Beginning Web Development, Silverlight, and ASP.NET AJAX

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CHAPTER 13 ■ AJAX SCRIPTS AND SERVICES
not the whole page). Should you set it to false, then full page refreshes will occur on
postbacks, unless you write JavaScript code that uses the XMLHttpRequest class to perform
asynchronous postbacks and then manually update the page. Typically for Ajax pages,
you will always set this to true. If your page is going to be relatively static (i.e., no postbacks
to the server), and you want to use a ScriptManager to deliver JavaScript, then you
can set it to false to reduce overhead.
Handling Errors in Partial Page Updates
In case you hit an error in the process of a partial page update, the ScriptManager has a
property called AllowCustomErrorsRedirect (defaulting to true), which determines that
the custom errors section of the Web.config file should be used when an error occurs—
namely that you can redirect the user to a friendly landing page instead of crashing them
out of your app.
Alternatively, you can catch and handle the AsyncPostBackError event, and/or set
the AsyncPostBackErrorMessage property to manage errors. To do this, you define the
AsyncPostBackError callback by setting the OnAsyncPostBackError attribute on the Script-
Manager control. Here’s an example:
Then, in your server code, you can implement a function that handles this error. In
this case, it takes the exception message and loads it into the AsyncPostBackErrorMessage
property, which is accessible from client-side script via the PageRequestManager (you’ll
see more of this in the next chapter).
protected void ScriptManager1_AsyncPostBackError(object sender,
AsyncPostBackErrorEventArgs e)
{
ScriptManager1.AsyncPostBackErrorMessage = e.Exception.Message;
}
For example, if you add an EndRequestHandler callback to the PageRequestManager,
like this:
Sys.WebForms.PageRequestManager.getInstance().add_endRequest(EndRequestHandler);
you can then access the AsyncPostBackErrorMessage from your JavaScript code as part of
the args sent back to the EndRequestHandler: