Architecture

                            Design Patterns

1) 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

2) 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.

3) 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.


      Factory Vs Abstract Factory

       Factory method uses inheritance to decide which object has to be instantiated
       Abstract factory uses delegation to decide instantiation of an object

      We can say Abstract factory uses factory method to complete the architecture. Abstract Factory is        one level higher in abstraction over Factory.

      1) Factory Patterns

             
                                      

    • The actual product section i.e.“Product” Class inherits from an abstract class  “Abstract Product”.
    • The creational aspect section i.e. “Concrete Creator” class which inherits from class "Creator”.

      Some rules the client will have to follow who will need the “Product” object?
  1. He will never refer directly to the actual “Product” object but can refer the“Product” object using “Abstract Product” class.
  2. He will never use “New” keyword to create the “Product” object but will use the “Creator” class that in turn will use the “Concrete Creator” class to create the actual “Product” object.  

        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 “Concrete Creator” and the client has reference to only “Creator”, so any implementation change in “Create Product” will not affect the client code. In short, now your creational aspect of object is completely encapsulated from the client’s logic.
  2) FAÇADE pattern
      
     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
            • 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.
3) Singleton pattern
  Singleton pattern ensures that a class has only one instance and provides a global point of access to it.
Following are the three steps needed to implement singleton pattern in .NET:-
     • First, create your class with static(Shared in vb.net) members.
          Public class ClsStaticClass
                  Private static clsCustomer objCustomer 
          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.

 4) Prototype pattern
     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 is three-tier architecture?
  The three-tier software architecture emerged in the 1990s to overcome the limitations of the twotier
  architecture.
  There are three layers when we talk about three-tier architecture:-

 User Interface (Client/Application Layer):-
   This is mostly the windows user interface or the Web interface but this has only the UI part.
Mid layer(Business 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 termed by the famous acronym "DAL



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

“Web services” uses “remoting” concepts internally. However, 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 when you are choosing between “Web
services” and “Remoting” keep the cross platform issue and the speed issue in mind.












General Questions
Does C# support multiple-inheritance? No.
Who is a protected class-level variable available to? It is available to any sub-class (a class inheriting this class).
Are private class-level variables inherited? Yes, but they are not accessible. Although they are not visible or accessible via the class interface, they are inherited.
Describe the accessibility modifier “protected internal”. It is available to classes that are within the same assembly and derived from the specified base class.
What’s the top .NET class that everything is derived from? System.Object.
What does the term immutable mean?The data value may not be changed. Note: The variable value may be changed, but the original immutable data value was discarded and a new data value was created in memory.
What’s the difference between System.String and System.Text.StringBuilder classes?System.String is immutable. System.StringBuilder was designed with the purpose of having a mutable string where a variety of operations can be performed.
What’s the advantage of using System.Text.StringBuilder over System.String?StringBuilder is more efficient in cases where there is a large amount of string manipulation. Strings are immutable, so each time a string is changed, a new instance in memory is created.
Can you store multiple data types in System.Array?No.
What’s the difference between the System.Array.CopyTo() and System.Array.Clone()?The Clone() method returns a new array (a shallow copy) object containing all the elements in the original array. The CopyTo() method copies the elements into another existing array. Both perform a shallow copy. A shallow copy means the contents (each array element) contains references to the same object as the elements in the original array. A deep copy (which neither of these methods performs) would create a new instance of each element's object, resulting in a different, yet identacle object.
How can you sort the elements of the array in descending order?By calling Sort() and then Reverse() methods.
What’s the .NET collection class that allows an element to be accessed using a unique key?HashTable.
What class is underneath the SortedList class?A sorted HashTable.
Will the finally block get executed if an exception has not occurred?­Yes.
What’s the C# syntax to catch any possible exception?A catch block that catches the exception of type System.Exception. You can also omit the parameter data type in this case and just write catch {}.
Can multiple catch blocks be executed for a single try statement?No. Once the proper catch block processed, control is transferred to the finally block (if there are any).
Explain the three services model commonly know as a three-tier application.Presentation (UI), Business (logic and underlying code) and Data (from storage or other sources).
Class Questions
What is the syntax to inherit from a class in C#? Place a colon and then the name of the base class.Example: class MyNewClass : MyBaseClass
Can you prevent your class from being inherited by another class? Yes. The keyword “sealed” will prevent the class from being inherited.
Can you allow a class to be inherited, but prevent the method from being over-ridden?Yes. Just leave the class public and make the method sealed.
What’s an abstract class?A class that cannot be instantiated. An abstract class is a class that must be inherited and have the methods overridden. An abstract class is essentially a blueprint for a class without any implementation.
When do you absolutely have to declare a class as abstract?1. When the class itself is inherited from an abstract class, but not all base abstract methods have been overridden. 2. When at least one of the methods in the class is abstract.
What is an interface class?Interfaces, like classes, define a set of properties, methods, and events. But unlike classes, interfaces do not provide implementation. They are implemented by classes, and defined as separate entities from classes.
Why can’t you specify the accessibility modifier for methods inside the interface?They all must be public, and are therefore public by default.
Can you inherit multiple interfaces?Yes. .NET does support multiple interfaces.
What happens if you inherit multiple interfaces and they have conflicting method names?It’s up to you to implement the method inside your own class, so implementation is left entirely up to you. This might cause a problem on a higher-level scale if similarly named methods from different interfaces expect different data, but as far as compiler cares you’re okay. To Do: Investigate
What’s the difference between an interface and abstract class?In an interface class, all methods are abstract - there is no implementation. In an abstract class some methods can be concrete. In an interface class, no accessibility modifiers are allowed. An abstract class may have accessibility modifiers.
What is the difference between a Struct and a Class?Structs are value-type variables and are thus saved on the stack, additional overhead but faster retrieval. Another difference is that structs cannot inherit.
Method and Property Questions
What’s the implicit name of the parameter that gets passed into the set method/property of a class? Value. The data type of the value parameter is defined by whatever data type the property is declared as.
What does the keyword “virtual” declare for a method or property? The method or property can be overridden.
How is method overriding different from method overloading? When overriding a method, you change the behavior of the method for the derived class. Overloading a method simply involves having another method with the same name within the class.
Can you declare an override method to be static if the original method is not static? No. The signature of the virtual method must remain the same. (Note: Only the keyword virtual is changed to keyword override)
What are the different ways a method can be overloaded? Different parameter data types, different number of parameters, different order of parameters.
If a base class has a number of overloaded constructors, and an inheriting class has a number of overloaded constructors; can you enforce a call from an inherited constructor to a specific base constructor?Yes, just place a colon, and then keyword base (parameter list to invoke the appropriate constructor) in the overloaded constructor definition inside the inherited class.
Events and Delegates
What’s a delegate? A delegate object encapsulates a reference to a method.
What’s a multicast delegate? A delegate that has multiple handlers assigned to it. Each assigned handler (method) is called.
ADO.NET and Database Questions
What is the role of the DataReader class in ADO.NET connections? It returns a read-only, forward-only rowset from the data source. A DataReader provides fast access when a forward-only sequential read is needed.
What are advantages and disadvantages of Microsoft-provided data provider classes in ADO.NET? SQLServer.NET data provider is high-speed and robust, but requires SQL Server license purchased from Microsoft. OLE-DB.NET is universal for accessing other sources, like Oracle, DB2, Microsoft Access and Informix. OLE-DB.NET is a .NET layer on top of the OLE layer, so it’s not as fastest and efficient as SqlServer.NET.
What is the wildcard character in SQL? Let’s say you want to query database with LIKE for all employees whose name starts with La. The wildcard character is %, the proper query with LIKE would involve ‘La%’.
Explain ACID rule of thumb for transactions.A transaction must be:1. Atomic - it is one unit of work and does not dependent on previous and following transactions.2. Consistent - data is either committed or roll back, no “in-between” case where something has been updated and something hasn’t.3. Isolated - no transaction sees the intermediate results of the current transaction).4. Durable - the values persist if the data had been committed even if the system crashes right after.
What connections does Microsoft SQL Server support? Windows Authentication (via Active Directory) and SQL Server authentication (via Microsoft SQL Server username and password).
Between Windows Authentication and SQL Server Authentication, which one is trusted and which one is untrusted? Windows Authentication is trusted because the username and password are checked with the Active Directory, the SQL Server authentication is untrusted, since SQL Server is the only verifier participating in the transaction.
What does the Initial Catalog parameter define in the connection string? The database name to connect to.
What does the Dispose method do with the connection object? Deletes it from the memory.To Do: answer better. The current answer is not entirely correct.
What is a pre-requisite for connection pooling? Multiple processes must agree that they will share the same connection, where every parameter is the same, including the security settings. The connection string must be identical.
Assembly Questions
How is the DLL Hell problem solved in .NET? Assembly versioning allows the application to specify not only the library it needs to run (which was available under Win32), but also the version of the assembly.
What are the ways to deploy an assembly? An MSI installer, a CAB archive, and XCOPY command.
What is a satellite assembly? When you write a multilingual or multi-cultural application in .NET, and want to distribute the core application separately from the localized modules, the localized assemblies that modify the core application are called satellite assemblies.
What namespaces are necessary to create a localized application? System.Globalization and System.Resources.
What is the smallest unit of execution in .NET?an Assembly.
When should you call the garbage collector in .NET?As a good rule, you should not call the garbage collector. However, you could call the garbage collector when you are done using a large object (or set of objects) to force the garbage collector to dispose of those very large objects from memory. However, this is usually not a good practice.
How do you convert a value-type to a reference-type?Use Boxing.
What happens in memory when you Box and Unbox a value-type?Boxing converts a value-type to a reference-type, thus storing the object on the heap. Unboxing converts a reference-type to a value-type, thus storing the value on the stack.