Request取值问题

<head runat="server">
    <title></title>
</head>
<body>
  <input id="ScrWidth" runat="server"  type="text" />
   <script language="JavaScript">
          var strScreenWidth = window.screen.width;
          var strScreenHeight = window.screen.height;
          document.getElementById("ScrWidth").value= strScreenWidth;
          alert(strScreenWidth);  
    </script>
    <form id="form1" runat="server">
    <div>
    
    </div>
 
    </form>
</body>
</html>在到台代码中得到的值为"";但alert(strScreenWidth);  可以有值
   protected void Page_Load(object sender, EventArgs e)
        {
            string strValue = ScrWidth.Value;
            strValue = Request.QueryString["ScrWidth"];
            string strHeigth = ScrWidth.Value;
                  }