This .NET class wraps the AcDbUnderlayDefinition ObjectARX class. 

UnderlayDefinition is an abstract class that handles the linkage to external underlay content. Underlays are similar to raster images, except that users can snap to the content of an underlay. 

An UnderlayDefinition object is referenced by zero or more UnderlayReferences. The UnderlayReference class is responsible for the placement of content within the drawing, while the UnderlayDefinition class handles the linkage to the underlay content. Instances of UnderlayReference-derived concrete classes are inserted in a block table record. Instances of UnderlayDefinition-derived concrete classes are inserted into a dictionary within the named object dictionary.

Autodesk.AutoCAD.DatabaseServices.DBObject
    Autodesk.AutoCAD.DatabaseServices.UnderlayDefinition
        Autodesk.AutoCAD.DatabaseServices.DgnDefinition
        Autodesk.AutoCAD.DatabaseServices.DwfDefinition
        Autodesk.AutoCAD.DatabaseServices.PdfDefinition

Public abstract Class UnderlayDefinition
Inherits DBObject

public abstract class UnderlayDefinition : DBObject;