Loade usercontrol i App_code og rendere indholdet

public class Class1 : System.Web.UI.Page
{
    public string Foo()
    {
        StringBuilder sb = new StringBuilder();
        StringWriter sw = new StringWriter(sb);
        HtmlTextWriter  wr = new HtmlTextWriter(sw);
        Control control = LoadControl("~/WebUserControl.ascx");
        control.RenderControl(wr);
        string s = sb.ToString();
    }
}

Tak for din kommentar, smid svar så er der point!

svar