In the first part, I showed how to set up the tools you need to build a BHO. I’ve now created a skeleton project that will compile into a very simple BHO, TinyBHO. I wasn’t able to find any good official sample code, all the articles involve starting with a template, and describe how to build on it, but the SurfHelper example by Xiaolin Zhang on CodeProject was a great help. Having a complete example I could build and run gave me the information I needed to build my own.

To build the BHO, download and unzip the project, open the TinyBHO.vcproj file in Visual Studio Express, and hit f7 to build it. The build process should compile, and then add the DLL to the registry, and the next time you run Internet Explorer, a message box should appear whenever a document’s loaded. This provides all the boilerplate to hook into browser events, and provides a stub that you can insert your  own document processing logic into.

If we want to build on this, we’ll need your own GUIDs for the interface and type library. Use a tool like uuidgen or one of the online versions to generate three IDs, and then load the TinyBHO.idl file into VS’s editor. Replace the first ID that begins with 03147ee0 with our first ID, it should only appear once near the top of the .idl file.

The second ID we need to replace starts with 7cd37f36, and appears once in both TinyBHO.idl and TinyBHO.rgs. The third begins with 00e71626, and defines the main class’s ID. It occurs four times in TinyBHO.rgs and once in TinyBHO.idl. I’d also recommend customizing the class and file names, but that shouldn’t be strictly necessary for the BHO to coexist with others the way that unique GUIDs are.

Now we’ve got the BHO personalized, we can start looking at CTinyBHOClass::Invoke(), the function that gets called whenever a browser event, such as a page load or key press, occurs. Right now, all it does is catch document load events, and pulls the document interface pointer from the event, and shuts down the event handler when the browser quits. For PeteSearch, I’ll be focused on key presses and document loads, since that’s where it does most of its work, but there’s a wide range of other events to grab if you want to implement something else, like a popup blocker. In Firefox terms, it’s pretty close to having called AddEventListener() on the browser, except that you get called on all events and have to check yourself to figure out what happened.

The trickiest thing about getting this working was that all the ATL COM handling code is such a black box. The registration part of the build phase was failing, and I eventually tracked it down to a misspelt resource name, but there was no logging or other useful debug information available, I just had to spend a long time inspecting the code looking for anything suspicious!

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Compared to writing Firefox extensions, there isn’t a whole lot of information about writing add-ons for Internet Explorer, and almost nothing about porting from Firefox to IE.  I’m converting PeteSearch over, and I’ll be sharing my notes on the process here.

There’s no way of writing a plugin using an interpreted environment like Firefox’s Javascript, instead you have to create a binary library in a DLL. These are known as Browser Helper Objects, and give you the ability to catch browser events like page loads, and work with the DOM, but don’t offer any UI access for things like toolbars. Hopefully PeteSearch doesn’t use anything outside of a BHO’s domain, since it works almost entirely on processing and altering existing pages.

Since we’ll need to compile a DLL, the first thing we need is a compiler. Luckily Microsoft has released a cutdown version of Visual Studio for free, though there are some limitations that I’ll describe. I chose to use C++ to create the DLL, because I know it well and it’s what most of the BHO examples use, but you could also use C# or Visual Basic.

After we’ve downloaded the compiler there’s some other components we need before we can build a DLL. Since the BHO is a win32 DLL, we need the platform SDK. To get it working with the express edition of Visual Studio, we’ll also need to muck about with some config files, as described in this MSDN article.

For dealing with COM code, ATL/WTL is very useful, so I’d also recommend downloading the open source WTL from MS, and following the steps in this CodeProject article.

Now we’re ready to compile a DLL, the best place to start is with some example code. There’s two good articles from Microsoft that describe writing BHOs, one from 1999 and a more recent 2006 version. Unfortunately they both start off using a wizard to create the base project, and the ATL wizard isn’t available for the express edition! They don’t have any complete examples downloadable, which is a bit puzzling. I’ve contacted one of the authors, Tony Schreiner, in the hope he can provide a complete example. If not, I may finagle a base ATL project from someone with the full studio package, then I’d be able to build it myself following the rest of the steps. Of course, I could purchase the standard studio for $300, but it seems worthwhile to work out a free way to write IE extensions.

René Nyffenegger has some example code that we can use instead, though I found I had to do some tweaking to get it to compile, such as defining a couple of logging macros, and sorting out some string literals that were expected to be wide but weren’t.

Now we should have the starting point for a BHO. I’ll cover the next steps, and show some example code, in a following article.

Edit – I’m trying this on a new Vista laptop, and there’s a few extra steps I noticed:

• Folder permissions for editing the ATL headers in the SDK are tricky in Vista. You need to make yourself the owner, and only after that sort out the permissions.
• You need to add the mfc include folder from the SDK too. I may have just forgotten this before.
• The registry-setting part of the build process doesn’t work. I’ll cover fixing that in my description of writing an installer, and update here once that’s done.

Edit – To fix the registry build stage, you can run Visual C++ as administrator by right-clicking on the app and choosing that option from the menu.

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