1. What is mean by Class?
Class is a structure that describes the state (property) and behavior (methods) of the object. It is a template or blueprint to create objects of the class. Class represents the noun and it can be used as type in programming language. E.g Car, Person etc
2. What is mean by Objects?
Object is an executable copy of a class. State and behavior of the objects are defined by class definition. We can create multiple objects for single class. It is also called as instance of the class. When an object is created from the class, memory will be allocated in RAM. e.g Car- Maruthi, Alto, Zen etc. Person- Ram, Sam, John etc
3. What is mean by Struture?
Structure is a light-weight class used to create user-defined types containing only public fields. Structure can't have implementation inheritance, but it can have interface inheritance. We cannot modify the default constructors as like a class. Structure is a value type holds their value in memory when they are declared.
4. What is difference between Class and Object?
Classes are the template or blueprint its state of how objects should be and behave, where as Object is an actual real term object. E.g CAR define state like it should have four wheel and body structure with moving, accelerating and break functionality. Maruthi, Alto or Zen is the real object which has different kind of state from one another.
5. What is difference between Class and Structure?
- Class is Reference type(Reference types hold a reference to an object in memory) - Structure is a Value type(Value types hold their value in memory when they are declared)
- User can modify default constructor and destructor of class- structure can't modify default constructor
- Class supports inheritance - Structure will not support inheritance
- Classes must be instantiated using the new operator - Structure can be instantiated without using the new operator
6. Which case we have to use Class and Structure?
Structure can be used for things that we no need for identity. Class can be used when we need the identity for an object.
7. What is the advantage of Structure over Class?
Since Stucture is a value type and if we use at the proper location, it will improve the performance.
9. What are advantages of using private constructor, method, property?
Due to security reason, methods and properties are not exposed outside the class using Private access modifier. For implementing Singleton pattern we go for Private Constructor, so we will not able to create instance. Separate method is used to create and return the instance.
10. What is mean by Partial class?
It is new features in .Net 2.0; partial classes mean that class definition can be split into multiple physical files. Logically, partial classes do not make any difference to the compiler. The compiler intelligently combines the definitions together into a single class at compile-time.
Example for Partial Class
partial class Employee
{
string m_Name;
public String Name
{
get { return m_Name; }
set { m_Name = value; }
}
}
partial class Employee
{
int m_Age;
public int Age
{
get { return m_Age; }
set { m_Age = value; }
}
}
public class ExampleofPartical
{
public void Method1()
{
Employee objClass1 = new Employee();
objClass1.Name="Name";
objClass1.Age = 12;
}
}
8. What are different access modifiers in .Net?
- Private - The type or member can only be accessed by code in the same class or struct.
- Protected - The type or member can only be accessed by code in the same class or struct, or in a derived class.
- Internal - The type or member can be accessed by any code in the same assembly, but not from another assembly.
- Procted Internal - The type or member can be accessed by any code in the same assembly, or by any derived class in another assembly.
- Public -The type or member can be accessed by any other code in the same assembly or another assembly that references it.
Note: In VB.Net 'Internal' is called as 'Friend'
11. What is mean by Partial method?
Partial methods are methods defined in a partial class that are (optionally) divided across two files. With partial methods one file contains the method signature - the method name, its return type, and its input parameters - while the body is (optionally) defined in a separate file. If the partial method's body is not defined then the compiler automatically removes the partial method signature and all calls to the method at compile-time.
Example for Partial method
{
string m_Name;
public String Name
{
get { return m_Name; }
set { m_Name = value; }
}
public partial string GetEmpDetails(string ID);
}
partial class Employee
{
int m_Age;
public int Age
{
get { return m_Age; }
set { m_Age = value; }
}
public partial string GetEmpDetails(string ID)
{
return "Employee1";
}
}
12. Why do we go for Partial method?
Partial methods are mainly useful in auto-generated code situations. A code generating tool might know that there are certain extension points that some users are going to be interested in customizing. For example, the objects created in LINQ to SQL have partial methods like OnLoaded, OnCreated, OnPropertyNameChanging, and OnPropertyNameChanged. The auto-generated code calls the OnCreated partial method from its constructor. If you want to run custom code when one of these objects is created you can create a partial class and define the body for the OnCreated partial method.
13. Why do we go for Partial class?
- Improve the readability of extremely large classes by partitioning related methods into separate files.
- Partial classes enable the code generator to generate code in one file while the developer who may need to extend the auto-generated logic can do so in a separate file, which eliminates the worry that the code generator might overwrite a developer's customizations.
14. Where we use Partial method and class?
Partial classes and partial methods are most commonly used in auto-generated code. It provides a simple and safe way to add new functionality or extend existing functionality of auto-generated code.
15. What are restrictions for Partial method?
- Partial definitions must preceded with the key word "Partial"
- Method signature should be same in both partial class file
- We cannot have partial implementation in one file and another implementation in other file. We can have declaration in one file and implementation in another file.
16. What is mean by Static class?
Static class is used to create attributes and methods that can be accessed without creating the instance of the class. Static classes are loaded automatically by the .NET Framework when application or assembly is loaded. 'Static' key word is used to mention the static class. e.g MyStaticClass.PI
Example for Static Class
public static class MyStaticClass
{
public static decimal PI = 3.14M;
public static int Add(int num1, int num2)
{
return num1 + num2;
}
public static string Append(string str1, string str2)
{
return str1 + str2;
}
}
MyStaticClass.PI
17. What is mean by Static method?
Static method can be accessed without creating the instance of the class. 'Static' key word is used to mention the static method. Static methods can be created inside the normal class or static class. If we create the static method inside the normal class, static method will not be able to access by creating instance of the class. e.g Math.Add()
18. Can we override static method?
No, compiler will not allow overriding the static method.
19. What are uses of static class and method?
- Compiler will not allow creating the instance of the class
- Static class also makes the implementation simpler and faster
- Cannot inherit a static class since it is sealed
20. What is static constructor?
A static constructor is used to initialize any static data, or to perform a particular action that needs performed once only. It is called automatically before the first instance is created or any static members are referenced.
Example:
public class MyStaticClass
{
static int count;
static MyStaticClass()
{
count = 0;
Console.WriteLine("Static class is initialized");
}
public static void MyMethod(string name)
{
Console.WriteLine("Static class is initialized " + name);
}
}
MyStaticClass.MyMethod("John");
Output:
Static class is initialized
Hello John
21. What are shared (VB.NET)/Static(C#) variables?
Static members are not associated with a particular instance of any class, which can be invoked directly from the class level, rather than from its instance
Example
public static double PI = 3.1457;
22. What is Nested Classes?
Classes with in classes are called as Nested class.
Example
public class MyClassLevel_1
{
public void Display()
{
Console.WriteLine("Level_1");
}
public class MyClassLevel_2
{
public void Display()
{
Console.WriteLine("Level_2");
}
public class MyClassLevel_3
{
public void Display()
{
Console.WriteLine("Level_3");
}
}
}
}
Creating instance of the nested class
MyClassLevel_1 L1 = new MyClassLevel_1();
MyClassLevel_1.MyClassLevel_2 L2 = new MyClassLevel_1.MyClassLevel_2();
MyClassLevel_1.MyClassLevel_2.MyClassLevel_3 L3 = new
MyClassLevel_1.MyClassLevel_2.MyClassLevel_3();
L1.Display();
L2.Display();
L3.Display();
Output
Level_1
Level_2
Level_3
23. What are difference between Singleton and Static class?
- Singleton can extend classes and implement interfaces, while a static class cannot implement the interface.
- Singleton can be initialized lazily or asynchronously while a static class is generally initialized when it is first loaded.
- Singleton class can be extended and it's methods can be overridden.
24. Why Main () method is static?
To ensure there is only one entry point to the application.
25. What is mean by inheritance?
Inheritance is one of the concepts of object-oriented programming, where a new class is created from an existing class. Inheritance class is often referred to as subclasses, comes from the fact that the subclass (the newly created class) contains the attributes and methods of the parent class. This can be used to create a highly specialized hierarchical class structure.
Example of Inheritance
class Circle
{
private double m_radius;
public double Radius
{
get { return m_radius; }
set { m_radius = value; }
}
public double Diameter
{
get { return Radius * 2; }
}
public double Circumference
{
get { return Diameter * 3.14; }
}
public double Area
{
get { return Radius * Radius * 3.14; }
}
}
class Sphere : Circle
{
new public double Area
{
get { return 4 * Radius * Radius * 3.14; }
}
public double Volume
{
get { return 4 * 3.14 * Radius * Radius * Radius / 3; }
}
}
26. Can we inherit multiple classes?
No, multiple inheritances are not supported in .Net. Because consider the provided example. Here there are two Parent class Paretn1 and Parent2. This is inherited by Child class, In GetData method, child call the parent class method PrintData(). In this case which method will be executed? It is very difficult for CLR to identify which method to call. It shows that we multiple inheritance create ambiguity to oops concept. In order to avoid this ambiguity we are going for multiple interface implementations.
public class Parent1
{
public string PrintData()
{
return "This is parent1";
}
}
public class Parent2
{
public string PrintData()
{
return "This is parent2";
}
}
public class Child1 : Parent1, Parent2
{
public string GetData()
{
return this.PrintData();
}
}
27. What is mean by Shadowing?
When the method is defined in base class are not override able and we need to provide different implementation for the same in derived class. In this kind of scenario we can use hide the base class implementation and provide new implementation using Shadows (VB.Net)/new(C#) keyword.
Example:
Public Class ParentClass
Public Sub Display()
Console.WriteLine("Parent class")
End Sub
End Class
Public Class ChildClass
Inherits ParentClass
Public Shadows Sub Display()
Console.WriteLine("Child class")
End Sub
End Class
Dim p As New ParentClass
Dim c As New ChildClass
Dim pc As ParentClass = New ChildClass
p.Display()
c.Display()
pc.Display()
Output:
Parent class
Child class
Parent class
28. How a base class method is hidden?
Using new keyword in the derived class, base class method can be hidden or suppressed. New implementation can be added to the derived class.
29. What does the keyword virtual mean in the method definition?
The method can be over-ridden.
30. How method overriding different from overloading?
If we are overriding the method, derived class method behavior is changed from the base class. In Overloading, method with same name by different signature is used.
Example:
{
public virtual void Display()
{
Console.WriteLine("ParentClass");
}
}
public class ChildClass : ParentClass
{
//Example for method override
public override void Display()
{
Console.WriteLine("ChildClass");
}
//Example for method overload
public void Display(string name)
{
Console.WriteLine(name);
}
//Example for method overload
public void Display(string name, string country)
{
Console.WriteLine("Name:"+name +"Country: "+ country );
}
}
ParentClass p = new ParentClass();
ChildClass c = new ChildClass();
ParentClass pc = new ChildClass();
p.Display();
c.Display();
pc.Display();
 
OutPut:
ParentClass
ChildClass
ChildClass
31. Can you declare the override method static while the original method is non-static?
No
32. What is mean by Sealed Class?
Class which cannot be inherited is called as sealed class. If we need to prevent a class from being inherited, use “Sealed†keyword. But sealed class can inherited from other classes.
Example:
public class MyBaseClass
{
public void Display()
{
Console.WriteLine("Base class");
}
}
//Compile Success: This class cannot be inherited
public sealed class MySealedClass:MyBaseClass
{
public void Display()
{
base.Display();
Console.WriteLine("Sealed class");
}
}
//Compilation Error: cannot derive from sealed type MySealedClass
public class MyChildClass : MySealedClass
{
}
33. 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.
34. Will sealed class allows inheritance, if not why?
Sealed means it is not inheritable
35. What are the advantages of Private constructor?
- Private constructor will prevent the user from creating the instance of the class which contains only static members.
- Private constructor are used for implementing the singleton pattern
36. While using inheritance, derived class construct will call base class constructor?
Yes, base class constructor will be called before child class constructor
37. Overloaded constructor will call default constructor internally?
No, overload constructor will not call default constructor
38. What is difference between Overrides and Overridable?
Overridable (VB.Net)/ virtual (C#) is used in parent class to indicate that a method can be overridden. Overrides(VB.Net)/ override(C#) is used in the child class to indicate that you are overriding a method.
39. What is Method overloading?
Method overloading occurs when a class contains two methods with the same name, but different signatures.
40. What is operator overloading?
Operator overloading is used to provide a custom functionality to existing operators. For Example +,-,* and / operators are used for mathematical functionality. But we can overload these operators to perform custom operation on classes or structure.
Example:
To concatenate the two strings we have to use Concat method
Dim str1, str2, str3 As String
str1 = "Hello"
str2 = "world"
str3 = String.Concat(str1, str2)
But .Net provides in build operator overloading for string we can use ‘+’ operator for concatenating the string value
str3=str1+str2
Similarly we can also implement operator overloading for classes or structure
Employee3= Employee1 + Employee2
41. What is mean by abstraction?
Abstraction is the process of showing necessary information and hiding unwanted information. Let us consider the "CalculateSalary" in your Employee class, which takes EmployeeId as parameter and returns the salary of the employee for the current month as an integer value. Now if someone wants to use that method. He does not need to care about how Employee object calculates the salary? An only thing he needs to be concern is name of the method, its input parameters and format of resulting member
42. What is mean by abstraction class?
Abstract classes contain one or more abstract methods that do not have implementation. An abstract class is a parent class that allows inheritance but can never be instantiated. Abstract classes allow specialization of inherited classes.
43. What id mean by Interface?
Interface defines the set of properties, signature of the methods and events. It does not include any implementation. Class which implements the interface can provide the behavior to the implemented method. For example two class MyEnglishClassand MyFreanchClass implementing same interface and provide two different set of behavior in their implementation.
public interface IMyInterface
{
string Hello(string name);
}
public class MyEnglishClass:IMyInterface
{
public string Hello(string name)
{
return "Hello " + name;
}
}
public class MyFrenchClass : IMyInterface
{
public String Hello(string name)
{
return "allo " + name;
}
}
44. What is difference between Abstract class and Interface?
- In Interface all the method must be abstract; in abstract class we can have both abstract and concrete methods.
- Access modifiers cannot be specified in interface because it should always be public; in Abstract class, we can specify the access modifier.
45. In which Scenario you will go for Abstract or Interface Class?
Abstract classes are useful when creating components because they allow you specify an invariant level of functionality in some methods, but leave the implementation of other methods until a specific implementation of that class is needed. They also version well, because if additional functionality is needed in derived classes, it can be added to the base class without breaking code.
Interfaces are often used to describe the peripheral abilities of a class, not its central identity, e.g. an Automobile class might implement the Recyclable interface, which could apply to many otherwise totally unrelated objects.
46. What is mean by polymorphism?
Polymorphism means the ability to take more than one form. An operation may exhibit different behaviors in different instances. The behavior depends on the data types used in the operation. Polymorphism is extensively used in implementing Inheritance.
47. What are different types of polymorphism?
There are two types of polymorphism
Static polymorphism - defining the method with same name and different signature is called as static polymorphism. In the below example there are three different Add() functionality this Add() will be executed based on the parameter passed.
Example :
public int Add(int a, int b)
{
return a + b;
}
public double Add(double a, double b)
{
return a + b;
}
public long Add(long a, long b)
{
return a + b;
}
Dynamic polymorphism – Dynamic polymorphism can be implemented using Virtual and Override keyword. By using polymorphism, each derived class can have its own behavior, Even though classes are derived or inherited from the same parent class
Example:
In the below example ClassB is inherited from ClassA. ClassB can have its own behavior by overriding the parent class method. Parent class method should be represented with virtual keyword to override the same method in derived class.
public class ClassA
{
public virtual void Display()
{
Console.WriteLine ( "ClassA");
}
}
public class ClassB:ClassA
{
public override void Display()
{
Console.WriteLine ( "ClassB");
}
}
static void Main(string[] args)
{
ClassA a = new ClassA();
ClassB b = new ClassB();
ClassA c = new ClassB();
a.Display();
b.Display();
c.Display();
Console.ReadLine();
}
OutPut:
ClassA
ClassB
ClassB
48. What you mean by Encapsulation?
Encapsulation is the procedure of covering up of data and functions into a single unit and protects the data from the outside world. Example “Class†only public functions and properties are exposed; functions implementation and private variables are hidden from outside world.
49. What is difference between data encapsulation and abstraction?
Abstraction refers to the act of representing essential features without including the background details or explanations. Storing data and functions in a single unit is called as encapsulation.
50. What is mean by Delegate?
Delegate is a type that holds a reference to a method or a function. . Once a delegate is assigned a method, it behaves exactly like that method. We can call the method using delegate instead of directly calling the method. Using delegate, we can also pass the parameter and get return value. Any method with matched the signature of the delegate can be assigned. Simply we can say .NET implements the concept of function pointers using delegate.
Example:
There are three step to following for using Delegate
- Declaration
- Instantiation
- Invocation
In the below example we have declared the new delegate “MyDelegateâ€, which accept string as parameter and return value as string. Two methods SayHello and SayBye function will be called using delegate.
//Declaring the delegate
delegate string MyDelegate(string name);
//function called by delegate dynamically
private static string SayHello(string name)
{
return "Hello " + name;
}
private static string SayBye(string name)
{
return "Bye " + name;
}
After declaration of delegate, we have initialized with SayHello function. Now this delegate will hold reference to specified function. Function will be called using Invoke () method of delegate. In this example we have called two methods (SayHello and SayBye) with same signature(parameter type and return type).
static void Main(string[] args)
{
//Initialllizing delegate with function name
MyDelegate delg = new MyDelegate(SayHello);
//Invoking function using delegate
Console.WriteLine(delg.Invoke("Sam"));
delg = new MyDelegate(SayBye);
//Invoking diffent function using same delegate
Console.WriteLine(delg.Invoke("Sam"));
Console.ReadLine();
}
OutPut:
Hello Sam
Bye Sam
51. What’s a multicast delegate?
It’s a delegate that stores the address of multiple methods and eventually fires off several methods. Multicast delegate must have a return type of void.
52. What is an Asynchronous delegate?
When you invoke a delegate asynchronously, no new thread is created. Instead, the CLR automatically assigns a free thread from a small thread pool that it maintains. Typically, this thread pool starts with one thread and increases to a maximum of about 25 free threads on a single-CPU computer. As a result, if you start 50 asynchronous operations, one after the other, the first 25 will complete first. As soon as one ends, the freed thread is used to execute the next asynchronous operation.
53. What is mean by Events?
Events are nothing but a publisher and subscriber model. Any subscriber who is interested in receiving notification from the publisher can subscribe the events. If source event is fired or publisher raises the event, a notification will be send to all subscribers. One publisher can have multiple subscribers. Internally events will try to make use of delegate for this publisher, subscription model.
Example:
In the below example, we have created new event called "SampleEvent" and this event will be fired once MyMethod() is called. Anyone who wants to subscribe to this event can create a instance of the MyClassWithEvent and add handler to the event. So when ever event is raised, add handler method will be called.
Public Class MyClassWithEvent
'Created New Event, which will return a message to all subscriber
Event SampleEvent(ByVal message As String)
'Event will be fired once this method is called
Public Sub MyMethod()
Console.WriteLine("MyMethod is called")
'Raising the event with message
RaiseEvent SampleEvent("Event is Raised from MyClassWithEvent")
End Sub
End Class
Module Module1
Sub Main()
Dim c As New MyClassWithEvent
'First subscriber of the event
AddHandler c.SampleEvent, AddressOf EventSubscriber1
'Second subscriber of the event
AddHandler c.SampleEvent, AddressOf EventSubscriber2
c.MyMethod()
Console.ReadLine()
End Sub
Private Sub EventSubscriber1(ByVal message As String)
Console.WriteLine("Subscriber 1")
Console.WriteLine("Message: " + message)
End Sub
Private Sub EventSubscriber2(ByVal message As String)
Console.WriteLine("Subscriber 2")
Console.WriteLine("Message: " + message)
End Sub
End Module
OutPut:
MyMethod is called
Subscriber 1
Message: Event is Raised from MyClassWithEvent
Subscriber 2
Message: Event is Raised from MyClassWithEvent
54. Can event’s have access modifiers?
Yes, Event’s can have access modifier, if we mention it as Protected events can be subscribed only within inherited class, If you mention it as Internal(C#)/Friends(VB.Net) it can be subscribed by all class inside the assembly. If you mention it as Private it can subscribed with in class where it is declared.
55. Can we have static/shared events?
Yes, we can have static(C#)/shared(VB.Net) event, but only shared method can raise shared events.
56. Can we have different access modifier for Get/Set of the properties?
Yes, in C# 3.0 and above, we can use different access modifier for Get/Set of the properties, but this is not possible in C#2.0 and lower
57. What is an indexer?
An indexer is an accessor that enables an object to be treated in the same way as an array. An indexer is considered when a class is better represented as a virtual container of data that can be retrieved or set using indices. Since an indexer is nameless, its signature is specified by the keyword “this†followed by its indexing parameters within square brackets.
Example:
In the below example we have created new index for class of type string. During get and set operation string manipulations are done.
public class MyClassForIndexer
{
private string m_Name = "This is example for indexer";
public string this[int index]
{
get
{
return m_Name.Substring( index);
}
set
{
m_Name = m_Name.Insert(index, value);
}
}
}
MyClassForIndexer ind = new MyClassForIndexer();
Console.WriteLine (ind[0]);
ind[7] = "Appended String";
Console.WriteLine(ind[0]);
Output:
This is example for indexer
This isAppended String example for indexer
58. What is ENUM?
ENUM means Enumeration; it is used to group related sets of constants. To create a enumeration you use the Enum statement
Example:
Enum Months
January = 1
Feburary = 2
March = 3
April = 4
May = 5
June = 6
July = 7
August = 8
September = 9
October = 10
November = 11
December = 12
End Enum
