aa.aspx.cs
class aa : Page
{
    public string GetStr(){
       return test;
    }
}

aaaa.ascx.cs
class aaaa:UserControl
{
     public aa GetControl(){//返回Page的Handle
           aa c = (aa)this.Parent;
           return aa;
     }
     public string GetPage_GetStr{
         return GetControl().GetStr();//调用Page的GetStr();
     }
}