C#

using System;
using System.Runtime.InteropServices;

namespace WebWatcher
{
    [ComVisible(true),
    InterfaceType(ComInterfaceType.InterfaceIsIUnknown),
    Guid("FC4801A3-2BA9-11CF-A229-00AA003D7352")]
    public interface IObjectWithSite
    {
        [PreserveSig]
        int SetSite([MarshalAs(UnmanagedType.IUnknown)]object site);
        [PreserveSig]
        int GetSite(ref Guid guid, out IntPtr ppvSite);
    }
}

using System;
using System.Collections.Generic;
using System.Text;
using SHDocVw;
using mshtml;
using System.IO;
using Microsoft.Win32;
using System.Runtime.InteropServices;

namespace WebWatcher
{
    [
    ComVisible(true),
    Guid("8a194578-81ea-4850-9911-13ba2d71efbd"),
    ClassInterface(ClassInterfaceType.None)
    ]
    public class BHO : IObjectWithSite
    {
        private SHDocVw.WebBrowser webBrowser;
       
        public void OnDocumentComplete(object pDisp, ref object URL)
        {
            System.Windows.Forms.MessageBox.Show("HelloWorld!");
        }

        #region WebWatcher Internal Functions
        public int SetSite(object site)
        {

            if (site != null)
            {
                webBrowser = (SHDocVw.WebBrowser)site;
                webBrowser.DocumentComplete += new DWebBrowserEvents2_DocumentCompleteEventHandler(this.OnDocumentComplete);
            }
            else
            {
                webBrowser.DocumentComplete -= new DWebBrowserEvents2_DocumentCompleteEventHandler(this.OnDocumentComplete);
                webBrowser = null;
            }

            return 0;

        }

        public int GetSite(ref Guid guid, out IntPtr ppvSite)
        {
            IntPtr punk = Marshal.GetIUnknownForObject(webBrowser);
            int hr = Marshal.QueryInterface(punk, ref guid, out ppvSite);
            Marshal.Release(punk);

            return hr;
        }
        #endregion
    }
}