C# Interview Questions
This is a list of questions I have gathered from other sources and created
myself over a period of time from my experience, many of which I felt where
incomplete or simply wrong. I have finally taken the time to go through each
question and correct them to the best of my ability. However, please feel free to
post feedback to challenge, improve, or suggest new questions. I want to thank
those of you that have contributed quality questions and corrections thus far.
There are some question in this list that I do not consider to be good questions for
an interview. However, they do exist on other lists available on the Internet so I felt
compelled to keep them easy access.
General Questions
1. Does C# support multiple-inheritance?
No.
2. Who is a protected class-level variable available to?
It is available to any sub-class (a class inheriting this class).
3. Are private class-level variables inherited?
Yes, but they are not accessible. Although they are not visible or accessible via
the class interface, they are inherited.
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.
5. What’s the top .NET class that everything is derived from?
System.Object.
6. 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.
7. 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.
8. 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.
9. Can you store multiple data types in System.Array?
No.
10. 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.
11. How can you sort the elements of the array in descending order?
By calling Sort() and then Reverse() methods.
12. What’s the .NET collection class that allows an element to be
accessed using a unique key?
HashTable.
13. What class is underneath the SortedList class?
A sorted HashTable.
14. Will the finally block get executed if an exception has not occurred?
Yes.
15. 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 {}.
16. 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 (if there are any).
17. 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.
To Do: Investigate
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 as.
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.
XML Documentation Questions
1. Is XML case-sensitive?
Yes.
2. What’s the difference between // comments, /* */ comments
and /// comments?
Single-line comments, multi-line comments, and XML documentation comments.
3. How do you generate documentation from the C# file commented
properly with a command-line compiler?
Compile it with the /doc switch.
Debugging and Testing Questions
1. What debugging tools come with the .NET SDK?
1. CorDBG – command-line debugger. To use CorDbg, you must compile the
original C# file using the /debug switch.
2. DbgCLR – graphic debugger. Visual Studio .NET uses the DbgCLR.
2. What does assert() method do?
In debug compilation, assert takes in a Boolean condition as a parameter, and
shows the error dialog if the condition is false. The program proceeds without
any interruption if the condition is true.
3. What’s the difference between the Debug class and Trace class?
Documentation looks the same. Use Debug class for debug builds, use Trace
class for both debug and release builds.
4. Why are there five tracing levels in
System.Diagnostics.TraceSwitcher?
The tracing dumps can be quite verbose. For applications that are constantly
running you run the risk of overloading the machine and the hard drive. Five
levels range from None to Verbose, allowing you to fine-tune the tracing
activities.
5. Where is the output of TextWriterTraceListener redirected?
To the Console or a text file depending on the parameter passed to the
constructor.
6. How do you debug an ASP.NET Web application?
Attach the aspnet_wp.exe process to the DbgClr debugger.
7. What are three test cases you should go through in unit testing?
1. Positive test cases (correct data, correct output).
2. Negative test cases (broken or missing data, proper handling).
3. Exception test cases (exceptions are thrown and caught properly).
8. Can you change the value of a variable while debugging a C#
application?
Yes. If you are debugging via Visual Studio.NET, just go to Immediate window.
ADO.NET and Database Questions
1. What is the role of the DataReader class in ADO.NET connections?
It returns a read-only, forward-only rowset from the data source. A DataReader
provides fast access when a forward-only sequential read is needed.
2. What are advantages and disadvantages of Microsoft-provided data
provider classes in ADO.NET?
SQLServer.NET data provider is high-speed and robust, but requires SQL Server
license purchased from Microsoft. OLE-DB.NET is universal for accessing other
sources, like Oracle, DB2, Microsoft Access and Informix. OLE-DB.NET is a .NET
layer on top of the OLE layer, so it’s not as fastest and efficient as
SqlServer.NET.
3. What is the wildcard character in SQL?
Let’s say you want to query database with LIKE for all employees whose name
starts with La. The wildcard character is %, the proper query with LIKE would
involve ‘La%’.
4. Explain ACID rule of thumb for transactions.
A transaction must be:
1. Atomic - it is one unit of work and does not dependent on previous and
following transactions.
2. Consistent - data is either committed or roll back, no “in-between” case
where something has been updated and something hasn’t.
3. Isolated - no transaction sees the intermediate results of the current
transaction).
4. Durable - the values persist if the data had been committed even if the
system crashes right after.
5. What connections does Microsoft SQL Server support?
Windows Authentication (via Active Directory) and SQL Server authentication (via
Microsoft SQL Server username and password).
6. Between Windows Authentication and SQL Server Authentication,
which one is trusted and which one is untrusted?
Windows Authentication is trusted because the username and password are
checked with the Active Directory, the SQL Server authentication is untrusted,
since SQL Server is the only verifier participating in the transaction.
7. What does the Initial Catalog parameter define in the connection
string?
The database name to connect to.
8. What does the Dispose method do with the connection object?
Deletes it from the memory.
To Do: answer better. The current answer is not entirely correct.
9. What is a pre-requisite for connection pooling?
Multiple processes must agree that they will share the same connection, where
every parameter is the same, including the security settings. The connection
string must be identical.
Assembly Questions
1. How is the DLL Hell problem solved in .NET?
Assembly versioning allows the application to specify not only the library it needs
to run (which was available under Win32), but also the version of the assembly.
2. What are the ways to deploy an assembly?
An MSI installer, a CAB archive, and XCOPY command.
3. What is a satellite assembly?
When you write a multilingual or multi-cultural application in .NET, and want to
distribute the core application separately from the localized modules, the
localized assemblies that modify the core application are called satellite
assemblies.
4. What namespaces are necessary to create a localized application?
System.Globalization and System.Resources.
5. What is the smallest unit of execution in .NET?
an Assembly.
6. When should you call the garbage collector in .NET?
As a good rule, you should not call the garbage collector. However, you could
call the garbage collector when you are done using a large object (or set of
objects) to force the garbage collector to dispose of those very large objects from
memory. However, this is usually not a good practice.
7. How do you convert a value-type to a reference-type?
Use Boxing.
8. What happens in memory when you Box and Unbox a value-type?
Boxing converts a value-type to a reference-type, thus storing the object on the
heap. Unboxing converts a reference-type to a value-type, thus storing the value
on the stack.
1. Explain the differences between Server-side and Client-side code?
ANS: Server side code will execute at server end all the business logic will execute
at server end where as client side code will execute at client side at browser end.
2. What type of code (server or client) is found in a Code-Behind class?
ANS : Server side.
3. Should validation (did the user enter a real date) occur server-side or
client-side? Why?
ANS : client side . there is no need to go to validate user input. If it relates to data
base validation we need to validate at server side.
4. What does the "EnableViewState" property do? Why would I want it on
or off?
ANS: IT keeps the data of the control during post backs.
if we turn off the values should not populate during server round trip.
5. What is the difference between Server.Transfer and
Response.Redirect? Why would I choose one over the other?
ANS: Server.Trnasfer will prevent round trip. it will redirect pages which or in the
same directory. NO way to pass the query strings . Thru http context we can able
to get the previous page control values.
Response.Redirect : There is a round trip to process the request. We can redirect
to any page external / internal other than aspx. We can pass the query string thru
which we can manage sessions.
6. Can you give an example of when it would be appropriate to use a web
service as opposed to a non-serviced .NET component
ANS : Web services are best suite for Hetrogenious environment.
Remoting is best suite for Homogenious environment. The systems that under CLR.
7. Let's say I have an existing application written using Visual Studio 6
(VB 6, InterDev 6) and this application utilizes Windows 2000 COM+
transaction services. How would you approach migrating this application
to .NET
We need to have Wrapper to communicate COM components in .net. and vis versa
CCW : Com Callable wrapper.
RCW : RUN time callable wrapper.
8. Can you explain the difference between an ADO.NET Dataset and anADO
Recordset?\
ANS : DIsconnected architechure . Maintainace relation schemas. MUtilple table
grouping.
Connected one .
9. Can you give an example of what might be best suited to place in the
Application_Start and Session_Start subroutines?
ANS: APplication_start need for global variable which are available over the
application.
Sesssion_Start : login dependent ( user dependent)
10. If I'm developing an application that must accomodate multiple
security levels though secure login and my ASP.NET web appplication is
spanned across three web-servers (using round-robbin load balancing)
what would be the best approach to maintain login-in state for the
users?
ANS : Database Support or Thru state service.
11. What are ASP.NET Web Forms? How is this technology different than
what is available though ASP (1.0-3.0)?
ANS : ASP . Interprepter.. use the script engine.
ASP.Net Compiled.
12. How does VB.NET/C# achieve polymorphism?
ANS : Function overloading.
Operator overloading.
Can you explain what inheritance is and an example of when you might
use it?
ANS : Heridity.
Use the existing functionality along with its own properities.
13. How would you implement inheritance using VB.NET/C#?
ANS: Derived Class : Basecalss
VB.NEt : Derived Class Inherits Baseclass
14. Whats an assembly
ANS : A Basic unit of executable code > Which contains : Manifest - Meta data
versioning , Culture , IL, Reference
15. Describe the difference between inline and code behind - which is
best in a loosely coupled solution Tightly coupled - INLINE
ANS: inline function bind at compile time can write in aspx page with in <% %> .
17. Explain what a diffgram is, and a good use for one
ANS : is an xml grammer. it talk about state of node in xml file.
18. Where would you use an iHTTPModule, and what are the limitations
of any approach you might take in implementing one
ANS: Preprocessing before going to IIS.
20. What are the disadvantages of viewstate/what are the benefits
ANS : IT can be hacked . page is size is heavy.
21 Describe session handling in a webfarm, how does it work and what
are the limits
ANS:
Session - mode
State sever
OUtprocess
sql
22. How would you get ASP.NET running in Apache web servers - why
would you even do this?
ANS: ---- Install Mod_AspDotNet
Add at the end of C:\Program Files\Apache Group\Apache2\conf\httpd.conf the
following lines
23. Whats MSIL, and why should my developers need an appreciation of
it if at all?
ANS : Microsoft Intermeidate lanaguage. which is the out put for all the .net
supported languages after comiplation will produce.
Appreciation for cross language support.
24. In what order do the events of an ASPX page execute. As a developer
is it important to undertsand these events?
ANS : INIT, PageLoad, Prerender , UNload.
25. Which method do you invoke on the DataAdapter control to load your
generated dataset with data?
Fill()
26. Can you edit data in the Repeater control?
NO
27. Which template must you provide, in order to display data in a
Repeater control?
ITemtemplate
28. How can you provide an alternating color scheme in a
Repeatercontrol?
AlternateItemTemplate
29. What property must you set, and what method must you call in your
code, in order to bind the data from some data source to the
Repeatercontrol?
Datasource,
DataBind
30. What base class do all Web Forms inherit from?
System.Web.UI.Page
31. What method do you use to explicitly kill a user s session?
abondon()
32 How do you turn off cookies for one page in your site?
disablecookies.
33. Which two properties are on every validation control?
control to validate, error message
34. What tags do you need to add within the asp:datagrid tags to bind
columns manually?
autogenerated columns is set to false
35. How do you create a permanent cookie?
Cooke = ne cookee().
cooke.adddate.
36. What tag do you use to add a hyperlink column to the DataGrid?
hyper link column
37. What is the standard you use to wrap up a call to a Web service
------------
38. Which method do you use to redirect the user to another page
without performing a round trip to the client?
server.transfer
39. What is the transport protocol you use to call a Web service SOAP
http
40. True or False: A Web service can only be written in .NET
false
41. What does WSDL stand for? webservice discription language. it is
used to generate for proxy( server object)
42. What property do you have to set to tell the grid which page to go to
when using the Pager object?
Page Index.
43. Where on the Internet would you look for Web services?
UDDI
44. What tags do you need to add within the asp:datagrid tags to bind
columns manually.
Autogenerate columns
45. 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?
datatext
datavalue
46. How is a property designated as read-only?
get
47. Which control would you use if you needed to make sure the values
in two different controls matched?
compare filed validator
48. True or False: To test a Web service you must create a windows
application or Web application to consume this service?
no
49. How many classes can a single .NET DLL contain?
as many as u want..
1. Use of property in an interface:
An interface can have methods, properties, events and indexers. When a
property in an interface is implemented in a class,
the advantage is that, through that property, the class can give access to its
private member varibles.
1. 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.
2. How do you inherit from a class in C#?
Place a colon and then the name of the base class.
3. Does C# support multiple inheritance?
No, use interfaces instead.
4. When you inherit a protected class-level variable, who is it available
to? Classes in the same namespace.
5. 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. But they are.
6. 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).
7. 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.
8. What’s the top .NET class that everything is derived from?
System.Object.
9. 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.
10. What does the keyword virtual mean in the method definition? The
method can be over-ridden.
11.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.
12. 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.
13.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.
14.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.
15.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.
16. 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.
17.What’s an interface class?
It’s an abstract class with public abstract methods all of which must be
implemented in the inherited classes.
18.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.
19. Can you inherit multiple interfaces? Yes, why not.
20.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.
21.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.
22.How can you overload a method?
Different parameter data types, different number of parameters, different
order of parameters.
23.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.
24.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.
25. 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.
Can you can specify values for for variables in interfaces in c#?
(In Java you can)
Answer) NO. I tried this in VS.net and it gave me the following error:
D:\VisualStudio\…\Interface.cs(10): Interfaces cannot contain fields
