Request.Form.Keys保存的是什么？

///1.这个Page_Load执行时Keys.Count为0,why?
/// 2.执行按钮事件，TextBox永远存在于Request.Form.Keys中
/// 3.CheckBox不管是服务器端控件还是Html控件，只在被选中时才包括在Keys中
/// 4.执行事件的那个Button也会包括在Request.Form.Keys中
/// 5.只要执行事件默认包括__VIEWSTATE与__EVENTVALIDATION
/// 博客园的高手谁能解释一下？我没有搜索到相关的信息，谢谢
单击Button1之后的效果：

<%@ Page Language="C#" AutoEventWireup="true" CodeFile="FormKeys.aspx.cs" Inherits="test_FormKeys" %>

<!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>Request.Form.Keys保存的是什么？</title>
</head>
<body>
    <form id="form1" runat="server">
    <div>
        <asp:Button ID="Button1" runat="server" Text="Button" OnClick="Button1_Click" />
        <asp:Button ID="Button2" runat="server" Text="Button" OnClick="Button2_Click" />
        <input name="chk1" id="chk1" type="checkbox" runat="server" />
        <asp:TextBox ID="TextBox1" runat="server">hello</asp:TextBox>
        <asp:CheckBox ID="chk2" runat="server" name="chk2" />
        <input id="Submit1" type="submit" value="submit" onclick="form1.submit();" />
    </div>
    </form>
</body>
</html>

using System;
using System.Data;
using System.Configuration;
using System.Collections;
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;
/// <summary>
/// 1.这个Page_Load执行时Keys.Count为0,why?
/// 2.执行按钮事件，TextBox永远存在于Request.Form.Keys中
/// 3.CheckBox不管是服务器端控件还是Html控件，只在被选中时才包括在Keys中
/// 4.执行事件的那个Button也会包括在Request.Form.Keys中
/// 5.只要执行事件默认包括__VIEWSTATE与__EVENTVALIDATION
/// 博客园的高手谁能解释一下？我没有搜索到相关的信息，谢谢
/// </summary>
public partial class test_FormKeys : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
            Show();
    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        Show();
    }
    protected void Button2_Click(object sender, EventArgs e)
    {
        Show();
    }
    private void Show()
    {
        Response.Write(Request.Form.Keys.Count.ToString() + "<br/>");
        for (int i = 0; i < Request.Form.Keys.Count; i++)
        {
            string ctl = Request.Form.GetKey(i);
            Response.Write(ctl + "<br/>");
        }
    }
}