Returns the number of rows in the level under the specified row, including non-viewable (invisible) rows if specified.
If DataGridRow is NULL, then it returns the count of top-level rows. If the DataGridRow has no children, it returns zero.
To ensure that you get an accurate row count, you can specify GetRowCount(TRUE), which will cause the application to scroll to the bottom of the datagrid, fetch all rows, and report the number of rows seen. This may take a lot of time, so only use this optional argument when needed.