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What is object oriented programming?

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.

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. TheSystem.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. Can multiple catch blocks be executed?
No, Multiple catch blocks can’t be executed. Once the proper catch code executed, the control is transferred to the finally block and then the code that follows the finally block gets executed.
14. What is the difference between public, static and void?
Public declared variables or methods are accessible anywhere in the application. Static declared variables or methods are globally accessible without creating an instance of the class. The compiler stores the address of the method as the entry point and uses this information to begin execution before any objects are created. And Void is a type modifier that states that the method or variable does not return any value.
15. Define Constructors?
A constructor is a member function in a class that has the same name as its class. The constructor is automatically invoked whenever an object class is created. It constructs the values of data members while initializing the class.

16. What is Jagged Arrays?
The array which has elements of type array is called jagged array. The elements can be of different dimensions and sizes. We can also call jagged array as Array of arrays.
17. What is the difference between ref & out parameters?
An argument passed as ref must be initialized before passing to the method whereas out parameter needs not to be initialized before passing to a method.
18. What is the use of using statement in C#?
The using block is used to obtain a resource and use it and then automatically dispose of when the execution of block completed.
19. What is serialization?
When we want to transport an object through network then we have to convert the object into a stream of bytes. The process of converting an object into a stream of bytes is called Serialization. For an object to be serializable, it should inherit ISerialize Interface.
De-serialization is the reverse process of creating an object from a stream of bytes.
20. Can “this” be used within a static method?
We can’t use ‘This’ in a static method because we can only use static variables/methods in a static method.
21. What is difference between constants and read-only?
Constant variables are declared and initialized at compile time. The value can’t be changed after wards. Read-only variables will be initialized only from the Static constructor of the class. Read only is used only when we want to assign the value at run time.
22. What is an interface class?
Interface is an abstract class which has only public abstract methods and the methods only have the declaration and not the definition. These abstract methods must be implemented in the inherited classes.
23. What are value types and reference types?
Value types are stored in the Stack whereas reference types stored on heap.
Value types:

24. What are Custom Control and User Control?

Custom Controls are controls generated as compiled code (Dlls), those are easier to use and can be added to toolbox. Developers can drag and drop controls to their web forms. Attributes can be set at design time. We can easily add custom controls to Multiple Applications (If Shared Dlls), If they are private then we can copy to dll to bin directory of web application and then add reference and can use them.
User Controls are very much similar to ASP include files, and are easy to create. User controls can’t be placed in the toolbox and dragged – dropped from it. They have their design and code behind. The file extension for user controls is ascx.

25. What are sealed classes in C#?

We create sealed classes when we want to restrict the class to be inherited. Sealed modifier used to prevent derivation from a class. If we forcefully specify a sealed class as base class then a compile-time error occurs.

26. What is the difference between Array and Arraylist?
In an array, we can have items of the same type only. The size of the array is fixed. An arraylist is similar to an array but it doesn’t have a fixed size.
27. Can a private virtual method be overridden?
No, because they are not accessible outside the class.
28. Describe the accessibility modifier “protected internal”.
Protected Internal variables/methods are accessible within the same assembly and also from the classes that are derived from this parent class.
29. What are the differences between System.String and System.Text.StringBuilder classes?
System.String is immutable. When we modify the value of a string variable then a new memory is allocated to the new value and the previous memory allocation released. System.StringBuilder was designed to have concept of a mutable string where a variety of operations can be performed without allocation separate memory location for the modified string.
30. What’s the difference between the System.Array.CopyTo() and System.Array.Clone() ?
Using Clone() method, we creates a new array object containing all the elements in the original array and using CopyTo() method, all the elements of existing array copies into another existing array. Both the methods perform a shallow copy.
31. How can we sort the elements of the array in descending order?
Using Sort() methods followed by Reverse() method.
32. Write down the C# syntax to catch exception?
To catch an exception, we use try catch blocks. Catch block can have parameter of system.Exception type.

try

{

GetAllData();

}

catch(Exception ex)

{

}

33. What’s the difference between an interface and abstract class?
Interfaces have all the methods having only declaration but no definition. In an abstract class, we can have some concrete methods. In an interface class, all the methods are public. An abstract class may have private methods.
34. What is the difference between Finalize() and Dispose() methods?
Dispose() is called when we want for an object to release any unmanaged resources with them. On the other hand Finalize() is used for the same purpose but it doesn’t assure the garbage collection of an object.
35. What are circular references?
Circular reference is situation in which two or more resources are interdependent on each other causes the lock condition and make the resources unusable.
36. What are generics in C#.NET?
Generics are used to make reusable code classes to decrease the code redundancy, increase type safety and performance. Using generics, we can create collection classes. To create generic collection, System.Collections.Generic namespace should be used instead of classes such as ArrayList in the System.Collections namespace. Generics promotes the usage of parameterized types.
37. What is an object pool in .NET?
An object pool is a container having objects ready to be used. It tracks the object that is currently in use, total number of objects in the pool. This reduces the overhead of creating and re-creating objects.
38. List down the commonly used types of exceptions in .Net?
ArgumentException, ArgumentNullException , ArgumentOutOfRangeException, ArithmeticException, DivideByZeroException ,OverflowException , IndexOutOfRangeException ,InvalidCastException ,InvalidOperationException , IOEndOfStreamException , NullReferenceException , OutOfMemoryException , StackOverflowException etc.
39. What are Custom Exceptions?
Sometimes there are some errors that need to be handeled as per user requirements. Custom exceptions are used for them and are used defined exceptions.
40. What are delegates?
Delegates are same are function pointers in C++ but the only difference is that they are type safe unlike function pointers. Delegates are required because they can be used to write much more generic type safe functions.
41. How do you inherit a class into other class in C#?
Colon is used as inheritance operator in C#. Just place a colon and then the class name.

42. What is the base class in .net from which all the classes are derived from?

1	System.Object

43. What is the difference between method overriding and method overloading?

In method overriding, we change the method definition in the derived class that changes the method behavior. Method overloading is creating a method with the same name within the same class having different signatures.

44. What are the different ways a method can be overloaded?

Methods can be overloaded using different data types for parameter, different order of parameters, and different number of parameters.

45. Why can’t you specify the accessibility modifier for methods inside the interface?

In an interface, we have virtual methods that do not have method definition. All the methods are there to be overridden in the derived class. That’s why they all are public.

46. How can we set class to be inherited, but prevent the method from being over-ridden?

Declare the class as public and make the method sealed to prevent it from being overridden.

47. What happens if the inherited interfaces have conflicting method names?

Implement is up to you as the method is inside your own class. There might be problem when the methods from different interfaces expect different data, but as far as compiler cares you’re okay.

48. What is the difference between a Struct and a Class?

Structs are value-type variables and classes are reference types. Structs stored on the stack, causes additional overhead but faster retrieval. Structs cannot be inherited.

49. How to use nullable types in .Net?

Value types can take either their normal values or a null value. Such types are called nullable types.

Int? someID = null;

If(someID.HasVAlue)

{

}

50. How we can create an array with non-default values?

We can create an array with non-default values using Enumerable.Repeat.

51. What is difference between is and as operators in c#?

“is” operator is used to check the compatibility of an object with a given type and it returns the result as Boolean.

“as” operator is used for casting of object to a type or a class.

52. What’s a multicast delegate?

A delegate having multiple handlers assigned to it is called multicast delegate. Each handler is assigned to a method.

53. What are indexers in C# .NET?

Indexers are known as smart arrays in C#. It allows the instances of a class to be indexed in the same way as array.

Eg:

1	public int this[int index] // Indexer declaration

54. What is difference between the “throw” and “throw ex” in .NET?

“Throw” statement preserves original error stack whereas “throw ex” have the stack trace from their throw point. It is always advised to use “throw” because it provides more accurate error information.

55. What are C# attributes and its significance?

C# provides developers a way to define declarative tags on certain entities eg. Class, method etc. are called attributes. The attribute’s information can be retrieved at runtime using Reflection.

56. How to implement singleton design pattern in C#?

In singleton pattern, a class can only have one instance and provides access point to it globally.

Eg: