" />

</asp:Content>

2、生成Html代碼

<form method="post" action="default.aspx" id="ctl01">
<div class="aspNetHidden">
<input type="hidden" name="__VIEWSTATE" id="__VIEWSTATE" value="/wEPDwUKMTAxODQ0NzY0N2Rk3QmLGwellfxyiCeAIAA4oqYSwo1xlvB7VFxjQu4b4vI=" />
</div>

<div class="aspNetHidden">
    <input type="hidden" name="__EVENTVALIDATION" id="__EVENTVALIDATION" value="/wEWAwKVtZf9DwLGp+bIDgLhkITeCIMB/c9fsbMu0t/9zhPZn6jrsl6JLzQfhKgCAW5b2NYc" />
</div>       
    <input type="submit" name="ctl00$MainContent$btnQuery1" value="query1" id="MainContent_btnQuery1" />
    <br />
    <input type="submit" name="ctl00$MainContent$btnQuery2" value="query2" id="MainContent_btnQuery2" />
</form>

3、點擊query1
submit表單，通過Request.Form,可以取到AllKeys,有三個key
1、__VIEWSTATE
2、__EVENTVALIDATION
3、ctl00$MainContent$btnQuery1
button控件引發PostBack,會把button的Id做為Request.Form的一個Key,通過枚舉key值，查找控件實例，如果控件是button控件，則判斷是由
該button控件引發事件

二、使用__doPostBack的javascript方法來觸發回傳

1、Default2.aspx
<%@ Page Title="" Language="C#" MasterPageFile="~/Site.Master" AutoEventWireup="true" CodeBehind="Default2.aspx.cs" Inherits="dopostback.Default2" %>
<asp:Content ID="Content1" ContentPlaceHolderID="HeadContent" runat="server">
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="MainContent" runat="server">
<asp:LinkButton ID="linkButton1" runat="server" Text="linkButton1" onclick="linkButton1_Click" ></asp:LinkButton>
</asp:Content>

2、Default2.aspx生成的Html
其中，__EVENTTARGET和__EVENTARGUMENT兩個隱藏域會被自動聲明，分別指向觸發事件的控件ID和相應的附加參數

（比如，該控件可觸發多個事件，則附加參數會指明是哪個事件被觸發而引起回傳）

ASP.NET正是通過這兩個隱藏域的值來判斷該如何將客戶端事件映射為對應的服務器端事件的

<form method="post" action="Default2.aspx" id="ctl01">
<div class="aspNetHidden">
<input type="hidden" name="__EVENTTARGET" id="__EVENTTARGET" value="" />
<input type="hidden" name="__EVENTARGUMENT" id="__EVENTARGUMENT" value="" />
<input type="hidden" name="__VIEWSTATE" id="__VIEWSTATE" value="/wEPDwUKMTExNzk1NzM2OWRkw4zwv+DAA5BZznQNZ/w0ocuq/niWGiRu8JdCZ3cteC8=" />
</div>

<script type="text/javascript">
//<![CDATA[
var theForm = document.forms['ctl01'];
if (!theForm) {
    theForm = document.ctl01;
}
function __doPostBack(eventTarget, eventArgument) {
    if (!theForm.onsubmit || (theForm.onsubmit() != false)) {
        theForm.__EVENTTARGET.value = eventTarget;
        theForm.__EVENTARGUMENT.value = eventArgument;
        theForm.submit();
    }
}
//]]>
</script>
<div class="aspNetHidden">
    <input type="hidden" name="__EVENTVALIDATION" id="__EVENTVALIDATION" value="/wEWAgKv5vfMCAL1wdXpD5IlsVoH5XmXvrw/IzUitXA/gZD45wTh4pUMTbi3gkVb" />
</div>           
<a id="MainContent_linkButton1" href="javascript:__doPostBack(&#39;ctl00$MainContent$linkButton1&#39;,&#39;&#39;)">linkButton1</a>
    </form>
</body>
</html>