A ToolWindow can be created by creating a control that derives from the ToolWindow class (which in turn derives from UserControl), or create/use a control and create a ToolWindow around it.

Each ToolWindow must have a unique key which can later be used to access it in its containing collection. When creating a ToolWindow by changing the derivation from UserControl to ToolWindow, the Key property must be set either in the designer or by setting the Key property in the constructor. When creating a ToolWindow from scratch, the base's constructor must be called or the Key property set in the constructor. Once a ToolWindow has been added to its associated DockLayoutManager's ToolWindows collection, its Key property can no longer be set.

System.Object
   System.MarshalByRefObject
      System.ComponentModel.Component
         System.Windows.Forms.Control
            System.Windows.Forms.ScrollableControl
               System.Windows.Forms.ContainerControl
                  System.Windows.Forms.UserControl
                     Xceed.DockingWindows.ToolWindow

Target Platforms: Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

Reference

ToolWindow Members
Xceed.DockingWindows Namespace