Out-of-Process versus In-Process

using System;
using System.Runtime.InteropServices;

using Autodesk.AutoCAD.Interop;
using Autodesk.AutoCAD.Runtime;
using Autodesk.AutoCAD.ApplicationServices;

[CommandMethod("ConnectToAcad")]
public static void ConnectToAcad()
{

  AcadApplication acAppComObj = null;
  const string strProgId = "AutoCAD.Application.18";
  //const는 상수를 만드는 것 같음...

  // Get a running instance of AutoCAD
  try
  {
      acAppComObj = (AcadApplication)Marshal.GetActiveObject(strProgId);
  }
  catch // An error occurs if no instance is running
  {
      try
      {
          // Create a new instance of AutoCAD
          // 실행중인 AutoCAD의 인스턴스 (instance)에 접근
        acAppComObj = (AcadApplication)Activator.CreateInstance(Type.GetTypeFromProgID(strProgId), true);
      }
      catch
      {
          // If an instance of AutoCAD is not created then message and exit
          System.Windows.Forms.MessageBox.Show("Instance of 'AutoCAD.Application'" +
                                               " could not be created.");

          return;
      }
  }

  // Display the application and return the name and version
  acAppComObj.Visible = true;
  System.Windows.Forms.MessageBox.Show("Now running " + acAppComObj.Name +
                                       " version " + acAppComObj.Version);

  // Get the active document
  AcadDocument acDocComObj;
  acDocComObj = acAppComObj.ActiveDocument;

  // Optionally, load your assembly and start your command or if your assembly
  // is demandloaded, simply start the command of your in-process assembly.
  acDocComObj.SendCommand("(command " + (char)34 + "NETLOAD" + (char)34 + " " +
                          (char)34 + "c:/myapps/mycommands.dll" + (char)34 + ") ");

  acDocComObj.SendCommand("MyCommand ");
}

모르겠다...
출처
http://docs.autodesk.com/ACD/2010/ENU/AutoCAD%20.NET%20Developer%27s%20Guide/index.html