A Acdx.Dll In My Reference Is Getting Vanished From The References Folder in visual studio If I Debug The Solution.

using System;
using System.IO;
using System.Text;
using System.Collections;
using System.Windows.Forms;
using System.Collections.Generic;
using System.Runtime.InteropServices;
using Autodesk.AutoCAD;
using Autodesk.AutoCAD.Geometry;
using Autodesk.AutoCAD.ComponentModel;
using Autodesk.AutoCAD.Customization;
using Autodesk.AutoCAD.ApplicationServices;
using Autodesk.AutoCAD.EditorInput;
using Autodesk.AutoCAD.GraphicsInterface;
using Autodesk.AutoCAD.GraphicsSystem;
using Autodesk.AutoCAD.LayerManager;
using Autodesk.AutoCAD.Runtime;
using Autodesk.AutoCAD.DataExtraction;
using Autodesk.AutoCAD.DatabaseServices;

Among these namespaces, if i add a ACDX.dll, intellisense is able to detect the Autodesk.AutoCAD.DataExtraction namespace and if i debug the solution then i am not able to see acdx.dll in my refrences tab and even the namespace Autodesk.AutoCAD.DataExtraction is throwing an error saying that  "The type or namespace name 'DataExtraction' does not exist in the namespace 'Autodesk.AutoCAD' (are you missing an assembly reference?)"	


I have the following references added in my project:
ac32bitappserver.dll
AcCoreMgd.dll
AcCui.dll
AcDbMgd.dll
acdbmgdbrep.dll
AcDx.dll
AcMgd.dll
acmnuparser.dll
AcMr.dll
AcTcMgd.dll
AcWindows.dll
AdWindows.dll
managedmc3.dll

Can u please help me out in resolving this issue.

Thanks you!