.NET

[#1]

using System;
using System.Threading;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.IO;
using System.Runtime.InteropServices;
using Autodesk.AutoCAD.Interop.Common;
using Autodesk.AutoCAD.Interop;
using Autodesk.AutoCAD.ApplicationServices;
using Autodesk.AutoCAD.DatabaseServices;
using Autodesk.AutoCAD.Runtime;
using Autodesk.AutoCAD.EditorInput;

namespace application
{
class Program
{
static void Main(string[] args)
{

Document acDoc = Application.DocumentManager.MdiActiveDocument;

}

}

Working Environment:-

1. Visual Studio 2010
2. AutoCAD 2011 
3. C#

After run following error will occure

"

An unhandled exception of type 'System.TypeLoadException' occurred in mscorlib.dll

Additional information: Method 'CopyTo' in type 'Autodesk.AutoCAD.ApplicationServices.DocumentCollection'

from assembly 'acmgd, Version=18.1.0.0, Culture=neutral, PublicKeyToken=null' does not have an implementation ."

[#2]

I see what you're trying to create a standalone application (exe-file). In this case you can not use AutoCAD. NET API (ie acmgd.dll, acdbmgd.dll, accoremgd.dll, etc)

[#3]

Although you can license the RealDWG SDK to add DWG capability to your own app. Brief explanation here - http://adndevblog.typepad.com/autocad/2012/03/readingwriting-dwg-files-from-a-standalone-application....