I have a gridview where the user can edit some data. I have one column that contains a drop down list and based on the selection in that ddl a comment box appears and requires text to be entered into it.

I'm not sure where to write this code however. This is basically the code I'm looking for but I can't find the controls (ddl or txtbox)

Code:
txtComments.visible=false
If ddlStatus.selectedvalue = "113" or ddlStatus.selectedvalue = "114" then
  txtComments.visible = true
End If

CustomValidator ...
If ddlStatus.selectedvalue = "113" or ddlStatus.selectedvalue = "114" then
  If txtComments.text = nothing then
     args.isvalid = false
  End If
End If

And somewhere on the gridview rowupdating event i need:
If me.isvalid = false then
  'do not update
End if
Any help?