Use the SplitContainer.Panel1MinSize property to prevent the splitter from moving too close to the left or top edge of the container. For example, you might want to prevent some of the display area of a System.Windows.Forms.TreeView from being covered.

If the SplitContainer.Orientation property is Vertical (the default), SplitContainer.Panel1MinSize returns the minimum distance in pixels of the splitter from the left edge of SplitContainer.Panel1. If the SplitContainer.Orientation property is Horizontal, SplitContainer.Panel1MinSize returns the minimum distance in pixels of the splitter from the top edge of SplitContainer.Panel1.