(B) What is Object Oriented Programming ?
It is a problem solving technique to develop software systems. It’s a technique to think real world in terms of objects. Object maps the software model to real world concept. These objects have responsibilities and provide services to application or other objects.
(B) What’s a Class ?
A class describes all the attributes of objects, as well as the methods that implement the behavior of member objects.Its a comprehensive data type which represent a blue print of objects.It’s a template of object.
(B) What’s a Object ?
It’s a basic unit of a system.An object is an entity that has attributes, behavior, and identity. Objects are members of a class.Attributes and behavior of an object are defined by the class definition.
(A) What’s the relation between Classes and Objects ?
They look very much same but are not same.Class is a definition , while object is a instance of the class created.Class is a blue print while objects are actual objects existing in real world.Example we have class CAR which has attributes and methods like Speed,Brakes,Type of Car etc.Class CAR is just a prototype , now we can create real time objects which can be used to provide functionality . Example we can create a Maruti car object with 100 km speed and urgent brakes.
(B) What are different properties provided by Objectoriented systems ?
Twist :- Can you explain different properties of Object Oriented Systems?
Note:- Difference between abstraction and encapsulation is one of the favorite interview question and quiet confusing as both the terminology look alike.Best is if you can brainstorm with your friends or do a little reading.
Following are characteristic’s of Object Oriented System’s :-
Abstraction
It allows complex real world to be represented in simplified manner.Example color is abstracted to RGB.By just making the combination of these three colors we can achieve any color in world.It’s a model of real world or concept.
Encapsulation
The process of hiding all the internal details of an object from the outside world.
Communication using messages
When application wants to achieve certain task it can only be done using combination of objects.A single object can not do all the task.Example if we want to make order processing form. We will use Customer object , Order object , Product object and Payment object to achieve this functionality.In short these objects should communicate with each other.This is achieved when objects send messages to each other.
Object lifetime
All objects have life time.Objects are created , initialized , necessary functionalities are done and later the object is destroyed.Every object have there own state and identity, which differ from instance to instance.
Class hierarchies (Inheritance and aggregation)
Twist :- What’s difference between Association , Aggregation and Inheritance relationships? In object oriented world objects have relation and hierarchies in between them.There are basically three kind of relationship in Object Oriented world :-
Association
This is the simplest relationship between objects.Example every customer has sales.So Customer object and sales object have a association relation between them.
Aggregation
This is also called as composition model.Example in order to make a “Accounts” class it has use other objects example “Voucher”,”Journal” and “Cash” objects.So accounts class is aggregation of these three objects.
Inheritance
Hierarchy is used to define more specialized classes based on a preexisting generalized class.Example we have VEHICLE class and we can inherit this class make more specialized class like CAR, which will add new attributes and use some existing qualities of the parent class.Its shows more of a parent-child relationship .This kind of hierarchy is called inheritance.
Polymorphism
When inheritance is used to extend a generalized class to a more specialized class,it includes behavior of the top clas(Generalized class).The inheriting class often implement a behavior that can be somewhat different than the generalized class, but the name of the behavior can be same.It is important that a given instance of an object use the correct behavior, and the property of polymorphism allows this to happen automatically.
(I) What are abstract classes ?
Following are features of a abstract class :-
√ You can not create a object of abstract class * How would you describe yourself ?
√ Abstract class is designed to act as a base class (to be inherited by other classes). Abstract class is a design concept in program development and provides a base upon which other classes are built.
√ Abstract classes are similar to interfaces. After declaring an abstract class, it cannot be instantiated on it's own, it must be inherited.
√ In VB.NET abstract classes are created using “MustInherit” keyword.In C# we have “Abstract” keyword.
√ Abstract classes can have implementation or pure abstract methods which should be implemented in the child class.
(B) What’s a Interface ?
Interface is a contract that defines the signature of the functionality. So if a class is implementing a interface it says to the outer world , that it provides specific behavior. Example if a class is implementing Idisposable interface that means it has a functionality to release unmanaged resources . Now external objects using this class knows that it has contract by which it can dispose unused unmanaged objects.
√ Single Class can implement multiple interfaces.
√ If a class implements a interface then it has to provide implementation to all its methods.
Below is the source code “IInterface” is the interface and “ClsDosomething”
implements the “IInterface”. This sample just displays a simple message box.
Public Interface IInterFace
Sub DoSomething()
End Interface
Public Class ClsDoSomething
Implements IInterFace
Public Sub DoSomething() Implements
WindowsInterFace.IInterFace.DoSomething
MsgBox(“Interface implemented”)
End Sub
End Class
OOPS (Part II)
(A) What is difference between abstract classes and interfaces?
Following are the differences between abstract and interfaces :-
√ Abstract classes can have concrete methods while interfaces have no methods implemented.
√ Interfaces do not come in inheriting chain , while abstract classes come in
inheritance.
(B) What is a delegate ?
Delegate is a class that can hold a reference to a method or a function.Delegate class has a signature and it can only reference those methods whose signature is compliant with the class. Delegates are type-safe functions pointers or callbacks. Below is a sample code which shows a example of how to implement delegates.
Public Class FrmDelegates
Inherits System.Windows.Forms.Form
Public Delegate Sub DelegateAddString()
Private Sub FrmDelegates_Load(ByVal sender As System.Object,
ByVal e As System.EventArgs) Handles MyBase.Load
End Sub
Private Sub AddString()
lstDelegates.Items.Add(“Running AddString() method”)
End Sub
Private Sub cmdDelegates_Click(ByVal sender As System.Object,
ByVal e As System.EventArgs) Handles cmdDelegates.Click
Dim objDelegateAddString As DelegateAddString
objDelegateAddString = AddressOf AddString
objDelegateAddString.Invoke()
End Sub
End Class
In the above there is a method called “AddString()” which adds a string to a listbox. You can also see a delegate declared as :-
Public Delegate Sub DelegateAddString()
This delegate signature is compatible with the “AddString” method.When i mean compatibility that means that there return types and passing parameter types are same. Later in command click of the button object of the Delegate is created and the method pointer is received from “AddressOf ” keyword.Then by using the “Invoke” method the method is invoked.
(B) What are event’s ?
As compares to delegates events works with source and listener methodology . So listener’s who are interested in receiving some events they subscribe to the source.Once this subscription is done the source raises events to all of it’s listener when needed.One source can have multiple listeners.
In example sample given below class “ClsWithEvents” is a event source class , which has a event “EventAddString()”.Now the listener’s who are interested in receiving this event’s they can subscribe to this event. In class “FrmWithEvents” you can see the handles clause which is associated with the “mobjClsWithEvents” objects.
Public Class ClsWithEvents
Event EventAddString(ByVal Value As String)
Public Sub AddString()
RaiseEvent EventAddString(“String added by Event”)
End Sub
End Class
Public Class FrmWithEvents
Inherits System.Windows.Forms.Form
Private WithEvents mobjClsWithEvents As New ClsWithEvents()
Private Sub FrmWithEvents_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
End Sub
Private Sub mobjClsWithEvents_EventAddString(ByVal Value As String) Handles mobjClsWithEvents.EventAddString
LstData.Items.Add(Value)
End Sub
Private Sub CmdRunEvents_Click(ByVal sender As System.Object,
ByVal e As System.EventArgs) Handles CmdRunEvents.Click
mobjClsWithEvents.AddString()
End Sub
End Class
(I) Do events have return type ?
No events do not have return type.
(A) Can event’s have access modifiers ?
Event’s are always public as they are meant to serve every one registering to it.But you can access modifiers in events.You can have events with protected keyword which will be accessible only to inherited classes.You can have private events only for object in that class.
(A) Can we have shared events ?
Yes you can have shared event’s note only shared methods can raise shared events.
(I) What is shadowing ?
When two elements in a program have same name , one of them can hide and shadow the other one.So in such cases the element which shadowed the main element is referenced. Below is a sample code , there are two classes “ClsParent” and “ClsShadowedParent”.In “ClsParent” there is a variable “x” which is a integer.”ClsShadowedParent” overrides “ClsParent” and shadows the “x” variable to a string.
Public Class ClsParent
Public x As Integer
End Class
Public Class ClsShadowedParent
Inherits ClsParent
Public Shadows x As String
End Class
(A) What’s difference between Shadowing and Overriding ?
Following are the differences between shadowing and overriding :-
√ Overriding redefines only the implementation while shadowing redefines the whole element.
√ In overriding derived classes can refer the parent class element by using “ME” keyword , but in shadowing you can access it by “MYBASE”.
(I) What’s difference between delegate and events?
√ Actually events use delegates in bottom. But they add an extra layer on the delegates, thus forming the publisher and subscriber model.
√ As delegates are function to pointers they can move across any clients. So any of the clients can add or remove events , which can be pretty confusing. But events give the extra protection by adding the layer and making it a publisher and subscriber model.
Just imagine one of your clients doing this
c.XyzCallback = null
This will reset all your delegates to nothing and you have to keep figuring where the error is.
(B) If we inherit a class do the private variables also get inherited ?
Yes the variables are inherited but can not be accessed directly by the class interface.
(B) What are different accessibility levels defined in .NET ?
Following are the five levels of access modifiers :-
√ Private : Only members of class have access.
√ Protected :-All members in current class and in derived classes can access the variables.
√ Friend (internal in C#) :- Only members in current project have access to the elements.
√ Protected friend (protected internal in C#) :- All members in current project
and all members in derived class can access the variables. √ Public :- All members have access in all classes and projects.
(I) Can you prevent a class from overriding ?
If you define a class as “Sealed” in C# and “NotInheritable” in VB.NET you can inherit the class any further.
(I) What’s the use of “MustInherit” keyword in VB.NET ?
If you want to create a abstract class in VB.NET it’s done by using “MustInherit” keyword.This acts only as base type and can not be inherited any further.You can not create a object of a class which is marked as “MustInherit”.
(I) Why can not you specify accessibility modifier in Interface ?
All elements in Interface should be public.So by default all interface elements are public by default.
(A) What are similarities between Class and structure ?
Following are the similarities between classes and structures :-
√ Both can have constructors, methods, properties, fields, constants,
enumerations, events, and event handlers.
√ Structures and classes can implement interface.
√ Both of them can have constructors without parameter and with parameter.
√ Both can have delegates and events.
(A) What’s the difference between Class and structure’s ?
Following are the key differences between them :-
√ Structure are value types and classes are reference types.So structures use stack and classes use heap.
√ Structures members can not be declared as protected , but class members can be. You can not do inheritance in structures.
√ Structures do not require constructors while classes require.
√ Objects created from classes are terminated using Garbage collector. Structures are not destroyed using GC.
(B) What does virtual keyword mean ?
That method and property can be overridden.
(B) What is Dispose method in .NET ?
.NET provides “Finalize” method in which we can clean up our resources.But relying on this is not always good so the best is to implement “Idisposable” interface and implement the “Dispose” method where you can put your clean up routines.
(B) Whats the use of “OverRides” and “Overridable” keywords ?
Overridable is used in parent class to indicate that a method can be overridden.Overrides is used in the child class to indicate that you are overriding a method
OOPS (Part III)
(A) Where are all .NET Collection classes located ?
System.Collection namespace has all the collection classes available in .NET.
(A) What is ArrayList ?
Array whose size can increase and decrease dynamically.Arraylist can hold item of different types.As Arraylist can increase and decrease size dynamically you do not have to use the REDIM keyword.You can access any item in array using the INDEX value of the array position.
(A) What’s a HashTable ?
Twist :- What’s difference between HashTable and ArrayList ?
You can access array using INDEX value of array , but how many times you know the real value of index.Hashtable provides way of accessing the index using a user identified KEY value , thus removing the INDEX problem.
(A) What are queues and stacks ?
Queue is for first-in, first-out (FIFO) structures. Stack is for last-in, first-out (LIFO) structures.
(B) What is ENUM ?
It’s used to define constants.
(B)What’s Operator Overloading in .NET?
It provides a way to define and use operators such as +, -, and / for user-defined classes or structs. It allows us to define/redefine the way operators work with our classes and structs. This allows programmers to make their custom types look and feel like simple types such as int and string. VB.NET till now does not support operator overloading. Operator overloading is done
by using the “Operator” keyword.
(B)What’s the significance of Finalize method in .NET?
.NET Garbage collector does almost all clean up activity for your objects. But unmanaged resources (ex: - Windows API created objects, File, Database connection objects, COM objects etc) is outside the scope of .NET framework we have to explicitly clean our resources. For these types of objects .NET framework provides Object.Finalize method which can be overridden and clean up code for unmanaged resources can be put in this section.
(A)Why is it preferred to not use finalize for clean up?
Problem with finalize is that garbage collection has to make two rounds in order to remove objects which have finalize methods. In this scenario there are three objects Object1, Object2 and Object3. Object2 has the finalize method overridden and remaining objects do not have the finalize method overridden.
Now when garbage collector runs for the first time it searches for objects whose memory has to freed. He sees three objects but only cleans the memory for Object1 and Object3. Object2 it pushes to the finalization queue. Now garbage collector runs for the second time. He see’s there are no objects to be freed and then checks for the finalization queue and at this moment it clears object2 from the memory. So if you notice that object2 was freed from memory in the second round and not first. That’s why the best practice is not to write clean up Non.NET resources in Finalize method rather use the DISPOSE.
I)How can we suppress a finalize method?
GC.SuppressFinalize ()
(B)What’s the use of DISPOSE method?
Dispose method belongs to IDisposable interface. We had seen in the previous section how bad it can be to override the finalize method for writing the cleaning of unmanaged resources. So if any object wants to release its unmanaged code best is to implement
IDisposable and override the Dispose method of IDisposable interface. Now once your class has exposed the Dispose method it’s the responsibility of the client to call the Dispose method to do the cleanup.
(A)How do I force the Dispose method to be called automatically, as clients can forget to call Dispose method?
Call the Dispose method in Finalize method and in Dispose method suppress the finalize method using GC.SuppressFinalize. Below is the sample code of the pattern. This is the best way we do clean our unallocated resources and yes not to forget we do not get the hit of running the Garbage collector twice.
Note:- It will suppress the finalize method thus avoiding the two trip.
Public Class ClsTesting
Implements IDisposable
Public Overloads Sub Dispose()Implements IDisposable.Dispose
' write ytour clean up code here
GC.SuppressFinalize(Me)
End Sub
Protected Overrides Sub Finalize()
Dispose()
End Sub
End Class
(I)In what instances you will declare a constructor to be private?
When we create a private constructor, we can not create object of the class directly from a client. So you will use private constructors when you do not want instances of the class to be created by any external client. Example UTILITY functions in project will have no instance and be used with out creating instance, as creating instances of the class would be waste of memory.
(I)Can we have different access modifiers on get/set methods of a property ?
No we can not have different modifiers same property. The access modifier on a property
applies to both its get and set accessors.
(I)If we write a goto or a return statement in try and catch block will the finally block execute ?
The code in the finally always runs even if there are statements like goto or a return statements.
(A)What is Indexer ?
An indexer is a member that enables an object to be indexed in the same way as an array.
(A)Can we have static indexer in C# ?
No.
(A)In a program there are multiple catch blocks so can it happen that two catch blocks are executed ?
No once the proper catch section is executed the control goes to finally block.So there
will not be any scenarios in which multiple catch blocks will be executed.
(A) What is the difference between System.String and System.StringBuilder classes?
System.String is immutable; System.StringBuilder can have mutable string where a variety of operations can be performed.
and working on ms SQL,if someone could suggest me good reference websites then that would be great.
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