The C1FlexGrid fully virtualizes rows and columns. Cells are created only when their row and column are visible to the user. In this case, a class that implements the ICellFactory interface is used to create the System.Windows.FrameworkElement objects that represent the cells. When cells are scrolled out of view, the same class is notified so it can properly dispose of the cell or recycle it for future use.

Reference

ICellFactory Members
C1.WPF.FlexGrid Namespace