Beginning Ajax With ASP.NET (2006).pdf

Chapter 9
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The use of an httpHandler or httpModule is similar in the respect that ASP.NET will process
the requests through either the handler or module before allowing the page to execute. To
signify this distinction, from now on we will refer to the use of either an httpHandler or
httpModule as “preprocessing.” Preprocessing provides a layer where the application will
examine requests coming to the server to determine if any Ajax support is required to render
to the page. Preprocessing configuration options are not enough alone to grant a page Ajax support,
but less work is required on a page level.
Some frameworks have a base page class that you inherit your ASP.NET web forms from instead
of the traditional System.Web.UI.Page class. The custom base class will encapsulate the required
plumbing that your page needs to handle Ajax interaction between the client and server.
The final option is to use a page control directive. These directives allow you to declaratively
add Ajax support to a page at design time. The page control directive can supersede the need
for preprocessing or the use of a base page class.
You will see each of these options implemented in coming examples.
Benefits
Preprocessing allows each page in your application the ability to cycle through your Ajax engine. This
can make turning on Ajax features of a page easier because there is less code you must write on the
page level.
Base classes give you all the benefits of inheritance by giving your page Ajax processing support “for free.”
The base class will often include useful services that make coding easier, but the true benefit is found in ease
of use. Often frameworks that provide a base class require very little extra coding to create an Ajax experience.
Another benefit to using a base class lies in the fact that you may inherit from the class. This extra layer
of abstraction allows you to make changes and provide services to your Ajax pages in one place.
Page control directives perhaps give you the best of both worlds, because you will avoid the disadvantages
(stated in the following section) of the preprocessing and base class options, while still giving your
page full Ajax support.
Drawbacks
In some experiments with frameworks that use preprocessing, we have found a few cases where the Ajax
httpHandler or httpModule interfered with applications that used their own custom handler or module.
The preprocessing configuration may not be the best option for you if you are working on a project
that already employs its own handlers or modules. Test rigorously to ensure a true working solution.
The drawbacks to base classes are classic in object-oriented development. If want to use a solution that
requires you inherit from the base class, you may run into a few problems. What if you are already inheriting
from your own base class? The .NET runtime does not support multiple inheritances, so you must
then make a choice; do you keep your base class or go with the base class from the third-party framework?
Other questions you might pose to yourself are: What if I change Ajax libraries? What will that
to do my inheritance structure? Although base classes at first may seem attractive, implementation by
inheritance, rather than composition may often architecturally limit you in the end.
Page control directives are nice because they are a light-touch addition to your page. However, since you
are not inheriting from the control, making global changes becomes more difficult. If you wanted to add
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