how i opening an iframe codebehind record asp.net?

i am perplexing set attributes an iframe html control code-behind aspx.cs file.

i came opposing says findcontrol non-asp controls using:

the aspx record contains:

<iframe id="contentpanel1" runat="server" />

and following code-behind record contains:

protected vacant page_load(object sender, eventargs e)
{
    htmlcontrol contentpanel1 = (htmlcontrol)this.findcontrol("contentpanel1");
    (contentpanel1 != null)
        contentpanel1.attributes["src"] = "http://www.stack .com";

}

except it's awaiting control, contentpanel1 null.

---

update 1

looking during rendered html:

<iframe id="ctl00_contentplaceholder1_contentpanel1"></iframe>

i attempted changing code-behind to:

htmlcontrol contentpanel1 = (htmlcontrol)this.findcontrol("ctl00_contentplaceholder1_contentpanel1");

if (contentpanel1 != null)
    contentpanel1.attributes["src"] = "http://www.clis.com";

but didn't help.

i am controlling masterpage.

---

update 2

changing aspx record to:

<iframe id="contentpanel1" name="contentpanel1" runat="server" />

also didn't help

---

answer

the answer obvious, unworthy even seeking uncanny question. have aspx code:

<iframe id="contentpanel1" runat="server" />

and wish opening iframe code-behind file, only opening it:

this.contentpanel1.attributes["src"] = "http://www.stack .com";