Object-Oriented Programming (Part - 1), .NET Interview Questions | Placement Papers - Technical & HR Questions - Interview Preparation PDF Download

1. What is object-oriented programming (OOP)?

OOP is a technique to develop logical modules, such as classes that contain properties, methods, fields, and events. An object is created in the program to represent a class. Therefore, an object encapsulates all the features, such as data and behavior that are associated to a class. OOP allows developers to develop modular programs and assemble them as software. Objects are used to access data and behaviors of different software modules, such as classes, namespaces, and sharable assemblies. .NET Framework supports only OOP languages, such as Visual Basic .NET, Visual C#, and Visual C++.

2. What is a class?

A class describes all the attributes of objects, as well as the methods that implement the behavior of member objects. It is a comprehensive data type, which represents a blue print of objects. It is a template of object. 

A class can be defined as the primary building block of OOP. It also serves as a template that describes the properties, state, and behaviors common to a particular group of objects.

A class contains data and behavior of an entity. For example, the aircraft class can contain data, such as model number, category, and color and behavior, such as duration of flight, speed, and number of passengers. A class inherits the data members and behaviors of other classes by extending from them.

3. What is an object?

They are instance of classes. It is a basic unit of a system. An object is an entity that has attributes, behavior, and identity. Attributes and behavior of an object are defined by the class definition.

4. What is the relationship between a class and an object?

A class acts as a blue-print that defines the properties, states, and behaviors that are common to a number of objects. An object is an instance of the class. For example, you have a class called Vehicle and Car is the object of that class. You can create any number of objects for the class named Vehicle, such as Van, Truck, and Auto.

The new operator is used to create an object of a class. When an object of a class is instantiated, the system allocates memory for every data member that is present in the class.

5. Explain the basic features of OOPs.

The following are the four basic features of OOP:

• Abstraction - Refers to the process of exposing only the relevant and essential data to the users without showing unnecessary information.
• Polymorphism - Allows you to use an entity in multiple forms.
• Encapsulation - Prevents the data from unwanted access by binding of code and data in a single unit called object.
• Inheritance - Promotes the reusability of code and eliminates the use of redundant code. It is the property through which a child class obtains all the features defined in its parent class. When a class inherits the common properties of another class, the class inheriting the properties is called a derived class and the class that allows inheritance of its common properties is called a base class.

6. What is the difference between arrays and collection?

Array:

1. You need to specify the size of an array at the time of its declaration. It cannot be resized dynamically.
2. The members of an array should be of the same data type.

Collection:

1. The size of a collection can be adjusted dynamically, as per the user's requirement. It does not have fixed size.
2. Collection can have elements of different types.

7. What are collections and generics?

A collection can be defined as a group of related items that can be referred to as a single unit. The System.Collections namespace provides you with many classes and interfaces. Some of them are - ArrayList, List, Stack, ICollection, IEnumerable, and IDictionary. Generics provide the type-safety to your class at the compile time. While creating a data structure, you never need to specify the data type at the time of declaration. The System.Collections.Generic namespace contains all the generic collections.

8. How can you prevent your class to be inherited further?

You can prevent a class from being inherited further by defining it with the sealed keyword.

9. What is the index value of the first element in an array?

In an array, the index value of the first element is 0 (zero).

10. Can you specify the accessibility modifier for methods inside the interface?

All the methods inside an interface are always public, by default. You cannot specify any other access modifier for them.

11. Is it possible for a class to inherit the constructor of its base class?

No, a class cannot inherit the constructor of its base class.

12. How is method overriding different from method overloading?

Overriding involves the creation of two or more methods with the same name and same signature in different classes (one of them should be parent class and other should be child). 

Overloading is a concept of using a method at different places with same name and different signatures within the same class.


13.What is the difference between a class and a structure?

Class:

1. A class is a reference type.
2. While instantiating a class, CLR allocates memory for its instance in heap.
3. Classes support inheritance.
4. Variables of a class can be assigned as null.
5. Class can contain constructor/destructor.

Structure:

1. A structure is a value type.
2. In structure, memory is allocated on stack.
3. Structures do not support inheritance.
4. Structure members cannot have null values.
5. Structure does not require constructor/destructor and members can be initialiazed automatically.

14. What are similarities between a class and a structure.

Structures and classes are the two most important data structures that are used by programmers to build modular programs by using OOP languages, such as Visual Basic .NET, and Visual C#. The following are some of the similarities between a class and a structure:

• Access specifiers, such as public, private, and protected, are identically used in structures and classes to restrict the access of their data and methods outside their body.
• The access level for class members and struct members, including nested classes and structs, is private by default. Private nested types are not accessible from outside the containing type.
• Both can have constructors, methods, properties, fields, constants, enumerations, events, and event handlers.
• Both structures and classes can implement interfaces to use multiple-inheritance in code.
• Both structures and classes can have constructors with parameter.
• Both structures and classes can have delegates and events.

15. What is a multicast delegate?

Each delegate object holds reference to a single method. However, it is possible for a delegate object to hold references of and invoke multiple methods. Such delegate objects are called multicast delegates or combinable delegates.

16. Can you declare an overridden method to be static if the original method is not static?

No. Two virtual methods must have the same signature.

17. Why is the virtual keyword used in code?

The virtual keyword is used while defining a class to specify that the methods and the properties of that class can be overridden in derived classes.

18. Can you allow a class to be inherited, but prevent a method from being overridden in C#?

Yes. Just declare the class public and make the method sealed.
 

19. Define enumeration?

Enumeration is defined as a value type that consists of a set of named values. These values are constants and are called enumerators. An enumeration type is declared using the enum keyword. Each enumerator in an enumeration is associated with an underlying type that is set, by default, on the enumerator. The following is an example that creates an enumeration to store different varieties of fruits:

enum Fruits {Mango, Apple, orange, Guava}; 

In the preceding example, an enumeration Fruits is created, where number 0 is associated with Mango, number 1 with Apple, number 2 with Orange, and number 3 with Guava. You can access the enumerators of an enumeration by these values.

20. In which namespace, all .NET collection classes are contained?

The System.Collections namespace contains all the collection classes.

21. Is it a good practice to handle exceptions in code?

Yes, you must handle exceptions in code so that you can deal with any unexpected situations that occur when a program is running. For example, dividing a number by zero or passing a string value to a variable that holds an integer value would result in an exception.

22. Explain the concept of constructor?

Constructor is a special method of a class, which is called automatically when the instance of a class is created. It is created with the same name as the class and initializes all class members, whenever you access the class. The main features of a constructor are as follows:

• Constructors do not have any return type.
• Constructors can be overloaded.
• It is not mandatory to declare a constructor; it is invoked automatically by .NET Framework.

23. Can you inherit private members of a class?

No, you cannot inherit private members of a class because private members are accessible only to that class and not outside that class.

24. Does .NET support multiple inheritance?

.NET does not support multiple inheritance directly because in .NET, a class cannot inherit from more than one class. .NET supports multiple inheritance through interfaces.
 

25. How has exception handling changed in .NET Framework 4.0?

In .NET 4.0, a new namespace, System.Runtime.ExceptionServices, has been introduced which contains the following classes for handling exceptions in a better and advanced manner:

• HandleProcessCorruptedStateExceptionsAttribute Class - Enables managed code to handle the corrupted state exceptions that occur in an operating system. These exceptions cannot be caught by specifying the try...catch block. To handle such exceptions, you can apply this attribute to the method that is assigned to handle these exceptions.
• FirstChanceExceptionEventArgs Class - Generates an event whenever a managed exception first occurs in your code, before the common language runtime begins searching for event handlers.

26. What is a delegate?

A delegate is similar to a class that is used for storing the reference to a method and invoking that method at runtime, as required. A delegate can hold the reference of only those methods whose signatures are same as that of the delegate. Some of the examples of delegates are type-safe functions, pointers, or callbacks.

27. What is the syntax to inherit from a class in C#?

When a class is derived from another class, then the members of the base class become the members of the derived class. The access modifier used while accessing members of the base class specifies the access status of the base class members inside the derived class.

The syntax to inherit a class from another class in C# is as follows: 
class MyNewClass : MyBaseclass