ASP.Net Interview Questions

1.       Describe the role of inetinfo.exe, aspnet_isapi. dll and aspnet_wp.exe in the page loading process?

 inetinfo.exe is the Microsoft IIS server running, handling ASP.NET requests among other things. When an ASP.NET request is received (usually a file with .aspx extension), the ISAPI filter aspnet_isapi. dll takes care of it by passing the request to the actual worker process aspnet_wp.exe.

2.      What’s the difference between Response.Write( ) and Response.Output. Write()?

Response.Output. Write() allows you to write formatted output.

3.      What methods are fired during the page load?

 Init() - when the page is instantiated

Load() - when the page is loaded into server memory

PreRender() - the brief moment before the page is displayed to the user as HTML

Unload() - when page finishes loading.

4.      When during the page processing cycle is ViewState available?

After the Init() and before the Page_Load(), or OnLoad() for a control.

5.      What namespace does the Web page belong in the .NET Framework class hierarchy?

System.Web.UI.Page

6.      Where do you store the information about the user’s locale?

System.Web.UI.Page.Culture

7.      What’s the difference between Codebehind=" MyCode.aspx. cs" and Src="MyCode. aspx.cs"?

CodeBehind is relevant to Visual Studio.NET only.

8.     What’s a bubbled event?

When you have a complex control, like DataGrid, writing an event processing routine for each object (cell, button, row, etc.) is quite tedious. The controls can bubble up their event handlers, allowing the main DataGrid event handler to take care of its constituents.

9.      Suppose you want a certain ASP.NET function executed on MouseOver for a certain button. Where do you add an event handler?

Add an OnMouseOver attribute to the button. Example: btnSubmit.Attribute s.Add("onmouseov er","someClientCodeHere() ;");

10.  What data types does the RangeValidator control support?

 Integer, String, and Date.

11.   Explain the differences between Server-side and Client-side code?

Server-side code executes on the server. Client-side code executes in the client's browser.

12. What type of code (server or client) is found in a Code-Behind class?

The answer is server-side code since code-behind is executed on the server. However, during the code-behind' s execution on the server, it can render client-side code such as JavaScript to be processed in the clients browser. But just to be clear, code-behind executes on the server, thus making it server-side code.

13.  Should user input data validation occur server-side or client-side? Why?

All user input data validation should occur on the server at a minimum. Additionally, client-side validation can be performed where deemed appropriate and feasible to provide a richer, more responsive experience for the user.

14.  What is the difference between Server.Transfer and Response.Redirect? Why would I choose one over the other?

Server.Transfer transfers page processing from one page directly to the next page without making a round-trip back to the client's browser. This provides a faster response with a little less overhead on the server. Server.Transfer does not update the clients URL history list or current URL. Response.Redirect is used to redirect the user's browser to another page or site. This performs as a trip back to the client where the client's browser is redirected to the new page. The user's browser history list is updated to reflect the new address.

15.   Can you explain the difference between an ADO.NET Dataset and an ADO Recordset?

Valid answers are:

A DataSet can represent an entire relational database in memory, complete with tables, relations, and views.

A DataSet is designed to work without any continuing connection to the original data source.

Data in a DataSet is bulk-loaded, rather than being loaded on demand.

There's no concept of cursor types in a DataSet.

DataSets have no current record pointer you can use For Each loops to move through the data.

You can store many edits in a DataSet, and write them to the original data source in a single operation.

Though the DataSet is universal, other objects in ADO.NET come in different versions for different data sources.

16.  What is the Global.asax used for?

The Global.asax (including the Global.asax. cs file) is used to implement application and session level events.

17.   What are the Application_ Start and Session_Start subroutines used for?

 This is where you can set the specific variables for the Application and Session objects.

18.  Can you explain what inheritance is and an example of when you might use it?

When you want to inherit (use the functionality of) another class. Example: With a base class named Employee, a Manager class could be derived from the Employee base class.

19.  What’s an assembly?

Assemblies are the building blocks of the .NET framework.

20.  Describe the difference between inline and code behind?

Inline code written alongside the html in a page. Code-behind is code written in a separate file and referenced by the .aspx page.

21.  What’s MSIL, and why should my developers need an appreciation of it if at all?

MSIL is the Microsoft Intermediate Language. All .NET compatible languages will get converted to MSIL. MSIL also allows the .NET Framework to JIT compiles the assembly on the installed computer.

22. Which method do you invoke on the DataAdapter control to load your generated dataset with data?

 The Fill() method.

23. Can you edit data in the Repeater control?

No, it just reads the information from its data source.

24. Which template must you provide, in order to display data in a Repeater control?

ItemTemplate.

25.  How can you provide an alternating color scheme in a Repeater control?

Use the AlternatingItemTemp late.

26. What property must you set, and what method must you call in your code, in order to bind the data from a data source to the Repeater control?

You must set the DataSource property and call the DataBind method.

27.  What base class do all Web Forms inherit from?

The Page class.

28. Name two properties common in every validation control?

 ControlToValidate property and Text property.

29.  Which property on a Combo Box do you set with a column name, prior to setting the DataSource, to display data in the combo box?

DataTextField property.

30. Which control would you use if you needed to make sure the values in two different controls matched?

CompareValidator control.

31.  How many classes can a single .NET DLL contain?

 It can contain many classes.

32.  What is ViewState?

ViewState allows the state of objects (serializable) to be stored in a hidden field on the page. ViewState is transported to the client and back to the server, and is not stored on the server or any other external source. ViewState is used to retain the state of server-side objects between post backs.

33. What is the lifespan for items stored in ViewState?

 Item stored in ViewState exist for the life of the current page. This includes postbacks (to the same page).

34.  What does the "EnableViewState" property do? Why would I want it on or off?

 It allows the page to save the users input on a form across postbacks. It saves the server-side values for a given control into ViewState, which is stored as a hidden value on the page before sending the page to the clients browser. When the page is posted back to the server the server control is recreated with the state stored in viewstate.

35.  What is CLS (Common Language Specification) ?

 It provides the set of specification which has to be adhered by any new language writer / Compiler writer for .NET Framework. This ensures Interoperability. For example: Within an ASP.NET application written in C#.NET language, we can refer to any DLL written in any other language supported by .NET Framework. As of now .NET supports around 32 languages.

36. What is CTS (Common Type System)?

 It defines about how Objects should be declared, defined and used within .NET. CLS is the subset of CTS.

37.  What is Boxing and UnBoxing?

Boxing is implicit conversion of ValueTypes to Reference Types (Object). UnBoxing is explicit conversion of Reference Types (Object) to its equivalent ValueTypes. It requires type-casting.

38. What is the difference between Value Types and Reference Types?

Value Types uses Stack to store the data whereas the later uses the Heap to store the data.

State Management in an ASP.Net application

Web Pages developed in ASP.Net are HTTP based and HTTP protocol is a stateless protocol. It means that web server does not have any idea about the requests from where they coming i.e. from same client or new clients. On each request web pages are created and destroyed.

So, how do we make web pages in ASP.Net which will remember about the user, would be able to distinguish b/w old clients(requests) and new clients(requests) and users previous filled information while navigating to other web pages in web site?

Solution of the above problem lies in State Management.

ASP.Net technology offers following state management techniques.

Client side State Management

·         Cookies

·         Hidden Fields

·         View State

·         Query String

Server side State Management

·         Session State

·         Application State

These state management techniques can be understood and by following simple examples and illustrations of the each techniques.

Client Side State Management

Cookies

A cookie is a small amount of data which is either stored at client side in text file or in memory of the client browser session. Cookies are always sent with the request to the web server and information can be retrieved from the cookies at the web server. In ASP.Net, HttpRequest object contains cookies collection which is nothing but list of HttpCookie objects. Cookies are generally used for tracking the user/request in ASP.Net for example, ASP.Net internally uses cookie to store session identifier to know whether request is coming from same client or not. We can also store some information like user identifier (UserName/Nick Name etc.) in the cookies and retrieve them when any request is made to the web server as described in following example. It should be noted that cookies are generally used for storing only small amount of data(i.e. 1-10 KB).

Code Sample

//Storing value in cookie 
HttpCookie cookie = new HttpCookie("NickName");
cookie.Value = "David";
Request.Cookies.Add(cookie);
//Retrieving value in cookie 
if (Request.Cookies.Count > 0 && Request.Cookies["NickName"] != null)
         lblNickName.Text = "Welcome" + Request.Cookies["NickName"].ToString();
else
         lblNickName.Text = "Welcome Guest";

Cookies can be permanent in nature or temporary. ASP.Net internally stores temporary cookie at the client side for storing session identifier. By default cookies are temporary and permanent cookie can be placed by setting "Expires" property of the cookie object.

Hidden Fields

A Hidden control is the control which does not render anything on the web page at client browser but can be used to store some information on the web page which can be used on the page.

HTML input control offers hidden type of control by specifying type as "hidden". Hidden control behaves like a normal control except that it is not rendered on the page. Its properties can be specified in a similar manner as you specify properties for other controls. This control will be posted to server in HttpControl collection whenever web form/page is posted to server. Any page specific information can be stored in the hidden field by specifying value property of the control.

ASP.Net provides HtmlInputControl that offers hidden field functionality.

Code Sample

//Declaring a hidden variable 
protected HtmlInputHidden hidNickName;
//Populating hidden variable
hidNickName.Value = "Page No 1";
//Retrieving value stored in hidden field.
string str = hidNickName.Value;

Note: Critical information should not be stored in hidden fields.

View State/Control State

ASP.Net technology provides View State/Control State feature to the web forms. View State is used to remember controls state when page is posted back to server. ASP.Net stores view state on client site in hidden field __ViewState in encrypted form. When page is created on web server this hidden control is populate with state of the controls and when page is posted back to server this information is retrieved and assigned to controls. You can look at this field by looking at the source of the page (i.e. by right clicking on page and selecting view source option.)

You do not need to worry about this as this is automatically handled by ASP.Net. You can enable and disable view state behavior of page and its control by specifying 'enableViewState' property to true and false. You can also store custom information in the view state as described in following code sample. This information can be used in round trips to the web server.

Code Sample

//To Save Information in View State 
ViewState.Add ("NickName", "David");
//Retrieving View state
String strNickName = ViewState ["NickName"];

Query String

Query string is the limited way to pass information to the web server while navigating from one page to another page. This information is passed in url of the request.  Following is an example of retrieving information from the query strings.

Code Sample

//Retrieving values from query string 
String nickname;
//Retrieving from query string
nickName = Request.Param["NickName"].ToString(); 

But remember that many browsers impose a limit of 255 characters in query strings. You need to use HTTP-Get method to post a page to server otherwise query string values will not be available.

Server Side State Management

Session State

Session state is used to store and retrieve information about the user as user navigates from one page to another page in ASP.Net web application. Session state is maintained per user basis in ASPNet runtime. It can be of two types in-memory and out of memory. In most of the cases small web applications in-memory session state is used. Out of process session state management technique is used for the high traffic web applications or large applications. It can be configured  with some configuration settings in web.conig file to store state information in ASPNetState.exe (windows service exposed in .Net or on SQL server.

In-memory Session state can be used in following manner

Code Sample

//Storing information in session state 
Session["NickName"] = "Ambuj";
//Retrieving information from session state
string str = Session["NickName"];

Session state is being maintained automatically by ASP.Net. A new session is started when a new user sent  first request to the server. At that time session state is created and user can use it to store information and retrieve it while navigating to different web pages in ASP.Net web application.

ASP.Net maintains session information using the session identifier which is being transacted b/w user machine and web server on each and every request either using cookies or querystring (if cookieless session is used in web application).

Application State

Application State is used to store information which is shared among users of the ASP.Net web application. Application state is stored in the memory of the windows process which is processing user requests on the web server. Application state is useful in storing small amount of often-used data. If application state is used for such data instead of frequent trips to database, then it increases the response time/performance of the web application.

In ASP.Net, application state is an instance of HttpApplicationState class and it exposes key-value pairs to store information. Its instance is automatically created when a first request is made to web application by any user and same state object is being shared across all subsequent users.

Application state can be used in similar manner as session state but it should be noted that many user might be accessing application state simultaneously so any call to application state object needs to be thread safe. This can be easily achieved in ASP.Net by using lock keyword on the statements which are accessing application state object. This lock keyword places a mutually exclusive lock on the statements and only allows a single thread to access the application state at a time. Following is an example of using application state in an application.

Code Sample

lock (this)
{
       Application["NickName"] = "David";
} 
//Retrieving value from application state
lock (this)
{
      string str = Application["NickName"].ToString();
}

So, In the above illustrations, we understood the practical concepts of using different state management techniques in ASP.Net technology.

.NET Interview Quetions

1.Does C# support multiple-inheritance?
No. But you can use Interfaces.

2.Where is a protected class-level variable available?
It is available to any sub-class derived from base class

3.Are private class-level variables inherited?
Yes, but they are not accessible.


4.Describe the accessibility modifier “protected internal”.
It is available to classes that are within the same assembly and derived from the specified base class.

6.Which class is at the top of .NET class hierarchy?
System.Object.

7.What does the term immutable mean?
The data value may not be changed.
Note: The variable value may be changed, but the original immutable data value was discarded and a new data value was created in memory.

8.What’s the difference between System.String and System.Text.StringBuilder classes?
System.String is immutable.
System.StringBuilder was designed with the purpose of having a mutable string where a variety of operations can be performed.

9.What’s the advantage of using System.Text.StringBuilder over System.String?

StringBuilder is more efficient in cases where there is a large amount of string manipulation. Strings are immutable, so each time a string is changed, a new instance in memory is created.

10.Can you store multiple data types in System.Array?
No.

11.What’s the difference between the System.Array.CopyTo() and System.Array.Clone()?
The Clone() method returns a new array (a shallow copy) object containing all the elements in the original array. The CopyTo() method copies the elements into another existing array. Both perform a shallow copy. A shallow copy means the contents (each array element) contains references to the same object as the elements in the original array. A deep copy (which neither of these methods performs) would create a new instance of each element's object, resulting in a different, yet identacle object.

12.How can you sort the elements of the array in descending order?
By calling Sort() and then Reverse() methods.

13.What’s the .NET collection class that allows an element to be accessed using a unique key?
HashTable.

14.What class is underneath the SortedList class?
A sorted HashTable.

15.Will the finally block get executed if an exception has not occurred?
Yes.

16.What’s the C# syntax to catch any possible exception?
A catch block that catches the exception of type System.Exception. You can also omit the parameter data type in this case and just write catch {}.

17.Can multiple catch blocks be executed for a single try statement?
No. Once the proper catch block processed, control is transferred to the finally block .

18.Explain the three services model commonly know as a three-tier application?
Presentation (UI), Business (logic and underlying code) and Data (from storage or other sources).

Class Questions

1.What is the syntax to inherit from a class in C#?
Place a colon and then the name of the base class.
Example: class MyNewClass : MyBaseClass

2.Can you prevent your class from being inherited by another class?
Yes. The keyword “sealed” will prevent the class from being inherited.

3.Can you allow a class to be inherited, but prevent the method from being over-ridden?
Yes. Just leave the class public and make the method sealed.

4.What’s an abstract class?
A class that cannot be instantiated. An abstract class is a class that must be inherited and have the methods overridden. An abstract class is essentially a blueprint for a class without any implementation.

5.When do you absolutely have to declare a class as abstract?

1. When the class itself is inherited from an abstract class, but not all base abstract methods have been overridden.

2. When at least one of the methods in the class is abstract.


6.What is an interface class?
Interfaces, like classes, define a set of properties, methods, and events. But unlike classes, interfaces do not provide implementation. They are implemented by classes, and defined as separate entities from classes.

7.Why can’t you specify the accessibility modifier for methods inside the interface?
They all must be public, and are therefore public by default.

8.Can you inherit multiple interfaces?
Yes. .NET does support multiple interfaces.

9.What happens if you inherit multiple interfaces and they have conflicting method names?
It’s up to you to implement the method inside your own class, so implementation is left entirely up to you. This might cause a problem on a higher-level scale if similarly named methods from different interfaces expect different data, but as far as compiler cares you’re okay.

10. What’s the difference between an interface and abstract class?
In an interface class, all methods are abstract - there is no implementation. In an abstract class some methods can be concrete. In an interface class, no accessibility modifiers are allowed. An abstract class may have accessibility modifiers.

11. What is the difference between a Struct and a Class?
Structs are value-type variables and are thus saved on the stack, additional overhead but faster retrieval. Another difference is that structs cannot inherit.

Method and Property Questions 

1. What’s the implicit name of the parameter that gets passed into the set method/property of a class?
Value. The data type of the value parameter is defined by whatever data type the property is declared .

2. What does the keyword “virtual” declare for a method or property?
The method or property can be overridden.

3. How is method overriding different from method overloading?
When overriding a method, you change the behavior of the method for the derived class. Overloading a method simply involves having another method with the same name within the class.

4. Can you declare an override method to be static if the original method is not static?
No. The signature of the virtual method must remain the same. (Note: Only the keyword virtual is changed to keyword override)

5. What are the different ways a method can be overloaded?
Different parameter data types, different number of parameters, different order of parameters.

6. If a base class has a number of overloaded constructors, and an inheriting class has a number of overloaded constructors; can you enforce a call from an inherited constructor to a specific base constructor?
Yes, just place a colon, and then keyword base (parameter list to invoke the appropriate constructor) in the overloaded constructor definition inside the inherited class.

Events and Delegates

1. What’s a delegate?
A delegate object encapsulates a reference to a method.

2. What’s a multicast delegate?
A delegate that has multiple handlers assigned to it. Each assigned handler (method) is called.

3. What’s the implicit name of the parameter that gets passed into the class’ set method?
Value, and it’s datatype depends on whatever variable we’re changing.

4. How do you inherit from a class in C#?
Place a colon and then the name of the base class.

5. Does C# support multiple inheritance?
No, use interfaces instead.

6. When you inherit a protected class-level variable, who is it available to?
Classes in the same namespace.

7. Are private class-level variables inherited?
Yes, but they are not accessible, so looking at it you can honestly say that they are not inherited.

8. Describe the accessibility modifier protected internal.?
It’s available to derived classes and classes within the same Assembly (and naturally from the base class it’s declared in).

9. C# provides a default constructor for me. I write a constructor that takes a string as a parameter, but want to keep the no parameter one. How many constructors should I write?
Two. Once you write at least one constructor, C# cancels the freebie constructor, and now you have to write one yourself, even if there’s no implementation in it.

10. What’s the top .NET class that everything is derived from?
System.Object.

11. How’s method overriding different from overloading?
When overriding, you change the method behavior for a derived class. Overloading simply involves having a method with the same name within the class.

12. What does the keyword virtual mean in the method definition?
The method can be over-ridden.

13. Can you declare the override method static while the original method is non-static?
No, you can’t, the signature of the virtual method must remain the same, only the keyword virtual is changed to keyword override.

14. Can you override private virtual methods?
No, moreover, you cannot access private methods in inherited classes, have to be protected in the base class to allow any sort of access.

15. Can you prevent your class from being inherited and becoming a base class for some other classes?
Yes, that’s what keyword sealed in the class definition is for. The developer trying to derive from your class will get a message: cannot inherit from Sealed class WhateverBaseClassName.
It’s the same concept as final class in Java.

16. Can you allow class to be inherited, but prevent the method from being over-ridden?
Yes, just leave the class public and make the method sealed.

17. What’s an abstract class?
A class that cannot be instantiated. A concept in C++ known as pure virtual method. A class that must be inherited and have the methods over-ridden.
Essentially, it’s a blueprint for a class without any implementation.

18. When do you absolutely have to declare a class as abstract (as opposed to free-willed educated choice or decision based on UML diagram)?
When at least one of the methods in the class is abstract. When the class itself is inherited from an abstract class, but not all base abstract methods have been over-ridden.

19. What’s an interface class?
It’s an abstract class with public abstract methods all of which must be implemented in the inherited classes.

20. Why can’t you specify the accessibility modifier for methods inside the interface?
They all must be public. Therefore, to prevent you from getting the false impression that you have any freedom of choice, you are not allowed to specify any accessibility, it’s public by default.

21. Can you inherit multiple interfaces?
Yes, why not.

22. And if they have conflicting method names?
It’s up to you to implement the method inside your own class, so implementation is left entirely up to you.
This might cause a problem on a higher-level scale if similarly named methods from different interfaces expect different data, but as far as compiler cares you’re okay.

23. What’s the difference between an interface and abstract class?
In the interface all methods must be abstract, in the abstract class some methods can be concrete. In the interface no accessibility modifiers are allowed, which is ok in abstract classes.

24. How can you overload a method?
Different parameter data types, different number of parameters, different order of parameters.

25. If a base class has a bunch of overloaded constructors, and an inherited class has another bunch of overloaded constructors, can you enforce a call from an inherited constructor to an arbitrary base constructor?
Yes, just place a colon, and then keyword base (parameter list to invoke the appropriate constructor) in the overloaded constructor definition inside the inherited class.

26. What’s the difference between System.String and System.StringBuilder classes?
System.String is immutable, System.StringBuilder was designed with the purpose of having a mutable string where a variety of operations can be performed.

27. Is it namespace class or class namespace?
The .NET class library is organized into namespaces. Each namespace contains a functionally related group of classes so natural namespace comes first.