How to thumbnail show preview of autocad dwg file ?

namespace : using System.IO

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.IO;

namespace thumbnailView
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            OpenFileDialog openFileDialog1 = new OpenFileDialog
            {
                InitialDirectory = @"D:\",             
                CheckFileExists = true,
                CheckPathExists = true,

                DefaultExt = "dwg",
                Filter = "dwg files (*.dwg)|*.dwg",
                FilterIndex = 2,
                RestoreDirectory = true,

                ReadOnlyChecked = true,
                ShowReadOnly = true
            };
            if (openFileDialog1.ShowDialog() == DialogResult.OK)
            {
                textBox1.Text = openFileDialog1.FileName;
            }  
        }

        private void textBox1_TextChanged(object sender, EventArgs e)
        {
            addDwgAsBmpImage(textBox1.Text);
        }

        private void addDwgAsBmpImage(string name)
        {
            try
            {
                FileStream fs = new FileStream(name, FileMode.Open, FileAccess.Read, FileShare.ReadWrite);
                BinaryReader br = new BinaryReader(fs);
                br.BaseStream.Seek(0x0D, SeekOrigin.Begin);
                Int32 imgPos = br.ReadInt32();
                br.BaseStream.Seek(imgPos, SeekOrigin.Begin);
                byte[] imgBSentinel = { 0x1F, 0x25, 0x6D, 0x07, 0xD4, 0x36, 0x28, 0x28, 0x9D, 0x57, 0xCA, 0x3F, 0x9D, 0x44, 0x10, 0x2B };
                byte[] imgCSentinel = new byte[16];
                imgCSentinel = br.ReadBytes(16);

                if (imgBSentinel.ToString() == imgCSentinel.ToString())
                {
                    UInt32 imgSize = br.ReadUInt32();
                    //Get number of images present
                    byte imgPresent = br.ReadByte();
                    // header
                    Int32 imgHeaderStart = 0;
                    Int32 imgHeaderSize = 0;
                    // bmp data
                    Int32 imgBmpStart = 0;
                    Int32 imgBmpSize = 0;
                    bool bmpDataPresent = false;
                    // wmf data
                    Int32 imgWmfStart = 0;
                    Int32 imgWmfSize = 0;
                    //bool wmfDataPresent = false;

                    //get each image present
                    for (int I = 0; I <= imgPresent; I++)
                    {
                        // Get image type
                        byte imgCode = br.ReadByte();
                        switch (imgCode)
                        {
                            case 1:
                                // Header data
                                imgHeaderStart = br.ReadInt32();
                                imgHeaderSize = br.ReadInt32();
                                break;
                            case 2:
                                // bmp data
                                imgBmpStart = br.ReadInt32();
                                imgBmpSize = br.ReadInt32();
                                bmpDataPresent = true;
                                break;
                            case 3:
                                //wmf data
                                imgWmfStart = br.ReadInt32();
                                imgWmfSize = br.ReadInt32();
                                //wmfDataPresent = true;
                                break;
                        }
                    }
                    if (bmpDataPresent)
                    {
                        br.BaseStream.Seek(imgBmpStart, SeekOrigin.Begin);
                        byte[] tempPixelData = new byte[imgBmpSize + 14];

                        // indicate it is a bit map
                        tempPixelData[0] = 0x42;
                        tempPixelData[1] = 0x4D;

                        //offBits
                        tempPixelData[10] = 0x36;
                        tempPixelData[11] = 0x04;

                        byte[] tempBuffData = new byte[imgBmpSize];
                        tempBuffData = br.ReadBytes(imgBmpSize);
                        tempBuffData.CopyTo(tempPixelData, 14);

                        MemoryStream memStream = new MemoryStream(tempPixelData);
                        Bitmap bmp = new Bitmap(memStream);
                        pictureBox1.Image = bmp;
                    } 
                }
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.ToString());
            }
        }
    }
}