using System;
using System.Drawing;
using System.Collections;
using System.ComponentModel;
using System.Windows.Forms;
using System.Data;
using SampleClasses;
using System.Runtime.InteropServices;
using Microsoft;
using Microsoft.VisualBasic;

using Autodesk.AutoCAD;
using Autodesk.AutoCAD.Interop;  
using Autodesk.AutoCAD.Runtime;
using Autodesk.AutoCAD.Geometry;
using Autodesk.AutoCAD.EditorInput;
using Autodesk.AutoCAD.DatabaseServices;
using Autodesk.AutoCAD.ApplicationServices;
using acadApp = Autodesk.AutoCAD.ApplicationServices.Application;
using AcadDB = Autodesk.AutoCAD.DatabaseServices;
using AcadRun = Autodesk.AutoCAD.Runtime;


  private void button2_Click(object sender, System.EventArgs e)
  {
   object G_OBJ = null;
   ObjectId G_OBJID;
   Entity G_ENT = null;
   Line G_LINE = null; 
   string G_ENTTYPE = "", s = "", es = "";
   try
   {
    Microsoft.VisualBasic.Interaction.AppActivate(cadApp.Caption);


    // Here I am Getting the ERROR  --> System.NullReferense Error Additional Info: Object Referense not set to an instance of object
    Database Db = HostApplicationServices.WorkingDatabase;


    Editor Ed = Autodesk.AutoCAD.ApplicationServices.Application.DocumentManager.MdiActiveDocument.Editor;
    Transaction Trans = Db.TransactionManager.StartTransaction();
    BlockTable bt = (BlockTable)Trans.GetObject(Db.BlockTableId, Autodesk.AutoCAD.DatabaseServices.OpenMode.ForWrite);
    BlockTableRecord btr = (BlockTableRecord)Trans.GetObject(bt[BlockTableRecord.ModelSpace],Autodesk.AutoCAD.DatabaseServices.OpenMode.ForWrite);
    PromptEntityResult p = Ed.GetEntity("click on an entity");
    G_OBJ =(Object) p;
    G_OBJID = p.ObjectId;
    if(G_OBJID.ToString() != "(0)")
    {
     G_ENT =  (Entity)Trans.GetObject(G_OBJID, Autodesk.AutoCAD.DatabaseServices.OpenMode.ForRead);
     G_ENTTYPE = G_ENT.GetType().ToString();
     if (G_ENTTYPE == "Autodesk.AutoCAD.DatabaseServices.Line")
     {
      G_LINE = (Line) G_ENT;
      Point3d p0 = G_LINE.StartPoint, p1 = G_LINE.EndPoint;
      double dX, dY, Len2D;
      dX = (p1.X - p0.X); dY = (p1.Y - p0.Y);
      Len2D = Math.Sqrt((dX * dX)+(dY * dY));
      s += "\n Length (2D = " + Len2D + ") ";
     }
     else es += "\n Selected Object is not a Line";
    }
   }
   catch(System.Exception EX){es += "\nErr.. in Get_Object : " + EX.Message.ToString();}
   MessageBox.Show(s);
   MessageBox.Show(es);
  }