Browser Helper Objects: The Browser the Way You Want It

Introduction

There are sometimes circumstances in which you need a more or less specialized version of the browser. Sometimes you work around this by developing a completely custom module built on top of the WebBrowser control, complete with buttons, labels, and whatever else the user interface requires. In this case, you're free to add to that browser any new, nonstandard feature you want. But what you actually have is just a new, nonstandard browser. The WebBrowser control is just the parsing engine of the browser. This means there still remains a number of UI-related tasks for you to do: adding an address bar, toolbar, history, status bar, channels, and favorites, just to name a few. So, to create a custom browser you have to write two types of code: the code that transforms the WebBrowser control into a full-fledged browser like Microsoft Internet Explorer, and the code that implements the new features you want it to support. Wouldn't it be nice if there was a straightforward way to customize Internet Explorer instead? Browser Helper Objects (BHO) do just that.

Program Customization

Historically speaking, the first way to customize the behavior of a program was through subclassing. By this means, you could change the way a given window in a program processed messages and actually obtain a different behavior. Although considered a brute-force approach, because the victim is largely unaware of what happens, it's been the only choice for a long time.

With the advent of the Microsoft Win32 API, interprocess subclassing was discouraged and made a bit harder to code. If you're brave-hearted, however, pointers have never scared you; above all, if you're used to living in symbiosis with system-wide hooks, you might even find it too simple. But this is not always the case. Despite the cleverness of the programming, the point is that each Win32 process runs in its own address space and breaking the process boundaries is somewhat incorrect. On the other hand, there might be circumstances that require you to do this with the best of intentions. More often, customization might be a specific feature the program itself allows by design.

In the latter case, the programs search for additional modules in well-known and prefixed disk zones, load, initialize, and then leave them free to do the job they have been designed to do. This is exactly what happens with the Internet Explorer browser and its helper objects.

Read more: MSDN

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