Yousef’s blog

IE7 Advanced toolbar buttons

February 24, 2007 on 3:47 pm | In Uncategorized |

Currently, there are several ways to create addons for Internet Explorer. The two I’ll be talking about in this post are BHOs (Browser Helper Objects) and Toolbar Buttons. To create a toolbar button, you simply add a value into the registry specifying whether you want to either launch a script, a function inside a COM library, executable or open an Explorer Toolband. When writing an advanced addon, most of the time you’d probably want to use a COM object.

As described in the Toolbar Button MSDN article, it’s fairly easy to get the COM object’s IOleCommandTarget::Exec function called when the user clicks the toolbar button. However, this method limits you to basic toolbar buttons. What if you’d like to create more advanced toolbar buttons, one with a dropdown menu for example?

This is a problem I ran into during the development of the first version of the Mapper IE7  addon. I’ve also seen this question being asked a few times at the IE Extension Development MSDN Forums. Several solutions to the problem are available on the internet (like this Codeproject article, or this newsgroup discussion). However, none of the solutions I found adds a real dropdown button into the toolbar, what I actually wanted to do.

My solution to the problem involves window subclassing. We basically need to hook into the toolbar’s WndProc to change its behavior. We also need to hook the toolbar’s parent window WndProc to listen for parent notifications such as WM_COMMAND messages. To set up these hooks, we need the hWnd of both windows. Functions to find these hWnds should be fairly easy to implement using Spy++ and the FindWindowEx function.

Note that since we’re adding the button to the toolbar from our code, we don’t need to add a Toolbar Button using the registry keys like the official API tells us to. Instead, we’re going to create a BHO. Since Mapper was written in C#, that’s what I used for the code attached to this article as well. There are several articles on the internet describing how to write a BHO in C#.

Let’s describe what we need to implement when we want to accomplish our goal. First, we need to set up the hooks in the BHO’s constructor. Next, we’ll need to listen for two different messages: TB_ADDBUTTONS and TBN_GETBUTTONINFO. The first one gets send to the toolbar itself, the second to its parent. It’s important to listen to both messages, because TB_ADDBUTTONS is important when the user customizes/resets the toolbar. TBN_GETBUTTONINFO should be getting called every time we start up Internet Explorer, but also when the user customizes the toolbar. When either of these functions gets called, we add our own button (or button info).

If you’d like to add your own image to the button (which you probably do), you also need to listen for TB_SETIMAGELIST. This way you can add your own icon into the toolbar’s image list. It has already been done in the attached code.

After the button has been successfully added we need to listen for user input. Obviously, we do this by listening for the WM_COMMAND and TBN_DROPDOWN messages. When the toolbar’s parent receives the TBN_DROPDOWN message, it’s time for us to show the drop down menu. You can do this using TrackPopupMenuEx (I did not include the code to do this though). Then you’ll receive WM_COMMAND messages when the user clicks any of your menu items.

Last but not least, we need to make sure we unsubclass the windows when our BHO unloads. Unfortunately, we can’t do this in the BHOs destructor. It seems that the destructor doesn’t always gets called. I think this has to do with .NET objects exposed as COM. Luckily, we can listen to WM_NCDESTROY, and remove the subclass there.

The basics of the hooking / message listening part are all done for you in the attached file. I didn’t do any BHO registering or COM stuff, since there are plenty of tutorials on that. If you’re familiar with Win32 programming, you should have no problem understanding what happens.

Download BHO.cs

Tags: addons, BHO, COM, dropdown, IE7, internet explorer, mapper