C#
Interview Questions And Answers
General Questions
Q1: Does C#
support multiple-inheritance?
Ans: No.
Q2: Who is a
protected class-level variable available to?
Ans: It is available to any sub-class (a class
inheriting this class).
Q3: Are private
class-level variables inherited?
Ans: Yes, but they are not accessible. Although
they are not visible or accessible via the class
interface, they are inherited.
Q4: Describe the
accessibility modifier “protected internal”.
Ans: It is available to classes that are within
the same assembly and derived from the
specified base class.
Q5: What’s the
top .NET class that everything is derived from?
Ans: System.Object.
Q6: What does the term immutable mean?
Ans: 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.
Q7: What’s the difference between System.String
and System.Text.StringBuilder classes?
Ans: System.String is immutable.
System.StringBuilder was designed with the
purpose of having a mutable string where a
variety of operations can be performed.
Q8: What’s the advantage of using System.Text.StringBuilder over System.String?
Ans: 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.
Q9: Can you store multiple data types in System.Array?
Ans: No.
Q10: What’s the difference between the System.Array.CopyTo() and System.Array.Clone()?
Ans: 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.
Q11: How can you sort the elements of the array
in descending order?
Ans: By calling Sort() and then Reverse()
methods.
Q12: What’s the .NET collection class that allows
an element to be accessed using a unique key?
Ans: HashTable.
Q13: What class is underneath the SortedList
class?
Ans: A sorted HashTable.
Q14: Will the
finally block get executed if an exception has
not occurred?
Ans: Yes.
Q15: What’s the C# syntax to catch any possible
exception?
Ans: 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 {}.
Q16: Can multiple catch blocks be executed for a
single try statement?
Ans: No. Once the proper catch block processed,
control is transferred to the finally block (if
there are any).
Q17: Explain the three services model commonly
know as a three-tier application.
Ans: Presentation (UI), Business (logic and
underlying code) and Data (from storage or other
sources).
Class Questions
Q18: What is the
syntax to inherit from a class in C#?
Ans: Place a colon and then the name of the base
class.
Example: class MyNewClass : MyBaseClass
Q19: Can you
prevent your class from being inherited by
another class?
Ans: Yes. The keyword “sealed” will prevent the
class from being inherited.
Q20: Can you
allow a class to be inherited, but prevent the
method from being over-ridden?
Ans: Yes. Just leave the class public and make
the method sealed.
Q21: What’s an
abstract class?
Ans: 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.
Q22: When do you
absolutely have to declare a class as abstract?
Ans: 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.
Q23: What is an
interface class?
Ans: 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.
Q24: Why can’t
you specify the accessibility modifier for
methods inside the interface?
Ans: They all must be public, and are therefore
public by default.
Q25: Can you
inherit multiple interfaces?
Ans: Yes. .NET does support multiple interfaces.
Q26: What
happens if you inherit multiple interfaces and
they have conflicting method names?
Ans: 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.
Q27: What’s the
difference between an interface and abstract
class?
Ans: 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.
Q28: What is the
difference between a Struct and a Class?
Ans: 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
Q29: What’s the
implicit name of the parameter that gets passed
into the set method/property of a class?
Ans: Value. The data type of the value parameter
is defined by whatever data type the property is
declared as.
Q30: What does
the keyword “virtual” declare for a method or
property?
Ans: The method or property can be overridden.
Q31: How is
method overriding different from method
overloading?
Ans: 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.
Q32: Can you
declare an override method to be static if the
original method is not static?
Ans: No. The signature of the virtual method must
remain the same. (Note: Only the keyword virtual
is changed to keyword override).
Q33: What are
the different ways a method can be overloaded?
Ans: Different parameter data types, different
number of parameters, different order of
parameters.
Q34: 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?
Ans: 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
Q35: What’s a
delegate?
Ans: A delegate object encapsulates a reference
to a method.
Q36: What’s a
multicast delegate?
Ans: A delegate that has multiple handlers
assigned to it. Each assigned handler (method)
is called.
XML Documentation
Questions
Q37: Is XML
case-sensitive?
Ans: Yes.
Q38: What’s the
difference between // comments, /* */ comments
and /// comments?
Ans: Single-line comments, multi-line comments,
and XML documentation comments.
Q39: How do you
generate documentation from the C# file
commented properly with a command-line compiler?
Ans: Compile it with the /doc switch.
Debugging and Testing
Questions
Q40: What
debugging tools come with the .NET SDK?
Ans: 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.
Q41: What does
assert() method do?
Ans: 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.
Q42: What’s the
difference between the Debug class and Trace
class?
Ans: Documentation looks the same. Use Debug
class for debug builds, use Trace class for both
debug and release builds.
Q43: Why are
there five tracing levels in System.Diagnostics.TraceSwitcher?
Ans: 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.
Q44: Where is
the output of TextWriterTraceListener
redirected?
Ans: To the Console or a text file depending on
the parameter passed to the constructor.
Q45: How do you
debug an ASP.NET Web application?
Ans: Attach the aspnet_wp.exe process to the
DbgClr debugger.
Q46: What are
three test cases you should go through in unit
testing?
Ans: 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).
Q47: Can you change the value of a variable while
debugging a C# application?
Ans: Yes. If you are debugging via Visual
Studio.NET, just go to Immediate window.
ADO.NET and Database
Questions
Q48: What is the
role of the DataReader class in ADO.NET
connections?
Ans: 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.
Q49: What are
advantages and disadvantages of
Microsoft-provided data provider classes in ADO.NET?
Ans: 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.
Q50: What is the
wildcard character in SQL?
Ans: 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%’.
Q51: Explain
ACID rule of thumb for transactions.
Ans: 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.
Q52: What
connections does Microsoft SQL Server support?
Ans: Windows Authentication (via Active
Directory) and SQL Server authentication (via
Microsoft SQL Server username and password).
Q53: Between
Windows Authentication and SQL Server
Authentication, which one is trusted and which
one is untrusted?
Ans: 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.
Q54: What does
the Initial Catalog parameter define in the
connection string?
Ans: The database name to connect to.
Q55: What does
the Dispose method do with the connection
object?
Ans: Deletes it from the memory.
To Do: answer better. The current answer is not
entirely correct.
Q56: What is a
pre-requisite for connection pooling?
Ans: 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
Q57: How is the
DLL Hell problem solved in .NET?
Ans: 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.
Q58: What are
the ways to deploy an assembly?
Ans: An MSI installer, a CAB archive, and XCOPY
command.
Q59: What is a
satellite assembly?
Ans: 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.
Q60: What
namespaces are necessary to create a localized
application?
Ans: System.Globalization and System.Resources.
Q61: What is the
smallest unit of execution in .NET?
Ans: an Assembly.
Q62: When should
you call the garbage collector in .NET?
Ans: 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.
Q63: How do you
convert a value-type to a reference-type?
Ans: Use Boxing.
Q64: What happens in memory when you Box and Unbox a value-type?
Ans: 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.
Tolal:0 Click:
Show All Comments
