This .NET class wraps the AcDbUnderlayItem ObjectARX class. 

This interface is implemented by host applications that wish to support underlays. UnderlayDefinition and AcDbUnderlayReference objects use this interface to accomplish their mission.

Autodesk.AutoCAD.DatabaseServices.UnderlayItem
    Autodesk.AutoCAD.DatabaseServices.DgnUnderlayItem

Public Class UnderlayItem
Inherits DisposableWrapper

public class UnderlayItem : DisposableWrapper;