How can I access an IFRAME from the codebehind file in ASP.NET?

<iframe id="contentPanel1" runat="server" />

protected void Page_Load(object sender, EventArgs e)
{
    HtmlControl contentPanel1 = (HtmlControl)this.FindControl("contentPanel1");
    if (contentPanel1 != null)
    	contentPanel1.Attributes["src"] = "http://www.stackoverflow.com";

}

<iframe id="ctl00_ContentPlaceHolder1_contentPanel1"></iframe>

HtmlControl contentPanel1 = (HtmlControl)this.FindControl("ctl00_ContentPlaceHolder1_contentPanel1");

if (contentPanel1 != null)
    contentPanel1.Attributes["src"] = "http://www.clis.com";

<iframe id="contentPanel1" name="contentPanel1" runat="server" />

<iframe id="contentPanel1" runat="server" />

this.contentPanel1.Attributes["src"] = "http://www.stackoverflow.com";

contentPanel1.Attribute = value;

protected virtual Control FindControlRecursive(Control root, String id)
    {
        if (root.ID == id) { return root; }
        foreach (Control c in root.Controls)
        {
            Control t = FindControlRecursive(c, id);
            if (t != null)
            {
                return t;
            }
        }
        return null;
    }