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Posted 10 Aug 2007

How to attach to Browser Helper Object (BHO) with C# in two minutes

, 10 Aug 2007
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This article will use screenshots step by step to show beginners how to attach to BHO with C# within just two minutes.


Microsoft has provided the Browser Helper Object (BHO) to let developers "drive" Internet Explorer. The first BHO was introduced in 1997 with IE 4.0. I have been writing programs on BHO for months. It could be quite depressing at the very beginning to learn all those things. I am writing this article to help beginners like me get familiar with BHO as soon as possible.


My personal interest is actually C++. C++ programs can be a lot less memory-consuming than C# programs. But C# does provide better service on BHO compared to C++. My first BHO program was written in C++. It took me quite a while to figure out how to handle BHO under VC++. But in C#, it only took me a few minutes. Also, C# has a lot of pleasant features such as foreach and type conversion.


To set up a BHO Hello World project, let us first start a C# Class Library, as BHO is written in a .dll attached to IE. You don't need Visual Studio 2005; C# Express is totally enough.

After we have this empty project, let's add a folder named BHO and an empty .cs file into the folder.

The first file has to been named IObjectWithSite to notify that this is a BHO project. To know more about this interface, please refer to

We also need to add two functions:

public interface IObjectWithSite
    int SetSite([MarshalAs(UnmanagedType.IUnknown)]object site);
    int GetSite(ref Guid guid, out IntPtr ppvSite);

Don't forget to add:

using System.Runtime.InteropServices;

Add another .cs file named BHO.cs.

Add a class called BHO in the newly added file (BHO.cs). The class contains the interface IObjectWithSite.

class BHO:IObjectWithSite

To use BHO, we need to have two references: SHDocVw.dll and MSHTML.dll. You can find them in the Windows\System32 folder. SHDocVw is a Microsoft Shell Doc Object and Control Library. MSHTML: All interfaces for accessing the Dynamic HTML (DHTML) Object Model are based on IDispatch and are the basis of access to the object model that is also used by scripts.

using SHDocVw;
using mshtml;
using System.Runtime.InteropServices;   
namespace BHO_HelloWorld
class BHO:IObjectWithSite


Having using SHDocVw is not enough, you need to add references to the project. Add SHDocVw. Later we are going to use MessageBox, so we also need to add a Windows Forms reference.

Now in BHO.cs: we add two variables into the class, WebBrowser and HTMLDocument. From their names, you can easily figure out what they do.

Besides the two methods we defined in the IObjectWithSite interface, we also need to add OnDocumentComplete. You can name the function anything you want as long as the parameters are the same as what's defined in the CDHtmlDialog class. Later you need to carefully attach it to an Event Handler. For consistency of code, we name it OnDocumentComplete. OnDocumentComplete is a function of the CDHtmlDialog class ( It will be triggered if the HTMLDocument downloading is complete, in other words, when your page is loaded. You can also use Navigate() or OnBeforeNavigate(). Please refer to to find out what you need exactly.

class BHO: IObjectWithSite
    WebBrowser webBrowser;
    HTMLDocument document;
    publiv void OnDocumentComplete(object pDisp, ref object URL)
    public void SetSite(object site)
    public int GetSite(ref Guid guid, out IntPtr ppvSite)

Under IObjectWithSite.cs: you need to point out the GUID of IE for the program, so it can attach to IE.


public interface IObjectWithSite
    int SetSite([MarshalAs(UnmanagedType.IUnknown)]object site);
    int GetSite(ref Guid guid, out IntPtr ppvSite);

Also, you need to assign a GUID for your own program in BHO.cs. You can use the System.Guid.NewGuid() method to get one, which is really neat compared to C++.


You cannot just leave SetSite and GetSite blank. fill them in. This step is to tell IE that the DocumentCompletent event is attached to OnDocumentComplete in our program.

public int SetSite(object site)
    if (site != null)
        webBrowser = (WebBrowser)site;
        webBrowser.DocumentComplete += 
          new DWebBrowserEvents2_DocumentCompleteEventHandler(
        webBrowser.DocumentComplete -= 
          new DWebBrowserEvents2_DocumentCompleteEventHandler(
        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);

    return hr;

Add one more reference:

using Microsoft.Win32;

In BHO.cs, we need to write two functions for register/unregister of this DLL.

public static string BHOKEYNAME = 
  "Software\\Microsoft\\Windows\\CurrentVersion\\Explorer\\Browser Helper Objects";

public static void RegisterBHO(Type type)
    RegistryKey registryKey = Registry.LocalMachine.OpenSubKey(BHOKEYNAME, true);

    if (registryKey == null)
        registryKey = Registry.LocalMachine.CreateSubKey(BHOKEYNAME);

    string guid = type.GUID.ToString("B");
    RegistryKey ourKey = registryKey.OpenSubKey(guid);

    if (ourKey == null)
        ourKey = registryKey.CreateSubKey(guid);

    ourKey.SetValue("Alright", 1);

public static void UnregisterBHO(Type type)
    RegistryKey registryKey = Registry.LocalMachine.OpenSubKey(BHOKEYNAME, true);
    string guid = type.GUID.ToString("B");

    if (registryKey != null)
        registryKey.DeleteSubKey(guid, false);

Now compile; under your Release folder, you will find the .dll of your project.

Then, use regasm /codebase "BHO HelloWorld.dll" to register the DLL. We have a problem here. REGASM will tell us me it's not registered. Why?

Because we didn't set the BHO class as public. That's why.

public class BHO:IObjectWiteSite

Now, do it again. It'll be successful.

Open your Registry. Find out the Browser Helper Object under LOCAL_MACHINE->SOFTWARE->MICROSOFT->WINDOWS->EXPLORER.

Now the program is officially attached to your BHO. We need to fill in the OnDocumentComplete function. It's really neat to use C#'s foreach loop rather than the for loop in C++. Because you don't need to care about the indexer. We will also see the type conversion is quite easy. This is an example of how find out the NAME attribute of an IHTMLInputElement.

An IHTMLInputElement is an Input element in an HTML page.

If the IHTMLInputElement does not have a name attribute, we will fetch the ID attribute. Then pop up the value.

public void OnDocumentComplete(object pDisp, ref object URL)
   document = (HTMLDocument)webBrowser.Document;
    foreach (
        IHTMLInputElement tempElement in document.getElementsByTagName("INPUT"))
  !=null?"it sucks, no name, try id"+

There you go, see?

Now, let's try using BeforeNavigate2().

As we can see, there is BeforeNavigate and BeforeNavigate2(). We go for the latter one. If you are interested, you can use the first one.

Add the function prototype:

public void OnBeforeNavigate2(object pDisp, ref object URL, ref object Flags, 
     ref object TargetFrameName, ref object PostData, 
     ref object Headers, ref bool Cancel) 

Set up the hook:

public int SetSite(object site)
    if (site != null)
        webBrowser = (WebBrowser)site;
        webBrowser.DocumentComplete += 
        new DWebBrowserEvents2_DocumentCompleteEventHandler(this.OnDocumentComplete);
           new DWebBrowserEvents2_BeforeNavigate2EventHandler(this.OnBeforeNavigate2);
        webBrowser.DocumentComplete -= 
          new DWebBrowserEvents2_DocumentCompleteEventHandler(this.OnDocumentComplete);
        webBrowser.BeforeNavigate2 -= 
          new DWebBrowserEvents2_BeforeNavigate2EventHandler(this.OnBeforeNavigate2);
        webBrowser = null;

    return 0;

Now we have to steal the password on an Input Password element:

public void OnBeforeNavigate2(object pDisp, ref object URL, ref object Flags, 
        ref object TargetFrameName, ref object PostData, ref object Headers,
        ref bool Cancel)
    document = (HTMLDocument)webBrowser.Document;

    foreach(IHTMLInputElement tempElement in document.getElementsByTagName("INPUT"))

See how easy you can get it.


In conclusion, it is really easy to handle BHO with C#. That is why there are so many IE add-ons. I hope this was useful to you. To save your time, you can use the project template I made. Download it and put it under your Visual Studio 2005\Templates\ProjectTemplates folder (it is usually under My Documents).


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

Web Developer Horizon Ideas
United States United States
My name is Jia Chen and I want you tell you about my childhood dream: being a problem-solver. My mom told me it was silly because it wasn't really a profession. Through the last decade, I have been a software engineer, a product manager, a repetitive student, a management consult and an entrepreneur. They appear far from my childhood dream. But I still think I am living it. Because the essence of it is to find problems and solve problems. Some times I may not solve new problems, but I always want to solve old problems in a new way.

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QuestionIE Extension not running Pin
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QuestionPre event of DocumentComplete Pin
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