form body
var formBody = new Sys.StringBuilder();
formBody.append(this._scriptManagerID + ''='' + this._postBackSettings.panelID + ''&'');

var count = form.elements.length;
for (var i = 0; i < count; i++)
{
//
// Traverse the input elements to construct the form body
//
}

if (this._additionalInput)
{
formBody.append(this._additionalInput);
this._additionalInput = null;
}

var request = new Sys.Net.WebRequest();
//
// prepare the web request object
//

var handler = this._get_eventHandlerList().getHandler("initializeRequest");
if (handler) {
var eventArgs = new Sys.WebForms.InitializeRequestEventArgs(
request, this._postBackSettings.sourceElement);
handler(this, eventArgs);
continueSubmit = !eventArgs.get_cancel();
}

//

this._request = request;
request.invoke();

if (evt) {
evt.preventDefault();
}
}

　　请注意加粗部分的代码。可以发现有两种数据需要被添加为隐藏的输入元素。其一是ScriptManager相关的信息（第一部分的红色代码），其二则是变量“_additionalInput”的内容。我们很容易得到前者的值，但是后者的内容究竟是什么呢？我们继续阅读代码：

function Sys$WebForms$PageRequestManager onFormElementClick(evt)
{
var element = evt.target;
if (element.disabled) {
return;
}

// Check if the element that was clicked on should cause an async postback
this._postBackSettings = this._getPostBackSettings(element, element.name);

if (element.name)
{
if (element.tagName === ''INPUT'')
{
var type = element.type;
if (type === ''submit'')
{
this._additionalInput =
element.name + ''='' + encodeURIComponent(element.value);
}
else if (type === ''image'')
{
var x = evt.offsetX;
var y = evt.offsetY;
this._additionalInput =
element.name + ''.x='' + x + ''&'' + element.name + ''.y='' + y;
}
}
else if ((element.tagName === ''BUTTON'') &&
(element.name.length !== 0) && (element.type === ''submit''))
{
this._additionalInput = element.name + ''='' + encodeURIComponent(element.value);
}
}
}

　　_onFormElmentClick方法会在用户点击form中特定元素时执行。方法会提供变量“_additionalInput”的内容，然后紧接着，我们之前分析过的_onFormSubmit方法会被调用。于是只要我们对WebRequest的body属性进行分析，就能够轻松地得知需要像form中额外添加哪些隐藏输入元素：

_addHiddenElement : function(name, value)
{
var hidden = document.createElement("input");
hidden.name = name;
hidden.value = value;
hidden.type = "hidden";
this._form.appendChild(hidden);
Array.add(this._hiddens, hidden);
},

_addAdditionalHiddenElements : function()
{
var prm = Sys.WebForms.PageRequestManager.getInstance();

this._hiddens = ;

this._addHiddenElement(prm._scriptManagerID, prm._postBackSettings.panelID);
this._addHiddenElement("__UpdatePanelUploading__", "true");

var additionalInput = null;
var element = this._sourceElement;

if (element.name)
{
var requestBody = this.get_webRequest().get_body();
var index = -1;

if (element.tagName === ''INPUT'')
{
var type = element.type;
if (type === ''submit'')
{
index = requestBody.