Custom Action - Get selected List item using C#

Hi,

I've created a custom action for one of my SharePoint lists and the custom action button is linked to an application page.

In the application page we're entering external data values which will then update that list item.

My question is how I can update the selected list item after I click on the custom action and fill in the external data fields in the application page.

I've tried:

SPContext.Current.ListItem - Returns null

SPContext.Current.ListItemID - Returns null

request.QueryString["ID"] - Returns null

Not sure how I can get the current/selected items URL.

Any help is greatly appreciated.

Thanks.

KP

KP