找到页面中用户控件所包含在用户控件中的控件FindControl("用户控件")

            System.Web.UI.UserControl uc = (System.Web.UI.UserControl)FindControl("ProductCategory1");
            DropDownList ddl1 = (DropDownList)uc.FindControl("ddlType1");
            DropDownList ddl2 = (DropDownList)uc.FindControl("ddlType2");
            DropDownList ddl3 = (DropDownList)uc.FindControl("ddlType3");
            DropDownList ddl4 = (DropDownList)uc.FindControl("ddlType4");