requiredfield validator dynamically

 Private Sub GridView7_RowDataBound1(sender As Object, e As System.Web.UI.WebControls.GridViewRowEventArgs) Handles GridView7.RowDataBound
        If e.Row.RowType = DataControlRowType.EmptyDataRow Then
            Exit Sub
        End If
        If e.Row.RowType = DataControlRowType.DataRow Then
            Dim ddl As DropDownList = TryCast(e.Row.FindControl("dropdownlist1"), DropDownList)

......

How can I add new requiredfield validator for the Ddl, dynamically? thanks