How to use System.Windows.Forms Namespace in ASP.NET

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ASP.NET

I have a source code for generating the snapshot of a webpage. The code is containing the WebBrowser class which is under the namespace System.Windows.Forms;

When i debug the code, it shows System.Windows.Forms as error in ASP.NET webpage.
What to do?

Here is the code what i found on web.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Windows.Forms;
using System.Threading;
using System.Drawing;
namespace WebSiteSnapshotThumbnails
{
    public class WebSiteSnapshot
    {
        #region Private Properties
        private String Url { get; set; }
        private Int32 BrowserWidth { get; set; }
        private Int32 BrowserHeight { get; set; }
        private Bitmap Bitmap { get; set; }
        #endregion

        #region Constructors
        
        public WebSiteSnapshot(string url, int browserWidth, int browserHeight)
        {
            this.Url = url;
            this.BrowserWidth = browserWidth;
            this.BrowserHeight = browserHeight;
        } 
        #endregion
        #region Public Methods
        
        public Bitmap GenerateWebSiteImage()
        {
            Thread m_thread = new Thread(new ThreadStart(_GenerateWebSiteImage));
            m_thread.SetApartmentState(ApartmentState.STA);
            m_thread.Start();
            m_thread.Join();
            return Bitmap;
        } 
        #endregion
        #region Private Methods
        private void _GenerateWebSiteImage()
        {
            WebBrowser m_WebBrowser = new WebBrowser();
            m_WebBrowser.ScrollBarsEnabled = false;
            m_WebBrowser.Navigate(Url);
            m_WebBrowser.DocumentCompleted += new WebBrowserDocumentCompletedEventHandler(WebBrowser_DocumentCompleted);
            while (m_WebBrowser.ReadyState != WebBrowserReadyState.Complete)
                Application.DoEvents();
            m_WebBrowser.Dispose();
        }
        private void WebBrowser_DocumentCompleted(object sender, WebBrowserDocumentCompletedEventArgs e)
        {
            WebBrowser m_WebBrowser = (WebBrowser)sender;
            m_WebBrowser.ClientSize = new Size(this.BrowserWidth, this.BrowserHeight);
            m_WebBrowser.ScrollBarsEnabled = false;
            Bitmap = new Bitmap(m_WebBrowser.Bounds.Width, m_WebBrowser.Bounds.Height);
            m_WebBrowser.BringToFront();
            m_WebBrowser.DrawToBitmap(Bitmap, m_WebBrowser.Bounds);
        } 
        #endregion
    }
}