Asp.net error creating usercontrol from WebMethod

Question

I'm referencing the article reference

To create a UserControl dynamically and pass it back to a page via jQuery.

When I just do a simple text in the UserControl "Hello World" everything works great... however, when I tried putting interactive controls on the control, I got the following message in my WebMethod (so the jQuery call stuff is irrelevant) (further testing showed that textboxes, and buttons cause the issue, but not labels)

Error executing child request for handler 'System.Web.UI.Page'

here's the webmethod:

[WebMethod]
public static string GetCtrl(string cname)
{
    StringWriter sw = new StringWriter();

    Page p = new Page();
    Control c = p.LoadControl("~/Controls/" + cname);
    p.Controls.Add(c);

    try
    {
        HttpContext.Current.Server.Execute(p, sw, false);
    }
    catch (Exception err)
    {
        return err.Message;
    }
    sw.Close();

    return sw.ToString();
}

The exception is happening during the Server.Execute call.

A solution may not be possible, I'd just like to know why interactive controls cause this.

EDIT: Taking advice below I looked deeper and found the inner exception said something to the effect that it must be within a form tag marked with runat="server", while I'm displaying this control on a aspx page within the form tag... my guess is that there's got to be some sort of 'asp.net magic' connection there for the control to operate... regardless, it'd be nice to know how to render a page with controls on it anyway (even if it wasn't usable in this situation)

So I looked at the output of the above code and for those curious:

<span id="ctl00_lbl1">abc: </span>

So the control is rendering properly (with just the label id='lbl1') on the page, however, the control is itself rendered, and the page content is not there at all... Is there a way to generate the page source as well? (I must not understand the way the Server.Execute function works)

Answer 1

I would try rendering the control and sending that string back...

Control ctrl;
StringBuilder sb = new StringBuilder();
StringWriter tw = new StringWriter(sb);
HtmlTextWriter hw = new HtmlTextWriter(tw);

ctrl.RenderControl(hw);
return sb.ToString();

Answer 2

I'd start by narrowing down the problem. For instance, what happens if you leave off the

p.Controls.Add(c);

What happens if you properly implement a using statement?

using (StringWriter sw = new StringWriter()) {
    // ...
    HttpContext.Current.Server.Execute(p, sw, false);
    return sw.ToString();
}

What happens, if you leave off the try/catch? It's costing you all the information in the exception other than the Message property, so why not just leave it off? Your caller might be expecting HTML in any case, so if you keep it, you might (later) want to wrap the output:

return "<p>" + ex.ToString() + "</p>";

Little by little, remove things until the problem goes away.

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Based on what you learned from the InnerException, I recommend that you add an HtmlForm control to the page, then add your loaded control as a child control of the HtmlForm.