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.NET Architecture Interview questions
 
 
  What are design patterns ?
  Design patterns are recurring solution to recurring problems in software architecture.
   
  Can you list down all patterns and there classification ?
  Note :- This is advanced question because any one who says to list down all patterns can only be crazy for what he is asking.But its always a win-a-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 of 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 representing another object.
  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.  
     
  What’s difference between Factory and Abstract Factory Pattern’s?  
  Note: - This is quiet a confusing architect question especially in design pattern section. Interviewer can take you for a nice ride …. So get the difference’s 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
method to complete the architecture. Abstract Factory is one level higher in abstraction over Factory.		  
  The below two class diagrams will provide overview of what the actual difference is. 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 a abstract class “AbstractProduct”.  
  The creational aspect section that’s “ConcreteCreator” class which inherits from class “Creator”.  
  Now there are some rules the client who will need the “Product” object will have to follow. 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 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.		  
  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.  
     
  What’s 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:-  
  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’s look at how in actually ASP.NET can we implement MVC pattern. During interview with theory they will be looking at more 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.  
     
  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:-  
  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 through out 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 singletonobject.  
     
  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 does change 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.  
  There’s where FAÇADE comes in to action. We have built a FAÇADE called as “FACADEORDER” which sits on the top of all these subsystem and fulfills 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 in 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.		  
     
  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		  
  The second question which of the 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  
     
  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 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 there own domain boundary. Let’s note some points of service :-  
  They are independent components and operate in there own boundary and own technology.  
  They have well defined interfaces which use XML and WSDL to describe themselves.  
  Services have URL where any one 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.  
     
  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
mainframe, application servers, internet technologies and Personal computers. Windows DNA is a 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.		  
  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.  
     
  What is aspect oriented programming?  
  Note :- This is something which is catching up the market so interviewer can ask you to see how you are in touch with the market.So probably this explanation can be quiet long but
bear with me it is worth of it		  
  I will try to be as short as possible as this book is not a reference book. Just to save you from interviewer I will give a short description of aspect oriented programming in .NET.  
  First let's try to define it which can probably save you during interview  
  Aspect-oriented software development is a new technology for separation of concerns (SOC) in software development. The techniques of AOSD make it possible to modularize crosscutting aspects of a system.  
  Ok that statement’s can save you for the first stage let’s get down actually what is it. Let’s revisit back how software development cycle evolved.  
  When we look back at times of Cobol where we used to break the modules in small functionalities and use reusability to its maximum.  
  Then came the time when we talked in terms of Objects where things where clearer as software was modeled in terms of real life examples. It worked fine and till today is the most accepted way of implementing and organizing project. So why AOP ?  
  Aspect oriented programming does not oppose OOP’s but rather supports it and make’s it more maintainable. So remove the logic from head the AOP is replacement of OOP. No its brother of OOP helping him to be better.  
  When we talk in terms of object’s it’s an entity which maps to real world domain. Object has attributes which represent the state of object and also define its behavior. By rule of object oriented programming object should be stand alone and communicate with other object’s using messages or defined interface.  
  One object should not communicate with other object directly rather communicate through defined interfaces. Every object satisfies some “Concern” in relation to the system.  
  Twist: - What is Concern in AOP?  
  “A concern is a particular goal, concept, or area of interest”  
  There are mainly two types of concern from an object perspective:-  
  Core / Main concerns which it should satisfy and is his work.  
  System concerns which are not related to business functionalities but software related concerns example audit trail, Error handling, Security etc  
  Ok now lets define crosscutting which is one of important aspect’s of AOP  
  Twist :- What is cross cutting in AOP ?  
  When one or many concerns span across module its called as cross cutting.Example in our audit trail example we will probably need to audit trail for customer as well as supplier.So Audit trail can span across other objects also that is termed as cross cutting.  
  If you see the
“Update” method of the customer class , its doing both of the concerns that is checking for customer code length and also maintaining the audit trail using the audit trail class.		  
  Public Class ClsCustomer
Private pstrCustcode As String
Private pstrCustName As String
Public Property Code() As String
Get
Return pstrCustcode
End Get
Set(ByVal Value As String)
pstrCustcode = Value
End Set
End Property
Public Property CustomerName() As String
Get
Return pstrCustName
End Get
Set(ByVal Value As String)
pstrCustName = Value
End Set
End Property
Public Function Update() As Boolean
‘ first / core concern
If pstrCustcode.Length() > 10 Then
Throw New Exception("Value can not be greater than 10")
End If
' usingthe customer audit trail to do auditing
‘ second concern / system concern
Dim pobjClsAuditTrail As New ClsAuditTrail
With pobjClsAuditTrail
.NewValue = "1001"
.OldValue = "1003"
.UserName = "shiv"
.Update()
End With
' then inserting the customer in database
End Function
End Class
Public Class ClsAuditTrail
Private pstrUserName As String
Private pstrOldValue As String
Private pstrNewValue As String
Private pdblLogTime As Double
Public Property UserName() As String
Get
Return pstrUserName
End Get
Set(ByVal Value As String)
pstrUserName = Value
End Set
End Property
Public Property OldValue() As String
Get
Return pstrOldValue
End Get
Set(ByVal Value As String)
pstrOldValue = Value
End Set
End Property
Public Property NewValue() As String
Get
Return pstrNewValue
End Get
Set(ByVal Value As String)
pstrNewValue = Value
End Set
End Property
Public Property LogTime() As Double
Get
Return pdblLogTime
End Get
Set(ByVal Value As Double)
pdblLogTime = Value
End Set
End Property
Public Sub Update()
' do the logging activity here
End Sub
End Class		  
  In short the customer class is doing lot of activity. There is lot of tangling of code. So how do we overcome this problem… Simple separate the System level concern (Audit Trail) from the core level concern ( Customer code check). This is achieved at this moment in .NET using attribute programming.  
  Here is the change to the customer class  
  Imports System.Reflection
Public Class ClsCustomer
Private pstrCustcode As String
Private pstrCustName As String
Public Property Code() As String
Get
Return pstrCustcode
End Get
Set(ByVal Value As String)
pstrCustcode = Value
End Set
End Property
Public Property CustomerName() As String
Get
Return pstrCustName
End Get
Set(ByVal Value As String)
pstrCustName = Value
End Set
End Property
<ClsAuditTrail("Shiv", "1001", "1003", 12)> _
Public Function Update() As Boolean
If pstrCustcode.Length() > 10 Then
Throw New Exception("Value can not be greater than 10")
End If
' usingthe customer audit trail to do auditing
' then inserting the customer in database
End Function
End Class		  
  And here is the change to the audit trail class  
  Imports System.Reflection
<AttributeUsage(AttributeTargets.All)> _
Public Class ClsAuditTrail
Inherits Attribute
Private pstrUserName As String
Private pstrOldValue As String
Private pstrNewValue As String
Private pdblLogTime As Double
Public Property UserName() As String
Get
Return pstrUserName
End Get
Set(ByVal Value As String)
pstrUserName = Value
End Set
End Property
Public Property OldValue() As String
Get
Return pstrOldValue
End Get
Set(ByVal Value As String)
pstrOldValue = Value
End Set
End Property
Public Property NewValue() As String
Get
Return pstrNewValue
End Get
Set(ByVal Value As String)
pstrNewValue = Value
End Set
End Property
Public Property LogTime() As Double
Get
Return pdblLogTime
End Get
Set(ByVal Value As Double)
pdblLogTime = Value
End Set
End Property
Public Sub New(ByVal pstrUserName As String, _
ByVal pstrOldValue As String, _
ByVal pstrnewValue As String, _
ByVal plng As Long)
Update()
End Sub
Public Sub Update()
' do the logging activity here
End Sub
End Class		  
  The fun part of the code is below , you can see that the code audit trail is now seperate
and the code check belongs to the customer code.		  
  <ClsAuditTrail("Shiv", "1001", "1003", 12)> _
Public Function Update() As Boolean
If pstrCustcode.Length() > 10 Then
Throw New Exception("Value can not be greater than 10")
End If
' usingthe customer audit trail to do auditing
' then inserting the customer in database
End Function		  
     
     
     
     
     
 
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