This .NET class wraps the AcDbDwfDefinition ObjectARX class. 

Represents DWF underlays in the drawing. Underlays are similar to raster images but their content is snappable.

Autodesk.AutoCAD.DatabaseServices.DBObject
    Autodesk.AutoCAD.DatabaseServices.UnderlayDefinition
        Autodesk.AutoCAD.DatabaseServices.DwfDefinition

Public Class DwfDefinition
Inherits UnderlayDefinition

public class DwfDefinition : UnderlayDefinition;