This .NET class wraps the AcDbDgnDefinition ObjectARX class. 

Represents DGN 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.DgnDefinition

Public Class DgnDefinition
Inherits UnderlayDefinition

public class DgnDefinition : UnderlayDefinition;