in the latest build when datagrid is databound and if multiselect = False
if (datagrid.SelectedRows.Count == 1) returns true only on first selection. When you select some other row in the grid this previous row and newly selected row is added to selectedrows (count is increasing) list. this happens despite the MultiSelect been set to False.
I could reproduce with dev build 1.3.62 but not with release 1.3.61.
In any case it is related to this issue: http://wisej.com/support/question/datagridview-selectedrows-not-updating-correctly
It will be fixed in the next release. Will update you when it´s online.