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ASP.Net AJAX Interview Questions


What is Ajax?
Ajax means "Asynchronous Javascript and XML". Ajax represents a set of commonly used techniques, like HTML/XHTML, CSS, Document Object Model(DOM), XML/XSLT, Javascript and the XMLHttpRequest object, to create RIA's (Rich Internet Applications).

ASP.NET AJAX update only a specified portion of data without refreshing the entire page. The ASP.NET AJAX works with the AJAX Library that uses object-oriented programming (OOP) to develop rich Web applications that communicate with the server using asynchronous postback.

What are the different controls of ASP.NET AJAX?
ASP.NET AJAX includes the following controls:
  • ScriptManager
  • ScriptManagerProxy
  • UpdatePanel
  • UpdateProgress
  • Timer
Describe the technology that makes up AJAX?  
AJAX is based on the following web standards:
  • XMLHttpRequest Object
  • JavaScript
  • HTML/DHTML
  • DOM
  • XML
  • CSS
What role does the ScriptManager play?  
The ScriptManager manages all ASP.Net AJAX resources on a page and renders the links for the ASP.Net AJAX client libraries, which lets you use AJAX functionality like PageMethods, UpdatePanels etc. It creates the PageRequestManager and Application objects, which are prominent in raising events during the client life cycle of an ASP.Net AJAX Web page. It also helps you create proxies to call web services asynchronously.
ScriptManager class is the heart of ASP.NET Ajax. Before elaborating more on ScriptManager, note that ScriptManager is class and a control (both) in Ajax.

The ScriptManager class in ASP.NET manages Ajax Script Libraries, partial page rendering functionality and client proxy class generation for web applications and services. By saying client proxy class, this means an instance of the Ajax runtime is created on the browser.

This class is defined in the System.Web.Extensions.dll. You will find this DLL in your system's Global Assembly Cache at C:\Windows\Assembly (For XP)

Can we use multiple ScriptManager on a page?
No. You can use only one ScriptManager on a page

What is the role of a ScriptManagerProxy?
A page can contain only one ScriptManager control. If you have a Master-Content page scenario in your application and the MasterPage contains a ScriptManager control, then you can use the ScriptManagerProxy control to add scripts to content pages.

Also, if you come across a scenario where only a few pages in your application need to register to a script or a web service, then its best to remove them from the ScriptManager control and add them to individual pages, by using the ScriptManagerProxy control. That is because if you added the scripts using the ScriptManager on the Master Page, then these items will be downloaded on each page that derives from the MasterPage, even if they are not needed, which would lead to a waste of resources.

How many types of triggers are there in update panel?
There are 2 types of triggers.
1. PostBackTrigger : It does a full postback. This is useful when any such control which placed within updatePanel but it cannot work asynchronously. Like File Upload Control.
2. AsyncPostBackTrigger :- It does partial post back asynchronously.

Explain the UpdatePanel?
The UpdatePanel enables you to add AJAX functionality to existing ASP.NET applications. It can be used to update content in a page by using Partial-page rendering. By using Partial-page rendering, you can refresh only a selected part of the page instead of refreshing the whole page with a postback.

Why do we use the UpdateProgress control in AJAX? The UpdateProgress control is somewhat related to the UpdatePanel control. The UpdateProgress control enables you to design a user-friendly interface when a Web page consists of a number of UpdatePanel controls for partial-page rendering.

The UpdateProgress control makes you aware of the status information about the partial-page updates in the UpdatePanel control.

Can Ajax be implemented in browsers that do not support the XmlHttpRequest object?
Yes. This is possible using remote scripts.

How to use multiple ScriptManager controls in a web page?
No. It is not possible to use multiple ScriptManager control in a web page. In fact, any such requirement never comes in because a single ScriptManager control is enough to handle the objects of a web page.

How to trigger a postback on an UpdatePanel from Javascript? Call the __doPostBack function. ASP.NET runtime always creates a javascript function named __doPostBack(eventTarget, eventArgument) when the web page is rendered. A control ID may be passed here to specifically invoke updation of the UpdatePanel.

What will happen with click of browser “back” button among asynchronous requests?
By default, browser will unload the web page and and will return to previous Web page, independent of any asynchronous requests.


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