Design Patterns Interview Questions

What are design patterns?

Design patterns are recurring solution to recurring problems in software architecture.

(A) Can you list down all patterns and their classification?

Note: - This is advanced question because anyone who asks to list down all patterns can

Only be crazy for what he is asking. But it is always a win-win situation for the interviewer.

There are three basic classification of patterns Creational, Structural and Behavioral

Patterns.

Creational Patterns

√ Abstract Factory:- Creates an instance of several families of classes

√ Builder: - Separates object construction from its representation

√ Factory Method:- Creates an instance of several derived classes

√ Prototype:- A fully initialized instance to be copied or cloned

√ Singleton: - A class in which only a single instance can exist

Note: - The best way to remember Creational pattern is by ABFPS (Abraham Became

First President of States).

Structural Patterns

√ Adapter:-Match interfaces of different classes.

√ Bridge:-Separates an object’s interface from its implementation.

√ Composite:-A tree structure of simple and composite objects.

√ Decorator:-Add responsibilities to objects dynamically.

√ Façade:-A single class that represents an entire subsystem.

√ Flyweight:-A fine-grained instance used for efficient sharing.

√ Proxy:-An object represents another object.

Note: To remember structural pattern best is (ABCDFFP)

Behavioral Patterns

√ Mediator:-Defines simplified communication between classes.

√ Memento:-Capture and restore an object’s internal state.

√ Interpreter:-A way to include language elements in a program.

√ Iterator:-Sequentially access the elements of a collection.

√ Chain of Resp:-A way of passing a request between a chain of objects.

√ Command:-Encapsulate a command request as an object.

√ State:-Alter an object’s behavior when its state changes.

√ Strategy:-Encapsulates an algorithm inside a class.

√ Observer:-A way of notifying change to a number of classes.

√ Template Method:-Defer the exact steps of an algorithm to a subclass.

√ Visitor:-Defines a new operation to a class without change.

Note: - Just remember Music……. 2 MICS On TV (MMIICCSSOTV).

Note: - No source code is provided for architecture section as much of the things can be clear From good UML diagrams.

What is the difference between Factory and Abstract Factory

Patterns?

Note: – This is quite a confusing architect question especially in design pattern section.

Interviewer can take you for a nice ride. So get the difference in your heart.

First read the definition provided in the first question about both these patterns. The

Common thing they have is that they belong to creational patterns. In short they hide the

Complexity of creating objects.

The main difference between factory and Abstract factory is factory method uses

Inheritance to decide which object has to be instantiated while abstract factory uses

Delegation to decide instantiation of object. We can say Abstract factory uses factory

259 methods to complete the architecture. Abstract Factory is one level higher in abstraction over Factory.

The two class diagrams below will provide overview of what is the actual difference.

First figure shows a sample implementation of Factory Patterns. In this figure there are

Two basic sections:-

√ the actual product section i.e. Class “Product” it inherits from an abstract

Class “AbstractProduct”.

√ the creational aspect section i.e. “ConcreteCreator” class which inherits

From class “Creator”.

√ now there is some rules the client will have to follow who

will need the “Product” object. He will never refer directly to the actual “Product” object

he will refer the “Product” object using “AbstractProduct”.

√ Second client will never use “New” keyword to create the “Product” object

but will use the “Creator” class which in turn will use the “ConcreteCreator”

class to create the actual “Product” object.

So what are the benefits from this architecture? All creational and initializing aspects are

now detached from the actual client. As your creational aspect is now been handled in

“ConcreteCreator” and the client has reference to only “Creator”, so any implementation

change in “CreateProduct” will not affect the client code. In short now your creational

aspect of object is completely encapsulated from the client’s logic.

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Now let’s look at the second class diagram which provides an overview of what actually

“Abstract factory” pattern is. It creates objects for families of classes. In short it describes

collection of factor methods from various different families. In short it groups related

factory methods. Example in this the class “Creator” is implemented using the “Abstract”

factory pattern. It now creates objects from multiple families rather one product.

Note :- Just stick up to this definition that Abstract factory classifies factory methods or

groups logically related factory method together.

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What is MVC pattern?

Twist: – How can you implement MVC pattern in ASP.NET?

The main purpose using MVC pattern is to decouple the GUI from the Data. It also gives

the ability to provide multiple views for the same Data. MVC pattern separates objects in

to three important sections:-

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√ Model: – This section is specially for maintaining data. It is actually where your

business logic, querying database, database connection etc. is actually

implemented.

√ Views: – Displaying all or some portion of data, or probably different view of

data. View is responsible for look and feel, Sorting, formatting etc.

√ Controller: – They are event handling section which affects either the model or

the view. Controller responds to the mouse or keyboard input to command

model and view to change. Controllers are associated with views. User

interaction triggers the events to change the model, which in turn calls some

methods of model to update its state to notify other registered views to refresh

their display.

Ok now this was all in theory. Let us look at how in actually ASP.NET we can implement

MVC pattern. During interview with theory question they will be looking at have you

really implemented MVC or its just talks. Following are the various sections of ASP.NET

which maps to MVC sections:-

√ Model: – This section is represented by Data view, Dataset, Typed Dataset,

Business components, business entity models etc. Now this section can then

be tied up to either windows application or web UI.

√ View: – ASPX, ASCX, or windows application UI like data grid etc. form the

view part of it.

√ Controller: – In ASP.NET the behind code is the controller as the events are

handled by that part. Controller communicates both with Model as well as

view.

I hope I was able to map you imagination of ASP.NET with the diagram given below.

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How can we implement singleton pattern in .NET?

Singleton pattern mainly focuses on having one and only one instance of the object running.

Example a windows directory service which has multiple entries but you can only have

single instance of it through out the network.

Note:- May of developers would jump to a conclusion saying using the “STATIC” keyword

we can have a single instance of object. But that’s not the real case there is something more

that has to be done. But please note we can not define a class as STATIC, so this will not

serve our actual purpose of implementing singleton pattern.

Following are the three steps needed to implement singleton pattern in .NET:-

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√ First create your class with static members.

Public class ClsStaticClass

Private shared objCustomer as clsCustomer

End class

This ensures that there is actually only one Customer object throughout the project.

√ Second define a private constructor to your class.

Note: – defining a private constructor to class does not allow a client to create objects directly.

√ Finally provide a static method to get access to your singleton object.

How do you implement prototype pattern in .NET?

Twist: – How to implement cloning in .NET ? What is shallow copy and deep copy ?

Cloning is achieved by using ICloneable of the System namespace. It has a “Clone” method

which actually returns the reference of the same copy. Clone method allows a Shallow

copy and not a deep copy. In Shallow copy if you make changes to the cloned object it

actually changes on the main object itself. So how is deep copy achieved, by using

“ISerializable” interface? So what you do is first serialize the object then deserialize back

to a complete new copy. Now any changes to this new copy do not reflect on the original

copy of the object, this is called as Deep copy.

What are the situations you will use a Web Service and Remoting in

projects?

Well “Web services” uses “remoting” concepts internally. But the major difference between

“web service” and “remoting” is that “web service” can be consumed by clients who are

not .NET platform. While remoting you need the client to be .NET compliant. Regarding

the speed issue “Remoting” is faster than “Web Services”. So I think when deciding the

architecture side of choosing between “Web services” and “Remoting” keep the cross

platform issue and the speed issue in mind.

Can you give a practical implementation of FAÇADE patterns?

Façade pattern sits on the top of lot of subsystems and makes access easy to interfaces

of these subsystems. Basic purpose of Façade is to make interfacing between many

modules and classes manageable.

Above is a simple live application of a Façade class. In this we have four subsystems :-

√ Customer

√ Product

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√ Payment

√ Invoicing

All the four modules when built at initial stage where built completely independent. The

main interaction between all these subsystems is customer placing order. This functionality

can be attained by using all these subsystems, which involves complex interaction between

them.

That is where FAÇADE comes in to action. We have built a FAÇADE called as

“FACADEORDER” which sits on the top of all these subsystem and fulfill our

functionality.

How can we implement observer pattern in .NET?

Observer patterns can be implemented using “Delegates” and “Events”. I leave this to

the readers to implement one sample code for observer patterns.

What is three tier architecture?

The three tier software architecture emerged in the 1990s to overcome the limitations of

the two tier architecture.

There are three layers when we talk about three tier architecture:-

User Interface (Client) :- This is mostly the windows user interface or the Web interface

but this has only the UI part.

Mid layer: – Middle tier provides process management where business logic and rules are

executed and can accommodate hundreds of users (as compared to only 100 users with

the two tier architecture) by providing functions such as queuing, application execution,

and database staging.

Data Access Layer: – This is also called by the famous acronym “DAL” component. It has

mainly the SQL statement which do the database operation part of the job.

The three tier architecture is used when an effective distributed client/server design is

needed that provides (when compared to the two tier) increased performance, flexibility,

maintainability, reusability, and scalability, while hiding the complexity of distributed

processing from the user.

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Have you ever worked with Microsoft Application Blocks, if yes then

which?

Application Blocks are C# and VB.NET classes distributed as Visual Studio projects

that can be downloaded from Microsoft’s Web site and used in any .NET application,

including ASP.NET Web applications. They are useful and powerful tools that can make

applications more maintainable, scalable and efficient

Secondly which application blocks has been used depends on really what you have

implemented. But there are two famous MAB which is making buzz around the industry:-

√ data access block

The Data Access Block provides static methods located in the SqlHelper class

that encapsulates the most common data access tasks performed with Microsoft

SQL server. If the term “static method” is new to you, it means that the class

methods can be called without instantiating an instance of the class. For

example, the method ExecuteReader () within the SqlHelper class can be called

by simply using the statement SqlHelper.ExecuteReader () — no object

instantiation of the SqlHelper class is required.

√ Exception management block.

The Exception Management Application Block provides a simple yet extensible

framework for handling exceptions. With a single line of application code you

can easily log exception information to the Event Log or extend it by creating

your own components that log exception details to other data sources or notify

operators, without affecting your application code. The Exception Management

Application Block can easily be used as a building block in your own .NET

application

Note: – It’s beyond the scope the book to look into details of application block. Best is go to

http://www.microsoft.com and search for these application block. Try to compile one or two

programs using their given classes and documentation.

What is Service Oriented architecture?

“Services” are components which expose well defined interfaces and these interfaces

communicate through XML messages. Using SOA you can build workflow, which uses

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interfaces of these components. SOA is typically useful when you are crossing

heterogeneous technical boundaries, organizations, domain etc.

In .NET SOA technically uses Web services to communicate with each service which is

crossing boundaries. You can look SOA which sits on top of web services and provides a

workflow.

SOA uses service components which operate in their own domain boundary. Let us note

some points of service :-

√ They are independent components and operate in their own boundary and

own technology.

√ They have well defined interfaces which use XML and WSDL to describe

themselves.

√ Services have URL where anyone can find them and clients can bind to these

URL to avail for the service.

√ Services have very loosely coupled architecture. In order to communicate to

service you only have to know the WSDL. Your client can then generate proxy

from the WSDL of the service.

Above figure describes a broader picture of what service oriented architecture will look

like. The basic fundamental of SOA is a web service. In above diagram you can see there

are two services available. One is the “Credit Card” service and other is “Address Check”

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web service. Both these services are provided by different company. Now we want to

build a functionality which needs to validate a credit card and also check that addresses

are proper. In short we will need functionalities of both the “CreditCard” and

“AddressCheck” service. Also note the “CreditCard” service has its own business layer

and DAL components, which can be in a proprietary language. It’s very much possible

that the whole Credit card service is made in .NET and the Address check is SAP

implementation or JAVA implementation. But because both the systems provide there

functionality using Web services which is nothing but basically XML message

communication. So we have made new service which sits like a FAÇADE on top of both

the web service and performs both functionalities in one common service. You will see I

have made a third service which sits on top of both the webservice and consumes them.

Also you can see that the UI part of the systems have access to Business layer and Web

service of there system. But the service which does both these check has only access to

the Web service.

Note:- It’s beyond the scope of this book to discuss about SOA. But just to keep you safe

during interview this book has tried to clear some basics of SOA.

What are different ways you can pass data between tiers?

There are many ways you can pass data between tiers :-

√ Dataset the most preferred one as they maintain data in XML format.

√ Datareader

√ Custom classes.

√ XML

What is Windows DNA architecture?

Note :- If you have worked with classic ASP this question can come to you.

The Windows Distributed interNet Applications Architecture (DNA) is a Microsoft

blueprint for robust, scalable, distributed business software. Windows DNA has evolved

over time and was not preplanned. It gives all combined advantages of Centralized

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mainframe, application servers, internet technologies and Personal computers. Windows

DNA is an evolution which started from mainframes (where all logic was centralized),

Fox pro ages ( where we talked in terms of two tier systems), VB6 / SQL SERVER (three

tier where we talked in terms of having one more tier which was mainly COM where

business logic resided), COM+ ( looking in terms of transactions and fulfilling ACID

rules) and finally the DNA.

Above shown is a Windows DNA model which is a blue print which Microsoft has

proposed. So if interviewer is asking you have you worked with Windows DNA, then

answer is yes. You will see that you always use these sections in project. Do not get

confused with the terminology DNA.