如何从ASP.NET窗体DataGrid中检索选定的行

I'm developing new features for an old project developped in ASP.Net Forms/VB.Net and .Net 3.5, one of this features is a batch operation which consists of letting user select multiple rows from a DataGrid (lets say it is GridA) the he press a button to start the operation, the server should display the list of selected items in another grid (lets say its GridB) in a Modal Popup (I'm using ModalPopupExtender from AjaxToolkit) the user should the fill some informations on the popup and then validate the operation using the 'Validate' Button.I'm developing new features for an old project