Disable GridView Controls in Edit Mode

I have a business requirement stating that when the Location Type is "C" the
user should not be able to update the StoreNumber. So, when the user clicks
the "edit" button in the GridView for a row, I need to check the value of
location type for that row and then disable the control for the Store Number.

The problem is I only need to do this when editing. I have put the logic in
the DataRowBound event. In the RowEditing event a application variable is
set indicating that the mode if "Edit".

Is there a better way to do this? Is there something already built into the
GridView that could make this checking easier? Thanks in advance for any
help!!