怎样才能动态给<iframe runat=server id=mainFrm name=mainFrm></iframe>的src进行付值？

用webmatrix写了一个
<%@ Page Language="C#" %>
<script runat="server">    // Insert page code here
    //
    void Page_Load(object sender,EventArgs e)
    {
       StringBuilder strBuilder = new StringBuilder();
       strBuilder.Append("<script language='javascript'>");
       strBuilder.Append("document.getElementById('iframe1').src='http://www.csdn.net'; <");
       strBuilder.Append("/script>");
       RegisterStartupScript("onload",strBuilder.ToString());
    }</script>
<html>
<head>
</head>
<body>
    <form runat="server">
        <!-- Insert content here -->
        <iframe runat=server id="iframe1"></iframe>
    </form>
</body>
</html>