convert your bound fields to template field as labels, and use a button instead of linkbutton.
then on g1_rowcommand event
If e.CommandName = "EDIT" Then
Dim btn As Button = DirectCast(e.CommandSource, Button)
Dim gvr As GridViewRow = DirectCast(btn.NamingContainer, GridViewRow)
'get value from invoice_num
Dim InvoiceNumLabel as Label = Directcast(gvr.FindControl("InvoiceNumlabel"), Label)
'write value to TextBox
InvoiceNLabelTextBox.text = InvoiceNumlabel.text.toString()
else
end if
I hope this helps