property Grid.SelectByDrag as Boolean
Specifies whether the user selects multiple items by dragging.

Boolean A boolean expression that specifies whether the user may select multiple items by drag and drop. 
By default, SelectByDrag property is True. Use the SelectByDrag property to disable selecting multiple items by dragging. The SelectByDrag property has effect only if the control supports multiple selection. The SingleSel property controls the number of items that the user may select. For instance, if the SingleSel property is True, the user can't select multiple items, and so a single item may be selected at the time. If the SingleSel property is False, the user can select multiple items using the mouse, keyboard or both. When the SelectByDrag property is True, the user may click the non text area to start select items by dragging. Use the SelectByDrag property on False when your control requires OLE drag and drop operations, like when you select multiple items and drag them to a new position. Use the OLEDropMode property to specify whether the OLE drag and drop operations inside the control is allowed. For instance, if the SelectByDrag and OLEDropMode properties are on, sometimes it is confused what control should do when user clicks and start to select items.