How to count rows or columns with data in a worksheet in Infragistics.Excel? For example, myWorkbook.Worksheets[0].Rows.Count
That count could be (and probably will be) inaccurate. It indicates the number of rows or columns which have been lazy loaded. A row could be accessed and have nothing changed on it. Just getting a row instance will force it to be created, and that will increment the count of the collection. Or a row can have something changed on it, then the change can be reset, and the count will still include that row.
ICollection<T> is implemented explicitly (no idea why, hint Mike), so just use
(ICollection<WorksheetColumn>)sheet.Columns).Count
(ICollection<WorksheetRow>)sheet.Rows).Count
There is currently no way to determine the number of rows or columns which only have data. I would recommend submitting a feature request for this: http://devcenter.infragistics.com/Protected/RequestFeature.aspx.