As Internet Explorer’s component architecture is based on COM, there are many different ways to extend its capabilities. These can be broken down into three broad categories of extensions as Browser Extensions, Content Extensions and Hosting and Reuse, as follows:
- Browser Extensions This category of end-user extensions adds additional functionality to Internet Explorer content. It includes features such as shortcut menu extensions, custom toolbars, Explorer Bars, and Browser Helper Objects (BHOs).
- Content Extensions These extend the types of content that can be parsed and displayed; their use depends on the content being loaded into the browser. This category includes ActiveX Controls and active documents.
- Hosting and Reuse By hosting and reusing the Internet Explorer components as part of your own application, you can build your own browser or add rich rendering and Internet capabilities.
This class of extensibility adds to the user interface (UI) of the browser and is not directly related to the viewable content of Web pages. It includes add-on functionality that users might install to enhance their browsing experience.
- Shortcut menu extensions. It is possible to extend Internet Explorer by adding options to the shortcut menu, which link the menu command to the task to be performed.
- Toolbars. Custom toolbars can complement Internet Explorer’s standard toolbars. An example is the Total-Toolbar
- Browser Helper Objects. A Browser Helper Object (BHO) runs within Internet Explorer and offers additional services, often without any obvious user interface. For example, a BHO might highlight terms of interest to the user, monitoring the user web habits.
- Download Manager. Use Explorer Bars to reserve part of the browser window, either the side or the bottom of the browser window.
Content Extensions:This category of extensions is invoked specifically by Internet Explorer content. It includes ActiveX controls, binary behaviors, and active documents.
- ActiveX Controls. ActiveX controls are a very powerful mechanism for extending the abilities of HTML.
Hosting and Reuse: Internet Explorer’s components can easily be reused thanks to its COM-based architecture. Two commonly used components are shdocvw.dll (the WebBrowser control) and mshtml.dll (“Trident”).
In the majority of situations, it is better to directly host shdocvw.dll than mshtml.dll. This is because shdocvw.dll supports in-place navigation, history, and so on-that is, the full capabilities of the browser. If you host mshtml.dll directly, you gain the use of an HTML and CSS parser and renderer, but you cannot take advantage of the browser’s other capabilities.
An example of a situation in which you might want to host mshtml.dll directly is the scenario of an e-mail application and HTML-based e-mail. In this scenario, the e-mail is displayed within the application, but if a user clicks on a hyperlink within the e-mail, the user would expect a separate browser instance to open rather than the e-mail application navigating in place. This is how applications such as Microsoft Outlook and Outlook Express handle HTML-based e-mail.