Those who are familiar with VBA it is much straight forward to access the objects using ThisDrawing.
I would like to share how to use the same logic using VB.NET in AutoCAD-2012
Imports Autodesk.AutoCAD.Interop
Imports Autodesk.AutoCAD.Interop.Common
Public Class ACADThisDrawing
    Public Shared ReadOnly Property ThisDrawing() As AcadDocument
            Return Autodesk.AutoCAD.ApplicationServices.Application.DocumentManager.MdiActiveDocument.AcadDocument
        End Get
    End Property
End Class
Source : the web