C# Interview Questions

C# Interview Questions:

1) What is C-Sharp (C#)?

C# is a type-safe, managed and object oriented language, which is compiled by .Net framework for generating intermediate language (IL).

2) Explain the features of C#?

Below are some of the features supported in C# -

• Constructors and Destructors
• Properties
• Passing Parameters
• Arrays
• Main
• XML Documentation and
• Indexers

3) List some of the advantages of C#?

Below are the advantages of C# -

• Easy to learn
• Object oriented
• Component oriented
• Part of .NET framework

4) What are IDE’s provided by Microsoft for C# development?

Below are the IDE’s used for C# development –

• Visual Studio Express (VCE)
• Visual Studio (VS)
• Visual Web Developer

5) Explain the types of comments in C#?

Below are the types of comments in C# -

• Single Line Comment Eg : //
• Multiline Comments Eg: /* */
• XML Comments Eg : ///

6) Explain sealed class in C#?

Sealed class is used to prevent the class from being inherited from other classes. So “sealed” modifier also can be used with methods to avoid the methods to override in the child classes.

7) Give an example of using sealed class in C#?

Below is the sample code of sealed class in C# -

class X {} 
sealed class Y : X {}

Sealed methods –

class A
{
 protected virtual void First() { }
 protected virtual void Second() { }
}
class B : A
{
 sealed protected override void First() {}
 protected override void Second() { }
}

If any class inherits from class “B” then method – “First” will not be overridable as this method is sealed in class B.

8) List out the differences between Array and ArrayList in C#?

• Array stores the values or elements of same data type but arraylist stores values of different datatypes.
• Arrays will use the fixed length but arraylist does not uses fixed length like array.

9) Why to use “using” in C#?

“Using” statement calls – “dispose” method internally, whenever any exception occurred in any method call and in “Using” statement objects are read only and cannot be reassignable or modifiable.

10) Explain namespaces in C#?

Namespaces are containers for the classes. We will use namespaces for grouping the related classes in C#. “Using” keyword can be used for using the namespace in other namespace.

11) Why to use keyword “const” in C#? Give an example.

“Const” keyword is used for making an entity constant. We can’t reassign the value to constant.

Eg: const string _name = "Test";

12) What is the difference between “constant” and “readonly” variables in C#?

• “Const” keyword is used for making an entity constant. We cannot modify the value later in the code. Value assigning is mandatory to constant variables.
• “readonly” variable value can be changed during runtime and value to readonly variables can be assigned in the constructor or at the time of declaration.

13) Explain “static” keyword in C#?

“Static” keyword can be used for declaring a static member. If the class is made static then all the members of the class are also made static. If the variable is made static then it will have a single instance and the value change is updated in this instance.

14) What is the difference between “dispose” and “finalize” variables in C#?

• Dispose - This method uses interface – “IDisposable” interface and it will free up both managed and unmanaged codes like – database connection, files etc.
• Finalize - This method is called internally unlike Dispose method which is called explicitly. It is called by garbage collector and can’t be called from the code.

15) How the exception handling is done in C#?

In C# there is a “try… catch” block to handle the error.

16) Can we execute multiple catch blocks in C#?

No. Once any exception is occurred it executes specific exception catch block and the control comes out.

17) Why to use “finally” block in C#?

“Finally” block will be executed irrespective of exception. So while executing the code in try block when exception is occurred, control is returned to catch block and at last “finally” block will be executed. So closing connection to database / releasing the file handlers can be kept in “finally” block.

18) What is the difference between “finalize” and “finally” methods in C#?

• Finalize – This method is used for garbage collection. So before destroying an object this method is called as part of clean up activity.
• Finally – This method is used for executing the code irrespective of exception occurred or not.

19) What is the difference between “throw ex” and “throw” methods in C#?

• “throw ex” will replace the stack trace of the exception with stack trace info of re throw point.
• “throw” will preserve the original stack trace info.

20) Can we have only “try” block without “catch” block in C#?

Yes we can have only try block without catch block but we have to have finally block.

21) List out two different types of errors in C#?

Below are the types of errors in C# -

• Compile Time Error
• Run Time Error

22) Do we get error while executing “finally” block in C#?

Yes. We may get error in finally block.

23) Mention the assembly name where System namespace lies in C#?

Assembly Name – mscorlib.dll

24) What are the differences between static, public and void in C#?

• Static classes/methods/variables are accessible throughout the application without creating instance. Compiler will store the method address as an entry point. 
• Public methods or variables are accessible throughout the application. 
• Void is used for the methods to indicate it will not return any value.

25) What is the difference between “out” and “ref” parameters in C#?

“out” parameter can be passed to a method and it need not be initialized where as “ref” parameter has to be initialized before it is used.

26) Explain Jagged Arrays in C#?

If the elements of an array is an array then it’s called as jagged array. The elements can be of different sizes and dimensions.

27) Can we use “this” inside a static method in C#?

No. We can’t use “this” in static method.

28) What are value types in C#?

Below are the list of value types in C# -

• decimal
• int
• byte
• enum
• double
• long
• float

29) What are reference types in C#?

Below are the list of reference types in C# -

• class
• string
• interface
• object

30) Can we override private virtual method in C#?

No. We can’t override private virtual methods as it is not accessible outside the class.

31) Explain access modifier – “protected internal” in C#?

“protected internal” can be accessed in the same assembly and the child classes can also access these methods.

32) In try block if we add return statement whether finally block is executed in C#?

Yes. Finally block will still be executed in presence of return statement in try block.

33) What you mean by inner exception in C#?

Inner exception is a property of exception class which will give you a brief insight of the exception i.e, parent exception and child exception details.

34) Explain String Builder class in C#?

This will represent the mutable string of characters and this class cannot be inherited. It allows us to Insert, Remove, Append and Replace the characters. “ToString()” method can be used for the final string obtained from StringBuilder. For example,

StringBuilder TestBuilder = new StringBuilder("Hello");
TestBuilder.Remove(2, 3); // result - "He"
TestBuilder.Insert(2, "lp"); // result - "Help"
TestBuilder.Replace('l', 'a'); // result - "Heap"

35) What is the difference between “StringBuilder” and “String” in C#?

• StringBuilder is mutable, which means once object for stringbuilder is created, it later be modified either using Append, Remove or Replace.
• String is immutable and it means we cannot modify the string object and will always create new object in memory of string type.

36) What is the difference between methods – “System.Array.Clone()” and “System.Array.CopyTo()” in C#?

• “CopyTo()” method can be used to copy the elements of one array to other. 
• “Clone()” method is used to create a new array to contain all the elements which are in the original array.

37) How we can sort the array elements in descending order in C#?

“Sort()” method is used with “Reverse()” to sort the array in descending order.

38) Explain circular reference in C#?

This is a situation where in, multiple resources are dependent on each other and this causes a lock condition and this makes the resource to be unused.

39) List out some of the exceptions in C#?

Below are some of the exceptions in C# -

• NullReferenceException
• ArgumentNullException
• DivideByZeroException
• IndexOutOfRangeException
• InvalidOperationException
• StackOverflowException etc.

40) Explain Generics in C#?

Generics in c# is used to make the code reusable and which intern decreases the code redundancy and increases the performance and type safety.
Namespace – “System.Collections.Generic” is available in C# and this should be used over “System.Collections” types.

41) Explain object pool in C#?

Object pool is used to track the objects which are being used in the code. So object pool reduces the object creation overhead.

42) What you mean by delegate in C#?

Delegates are type safe pointers unlike function pointers as in C++. Delegate is used to represent the reference of the methods of some return type and parameters.

43) What are the types of delegates in C#?

Below are the uses of delegates in C# -

• Single Delegate
• Multicast Delegate
• Generic Delegate

44) What are the three types of Generic delegates in C#?

Below are the three types of generic delegates in C# -

• Func
• Action
• Predicate

45) What are the differences between events and delegates in C#?

Main difference between event and delegate is event will provide one more of encapsulation over delegates. So when you are using events destination will listen to it but delegates are naked, which works in subscriber/destination model.

46) Can we use delegates for asynchronous method calls in C#?

Yes. We can use delegates for asynchronous method calls.

47) What are the uses of delegates in C#?

Below are the list of uses of delegates in C# -

• Callback Mechanism
• Asynchronous Processing
• Abstract and Encapsulate method
• Multicasting

48) What is Nullable Types in C#?

Variable types does not hold null values so to hold the null values we have to use nullable types. So nullable types can have values either null or other values as well.

Eg: Int? mynullablevar = null;

49) Why to use “Nullable Coalescing Operator” (??) in C#?

Nullable Coalescing Operator can be used with reference types and nullable value types. So if the first operand of the expression is null then the value of second operand is assigned to the variable. For example,

double? myFirstno = null;
double mySecno;
mySecno = myFirstno ?? 10.11;

50) What is the difference between “as” and “is” operators in C#?

• “as” operator is used for casting object to type or class.
• “is” operator is used for checking the object with type and this will return a Boolean value.

51) Define Multicast Delegate in C#?

A delegate with multiple handlers are called as multicast delegate. The example to demonstrate the same is given below

public delegate void CalculateMyNumbers(int x, int y);
int x = 6;
int y = 7;
CalculateMyNumbers addMyNumbers = new CalculateMyNumbers(FuncForAddingNumbers);
CalculateMyNumbers multiplyMyNumbers = new CalculateMyNumbers(FuncForMultiplyingNumbers);
CalculateMyNumbers multiCast = (CalculateMyNumbers)Delegate.Combine (addMyNumbers, multiplyMyNumbers);
multiCast.Invoke(a,b);

52) What is the difference between CType and Directcast in C#?

• CType is used for conversion between type and the expression.
• Directcast is used for converting the object type which requires run time type to be the same as specified type.

53) Is C# code is unmanaged or managed code?

C# code is managed code because the compiler – CLR will compile the code to Intermediate Language.

54) Why to use lock statement in C#?

Lock will make sure one thread will not intercept the other thread which is running the part of code. So lock statement will make the thread wait, block till the object is being released.

55) Explain Hashtable in C#?

It is used to store the key/value pairs based on hash code of the key. Key will be used to access the element in the collection. For example,

Hashtable myHashtbl = new Hashtable();
myHashtbl.Add("1", "TestValue1");
myHashtbl.Add("2", "TestValue2");

56) How to check whether hash table contains specific key in C#?

Method – “ContainsKey” can be used to check the key in hash table. Below is the sample code for the same –

Eg: myHashtbl.ContainsKey("1");

57) What is enum in C#?

enum keyword is used for declaring an enumeration, which consists of named constants and it is called as enumerator lists. Enums are value types in C# and these can’t be inherited. Below is the sample code of using Enums

Eg: enum Fruits { Apple, Orange, Banana, WaterMelon};

58) Which are the loop types available in C#?

Below are the loop types in C# -

For
While
Do.. While

59) What is the difference between “continue” and “break” statements in C#?

• “continue” statement is used to pass the control to next iteration. This statement can be used with – “while”, “for”, “foreach” loops.
• “break” statement is used to exit the loop.

60) Write a sample code to write the contents to text file in C#?

Below is the sample code to write the contents to text file –

Using System.IO;
File.WriteAllText(”mytextfilePath”, “MyTestContent”);\

61) What you mean by boxing and unboxing in C#?

Boxing – This is the process of converting from value type to reference type. For example,

int myvar = 10;
object myObj = myvar;

UnBoxing – It’s completely opposite to boxing. It’s the process of converting reference type to value type. For example,

int myvar2 = (int)myObj;

62) Explain Partial Class in C#?

Partial classes concept added in .Net Framework 2.0 and it allows us to split the business logic in multiple files with the same class name along with “partial” keyword.

63) Explain Anonymous type in C#?

This is being added in C# 3.0 version. This feature enables us to create an object at compile time. Below is the sample code for the same –

Var myTestCategory = new { CategoryId = 1, CategoryName = “Category1”};

64) Name the compiler of C#?

C# Compiler is – CSC.

65) Explain the types of unit test cases?

Below are the list of unit test case types –

• Positive Test cases
• Negative Test cases
• Exception Test cases

66) Explain Copy constructor in C#?

If the constructor contains the same class in the constructor parameter then it is called as copy constructor.

class MyClass
{
 public string prop1, prop2;
 public MyClass(string a, string b)
 {
 prop1 = a;
 prop2 = b;
 }
 
 public MyClass(MyClass myobj) // Copy Constructor
 {
 prop1 = myobj.prop1;
 prop2 = myobj.prop2;
 }
}

67) Explain Static constructor in C#?

If the constructor is declared as static then it will be invoked only once for all number of instances of a class. Static constructor will initialize the static fields of a class.

class MyClass
{
 public string prop1, prop2;
 public MyClass(string a, string b)
 {
 prop1 = a;
 prop2 = b;
 }

Static MyClass()
 {
 Console.WriteLine(“Static Constr Test”);
 }
 public MyClass(MyClass myobj) // Copy Constructor
 {
 prop1 = myobj.prop1;
 prop2 = myobj.prop2;
 }
}

68) Which string method is used for concatenation of two strings in c#?

“Concat” method of String class is used to concatenate two strings. For example,

string.Concat(firstStr, secStr)

69) Explain Indexers in C#?

Indexers are used for allowing the classes to be indexed like arrays. Indexers will resemble the property structure but only difference is indexer’s accessors will take parameters. For example,

class MyCollection<T>
{
 private T[] myArr = new T[100];
 public T this[int t]
 {
 get
 {
 return myArr[t];
 }
 set
 {
 myArr[t] = value;
 }
 }
}

70) What are the collection types can be used in C#?

Below are the collection types in C# -

• ArrayList
• Stack
• Queue
• SortedList
• HashTable
• Bit Array

71) Explain Attributes in C#?

• Attributes are used to convey the info for runtime about the behavior of elements like – “methods”, “classes”, “enums” etc.
• Attributes can be used to add metadata like – comments, classes, compiler instruction etc.

72) List out the pre defined attributes in C#?

Below are the predefined attributes in C# -

• Conditional
• Obsolete
• Attribute Usage

73) What is Thread in C#?

Thread is an execution path of a program. Thread is used to define the different or unique flow of control. If our application involves some time consuming processes then it’s better to use Multithreading., which involves multiple threads.

74) List out the states of a thread in C#?

Below are the states of thread –

• Unstarted State
• Ready State
• Not Runnable State
• Dead State

75) Explain the methods and properties of Thread class in C#?

Below are the methods and properties of thread class –

• CurrentCulture
• CurrentThread
• CurrentContext
• IsAlive
• IsThreadPoolThread
• IsBackground
• Priority

76) What is a class ?

A class is the generic definition of what an object is. A Class describes all the attributes of the object, as well as the methods that implement the behavior of the member object. In other words, class is a template of an object. For ease of understanding a class, we will look at an example. In the class Employee given below, Name and Salary are the attributes of the class Person. The Setter and Getter methods are used to store and fetch data from the variable.

public class Employee
{
private String name;
private String Salary;
public String getName()
{
  return name;
}
public void setName(String name)
{
  this.name = name; 
}
public String getSalary ()
{
  return Salary;
}
public void setSalary (String Salary)
{
  this. Salary = Salary;
}
} 

77) What is an Object?

An object is an instance of a class. It contains real values instead of variables. For example, let us create an instance of the class Employee called “John”.

Employee John= new Employee();
Now we can access all the methods in the class “Employee” via object “John” as shown below.
John.setName(“XYZ”); 

78) What are the Access Modifiers in C# ?

Different Access Modifier are - Public, Private, Protected, Internal, Protected Internal

• Public – When a method or attribute is defined as Public, it can be accessed from any code in the project. For example, in the above Class “Employee” getName() and setName() are public.
• Private - When a method or attribute is defined as Private, It can be accessed by any code within the containing class only. For example, in the above Class “Employee” attributes name and salary can be accessed within the Class Employee Only. If an attribute or class is defined without access modifiers, it's default access modifier will be private.
• Protected - When attribute and methods are defined as protected, it can be accessed by any method in the inherited classes and any method within the same class. The protected access modifier cannot be applied to classes and interfaces. Methods and fields in a interface can't be declared protected.
• Internal – If an attribute or method is defined as Internal, access is restricted to classes within the current project assembly.
• Protected Internal – If an attribute or method is defined as Protected Internal, access is restricted to classes within the current project assembly and types derived from the containing class. 

79) Explain Static Members in C# ?

If an attribute's value had to be same across all the instances of the same class, the static keyword is used. For example, if the Minimum salary should be set for all employees in the employee class, use the following code.

private static double MinSalary = 30000;

To access a private or public attribute or method in a class, at first an object of the class should be created. Then by using the object instance of that class, attributes or methods can be accessed. To access a static variable, we don't want to create an instance of the class containing the static variable. We can directly refer that static variable as shown below.

double var = Employee.MinSalary ;

80) What is Reference Type in C# ?

Let us explain this with the help of an example. In the code given below,

Employee emp1;
Employee emp2 = new Employee();
emp1 = emp2;

Here emp2 has an object instance of Employee Class. But emp1 object is set as emp2. What this means is that the object emp2 is referred in emp1, rather than copying emp2 instance into emp1. When a change is made in emp2 object, corresponding changes can be seen in emp1 object. 

81) What is overriding in c# ?

To override a base class method which is defined as virtual, Override keyword is used. In the above example, method DriveType is overridden in the derived class.

82) What is Method Hiding in C# ?

If the derived class doesn't want to use methods in the base class, derived class can implement it's own version of the same method with same signature. For example, in the classes given below, DriveType() is implemented in the derived class with same signature. This is called Method Hiding.

class Car
{
 public void DriveType()
 {
  Console.WriteLine("Right Hand Drive");
 }
}

class Ford : Car
{
 public void DriveType()
 {
  Console.WriteLine("Right Hand ");
 }
}

83) What is Abstract Class in C#?

If we don't want a class to be instantiated, define the class as abstract. An abstract class can have abstract and non abstract classes. If a method is defined as abstract, it must be implemented in derived class. For example, in the classes given below, method DriveType is defined as abstract. 

abstract class Car
{
 public Car()
 {
  Console.WriteLine("Base Class Car");
 }
 public abstract void DriveType();
}

class Ford : Car
{
 public void DriveType()
 {
  Console.WriteLine("Right Hand ");
 }
}

Method DriveType get implemented in derived class.

84) What is Sealed Classes in c# ?

If a class is defined as Sealed, it cannot be inherited in derived class. Example of a sealed class is given below.

public sealed class Car
{
 public Car()
 {
  Console.WriteLine("Base Class Car");
 }

 public void DriveType()
 {
  Console.WriteLine("Right Hand ");
 }
} 

85) What is an Interface in C# ?

An interface is similar to a class with method signatures. There wont be any implementation of the methods in an Interface. Classes which implement interface should have an implementation of methods defined in the abstract class.

86) What is a Constructor in C# ?

Constructor is a special method that get invoked/called automatically, whenever an object of a given class gets instantiated. In our class car, constructor is defined as shown below

public Car()
{
 Console.WriteLine("Base Class Car");
}

When ever an instance of class car is created from the same class or its derived class(Except Few Scenarios), Constructor get called and sequence of code written in the constructor get executed.

interface Breaks
{
 void BreakType();
}

interface Wheels
{
 void WheelType();
}

class Ford : Breaks, Wheels
{
 public Ford()
 {
  Console.WriteLine("Derived Class Ford");
 }
 public void Price()
 {
  Console.WriteLine("Ford Price : 100K $");
 }
 public void BreakType()
 {
  Console.WriteLine("Power Break");
 }
 public void WheelType()
 {
  Console.WriteLine("Bridgestone");
 }
}

87) What is a Destructor in C# ? 

Destructor is a special method that get invoked/called automatically whenever an object of a given class gets destroyed. Main idea behind using destructor is to free the memory used by the object.