3 C# Interview Questions

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) Define Property in C# ?

Properties are a type of class member, that are exposed to the outside world as a pair of Methods. For example, for the static field Minsalary, we will Create a property as shown below.

private double minimumSalary;
public static double MinSalary
{
 get
 {
   return minimumSalary;
 }
 set
 {
   minimumSalary = value;
 }
}

So when we execute the following lines code

double minSal = Employee.MinSalary;

get Method will get triggered and value in minimumSalary field will be returned. When we execute,

Employee. MinSalary = 3000;

set Method will get triggered and value will be stored in minimumSalary field.

82) Explain Overloading in C# ?

When methods are created with the same name, but with different signature its called overloading. For example, WriteLine method in console class is an example of overloading. In the first instance, it takes one variable. In the second instance, “WriteLine” method takes two variable.

Console.WriteLine(x);
Console.WriteLine("The message is {0}", Message);

Different types of overloading in C# are

• Constructor overloading
• Function overloading
• Operator overloading

83) What is Constructor Overloading in C# .net ?

In Constructor overloading, n number of constructors can be created for the same class. But the signatures of each constructor should vary. For example

public class Employee 
{
 public Employee() 
 { }
 public Employee(String Name) 
 { }
}

84) What is Function Overloading in C# .net ?

In Function overloading, n number of functions can be created for the same class. But the signatures of each function should vary. For example

public class Employee 
{
 public void Employee() 
 { }
 public void Employee(String Name) 
 { }
}

85) What is Operator Overloading in C# .net ?

We had seen function overloading in the previous example. For operator Overloading, we will have a look at the example given below. We had defined a class rectangle with two operator overloading methods.

class Rectangle
{
 private int Height;
 private int Width;

 public Rectangle(int w,int h)
 {
   Width=w;
   Height=h;
 } 
 public static bool operator >(Rectangle a,Rectangle b)
 {
   return a.Height > b.Height ;
 }
 public static bool operator <(Rectangle a,Rectangle b)
 {
   return a.Height < b.Height ;
 } 
}

Let us call the operator overloaded functions from the method given below. When first if condition is triggered, the first overloaded function in the rectangle class will be triggered. When second if condition is triggered, the second overloaded function in the rectangle class will be triggered. 

public static void Main()
{
Rectangle obj1 =new Rectangle();
Rectangle obj2 =new Rectangle();

 if(obj1 > obj2)
 {
  Console.WriteLine("Rectangle1 is greater than Rectangle2");
 } 

 if(obj1 < obj2)
 {
  Console.WriteLine("Rectangle1 is less than Rectangle2");
 }
}

86) What is Data Encapsulation ?

Data Encapsulation is defined as the process of hiding the important fields from the end user. In the above example, we had used getters and setters to set value for MinSalary. The idea behind this is that, private field “minimumSalary” is an important part of our classes. So if we give a third party code to have complete control over the field without any validation, it can adversely affect the functionality. This is inline with the OOPS Concept that an external user should know about the what an object does. How it does it, should be decided by the program. So if a user set a negative value for MinSalary, we can put a validation in the set method to avoid negative values as shown below

set
{
 if(value > 0)
 {
  minSalary = value;
 }
}

87) Explain Inheritance in C# ?

In object-oriented programming (OOP), inheritance is a way to reuse code of existing objects. In inheritance, there will be two classes - base class and derived classes. A class can inherit attributes and methods from existing class called base class or parent class. The class which inherits from a base class is called derived classes or child class. For more clarity on this topic, let us have a look at 2 classes shown below. Here Class Car is Base Class and Class Ford is derived class.

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

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

class Ford : Car
{
 public Ford()
 {
  Console.WriteLine("Derived Class Ford");
 }

 public void Price()
 {
  Console.WriteLine("Ford Price : 100K $");
 }
}

When we execute following lines of code ,

Ford CarFord = new Ford();
CarFord.DriveType();
CarFord.Price();

Output Generated is as given below.

Base Class Car
Derived Class Ford
Right Hand Drive
Ford Price : 100K $

What this means is that, all the methods and attributes of Base Class car are available in Derived Class Ford. When an object of class Ford is created, constructors of the Base and Derived class get invoked. Even though there is no method called DriveType() in Class Ford, we are able to invoke the method because of inheriting Base Class methods to derived class.

88) Can Multiple Inheritance implemented in C# ?

In C#, derived classes can inherit from one base class only. If you want to inherit from multiple base classes, use interface.
 

89) What is Polymorphism in C# ?

The ability of a programming language to process objects in different ways depending on their data type or class is known as Polymorphism. There are two types of polymorphism

• Compile time polymorphism. Best example is Overloading
• Runtime polymorphism. Best example is Overriding

90) Explain the use of Virtual Keyword in C# ?

When we want to give permission to a derived class to override a method in base class, Virtual keyword is used. For example. lets us look at the classes Car and Ford as shown below.

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

class Ford : Car
{
 public Ford()
 {
  Console.WriteLine("Derived Class Ford");
 }
 public void Price()
 {
  Console.WriteLine("Ford Price : 100K $");
 }
 public override void DriveType()
 {
  Console.WriteLine("Right Hand ");
 }
}

When following lines of code get executed 

Car CarFord = new Car();
CarFord.DriveType();
CarFord = new Ford();
CarFord.DriveType();

Output is as given below.

Base Class Car
Right Hand Drive
Base Class Car
Derived Class Ford
Right Hand

91) 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.

92) 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 ");
 }
}

93) 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.

94) 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 ");
 }
} 

95) 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.

96) 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");
 }
}

97) 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.