Some imp ASP.NET Interview Questions and Answers

“ASP.NET is a web application development framework for building web sites and web applications that follows object oriented programming approach”.

Following are the top 10 commonly asked Interview Questions with detailed answers on ASP.NET:

What is the concept of Postback in ASP.NET?
A postback is a request sent from a client to server from the same page user is already working with.
ASP.NET was introduced with a mechanism to post an HTTP POST request back to the same page. It’s basically posting a complete page back to server (i.e. sending all of its data) on same page. So, the whole page is refreshed.
Another concept related to this approach is “Callback” that is also asked sometimes during a technical interview question. Click here to understand Postback Vs Callback in ASP.NET.

Difference between ASP.NET WebForms and ASP.NET MVC?
ASP.NET Web Forms uses Page controller pattern approach for rendering layout. In this approach, every page has it’s own controller i.e. code-behind file that processes the request. On the other hand, ASP.NET MVC uses Front Controller approach. In this approach a common controller for all pages, processes the requests.
Please follow for detailed information on WebForms Vs MVC.

Please briefly explain ASP.NET Page life Cycle?
ASP.NET page passes through a series of steps during its life cycle. Following is the high-level explanation of life cycle stages/steps.

Initialization: Controls raise their Init event in this stage.Objects and variables are initializes for complete lifecyle of request.

LoadViewState: is a post back stage and loads the view state for the controls that enabled its view state property.

LoadPostBackData: is also a post back stage and loads the data posted for the controls and update them.

Load: In this stage page as well as all the controls raise their Load event. Till this stage all the controls are initialized and loaded. In most of the cases, we are coding this event handler.

RaisePostBackEvent: is again a postback stage. For example, it’s raise against a button click event. We can easily put our code here to perform certain actions.

SaveViewState: Finally, controls state is saved in this stage before Rendering HTML.

Render: This is the stage where HTML is generated for the page.

Dispose: Lastly, all objects associated with the request are cleaned up.
For very detailed explanation of Page Life Cycle is explained here.

What is the difference between custom controls and user controls?
Custom controls are basically compiled code i.e. DLLs. These can be easily added to toolbox, so it can be easily used across multiple projects using drag and drop approach. These controls are comparatively hard to create.
But User Controls (.ascx) are just like pages (.aspx). These are comparatively easy to create but tightly couple with respect to User Interface and code. In order to use across multiple projects, we need to copy and paste to the other project as well.

What is the concept of view state in ASP.NET?
As in earlier question, we understood the concept of postback. So, in order to maintain the state between postbacks, ASP.NET provides a mechanism called view state. Hidden form fields are used to store the state of objects on client side and returned back to server in subsequent request (as postback occurs).

Difference between Response.Redirect and Server.Transfer?
In case of Response.Redirect, a new request is generated from client-side for redirected page. It’s a kind of additional round trip. As new request is generated from client, so the new URL is visible to user in browser after redirection.
While in case of Server.Transfer, a request is transferred from one page to another without making a round trip from client. For the end user, URL remains the same in browser even after transferring to another page.

Please briefly explain the usage of Global.asax?
Global.asax is basically ASP.NET Application file. It’s a place to write code for Application-level events such as Application start, Application end, Session start and end, Application error etc. raised by ASP.NET or by HTTP Modules.

There is a good list of events that are fired but following are few of the important events in Global.asax:

  • Application_Init occurs in case of application initialization for the very first time.
  • Application_Start fires on application start.
  • Session_Start fires when a new user session starts
  • Application_Error occurs in case of an unhandled exception generated from application.
  • Session_End fires when user session ends.
  • Application_End fires when application ends or time out.

What are the different types of Validation controls in ASP.NET?

In order to validate user input, ASP.NET provides validation server controls. All validation controls inherits from BaseValidator class which contains the common validation properties and methods like ControlToValidate, Enabled, IsValid, EnableClientScript, ValidationGroup,Validate() etc.

ASP.Net provides a range of validation controls:

  • RequiredFieldValidator validates compulsory/required input.
  • RangeValidator validates the range. Validates that input falls between the given range values.
  • CompareValidator validates or compares the input of a control with another control value or with a fixed value.
  • RegularExpressionValidator validates input value against a defined regular expression pattern.
  • CustomValidator allows to customize the validation logic with respect to our application logic.
  • ValidationSummary displays all errors on page collectively.

What are the types of Authentication in ASP.NET?

There are three types of authentication available in ASP.NET:

  • Windows Authentication: This authentication method uses built-in windows security features to authenticate user.
  • Forms Authentication: authenticate against a customized list of users or users in a database.
  • Passport Authentication: validates against Microsoft Passport service which is basically a centralized authentication service.

What are Session state modes in ASP.NET?

ASP.NET supports different session state storage options:

  • In-Process is the default approach. It stores session state locally on same web server memory where the application is running.
  • StateServer mode stores session state in a process other than the one where application is running. Naturally, it has added advantages that session state is accessible from multiple web servers in a Web Farm and also session state will remain preserved even web application is restarted.
  • SQLServer mode stores session state in SQL Server database. It has the same advantages as that of StateServer.
  • Custom modes allows to define our custom storage provider.
  • Off mode disables session storage.

What are HttpHandlers and HttpModules in ASP.NET?

In order to fully comprehend the concept of HttpHandlers and HttpModules, I have written a detailed ASP.NET Tutorial. Here I am defining both the concepts as follows:

HttpHandler: ASP.NET Engine uses HttpHandlers to handle specific requests on the basis of it’s extensions. ASP.NET Page Handler handles all requests coming for (.aspx) pages. We can define our own custom HttpHandler to handle a specific request with a specific extension, say .jpeg, .gif, or .ahmad. But there will always be only one handler for a specific request.

HttpModule: ASP.NET Engine uses HttpModules to inject some specific functionality along with ASP.NET default functionality for all incoming requests regardless of its extensions. There are a number of built-in modules already available in ASP.NET HTTP Pipeline. But we can write our own custom HTTP module to perform some additional functionality (for example, URL rewriting or implementing some security mechanism) for all incoming requests.


What is State Management?

HTTP is a stateless protocol by nature. So, we need some mechanism to preserve state (i.e. state of a webpage, a control or an object etc.) between subsequent requests to server from one or more clients. And this mechanism is referred as State Management.

What are the State Management Techniques used in ASP.NET?

State Management techniques used in ASP.NET can be categorized in two types:

  1. Client-Side State Management
    • View State
    • Control State
    • Hidden Fields
    • Cookies
    • Query String
  2. Server-Side State Management
    • Application State
    • Session State
    • Profile Properties

What is ViewState? or Explain ViewState as State Management Technique?

ViewState is one of the Client-Side State Management techniques that provides page-level state management, which means state is preserved between subsequent requests to same page. By using this technique, state of the page along with its controls is stored in a hidden form field  i.e. “__VIEWSTATE” and this field is again available on server when page is posted back with HTTP Request.
You can find this hidden field by looking into view source of an .ASPX page as:

<input type=”hidden” name=”__VIEWSTATE” value=”wEPDwUKMTM4OTIxNTEzNA9kFgJmD2QWAgIBD2QWAgIDDxYCHgVzdHlsZQV” />

ViewState data is encoded in Base64 String encoded format.

Can we Enable/Disable ViewState?

Yes, ViewState can be enabled or disable at different levels:

  • Control Level
    ViewState for a specific control can be enabled or disabled by setting EnableViewState property as follows:

    aControl.EnableViewState = false;
  • Page Level
    We can enable/disable ViewState for a complete page as follows:

    <%@ Page Language=”C#” EnableViewState=”false” %>
  • Application Level
    For whole application, we can enable/disable views in configuration file as follows:

    <pages enableViewState=”false”>

What is the difference between Session.Clear() and Session.Abandon() in ASP.NET?

As we understand that Session is a Collection and it stores data as Key/Value pair. So,
Session.Clear() clears all the session values but doesn’t destroy the Session. however,
 Session.Abandon() destroys the session object.
In other words, Session.Clear() is like deleting all files inside a folder (say “Root”) but Session.Abandon() means deleting the “Root” folder.

What is the difference between Application and Session State?

Application state is basically a common data repository for an application’s all users and their all sessions. On the other hand, Session state is specific to a single user session.
So, we can store data in application state object that is common for all users of a particular application as follows:

//Set Value
Application[“UsersCounter”] = Convert.ToInt32(Application[“UsersCounter”]) + 1;
//Retrieve Value
lblUsersCounter.Text = Application[“UsersCounter”].ToString();

It’s recommended to store smaller size values in application object.

Session object can store data for a specific session of user. Storage and retrieval is also simple just as for application object.

//Set Value
Session[“ProductsCount”] = Convert.ToInt32(Session[“ProductsCount”]) + 1;
//Retrieve Value
lblProductsCounter.Text = Session[“ProductsCount”].ToString();

Interview Questions about Session State Modes and Session_Start/Session_End events in Global.asax are alreadyexplained here.

What is the difference between Label Control and Literal Control?

A Label control in ASP.NET renders text inside <span> tags while a Literal Control renders just the text without any tags.
With Label controls we can easily apply styles using it’s CssClass property, however, if we don’t want to apply style/formatting, it’s better to go for a Literal control.

Hyperlink Vs LinkButton in ASP.NET?

A Hyperlink just redirects to a given URL identified by “NavigateURL” property. However a LinkButton which actually displays a Hyperlink style button causes a postback to the same page but it doesn’t redirect to a given URL.

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