This is the base class from which all toolbar items derive.

Autodesk.AutoCAD.Customization.ElementBase
    Autodesk.AutoCAD.Customization.VersionableElement
        Autodesk.AutoCAD.Customization.CustomizationElement
            Autodesk.AutoCAD.Customization.ToolbarItemBase
                Autodesk.AutoCAD.Customization.ToolbarButton
                Autodesk.AutoCAD.Customization.ToolbarControl
                Autodesk.AutoCAD.Customization.ToolbarFlyout

Public Class ToolbarItemBase
Inherits CustomizationElement

public class ToolbarItemBase : CustomizationElement;