Top 10 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 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. 

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.


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.