Month: April 2013

Code First Approach using Entity Framework 4.1, Inversion of Control, Unity Framework, Repository & Unit of Work Pattern and MVC3 Razor View


Download Source Code

Introduction :
In my previous article I discussed about developing a simple basic multilayered architecture for a .Net application.However there were few points that I skipped considering that the article was for beginners.When we talk about an application architecture there are certain points that need to be put into consideration before initiating,
  • Is the architecture loosely coupled?
  • Is it to be service based?
  • Where should the entities reside?
  • What should be Mode of Communication with the database?
  • Do it require design patterns? If yes,What patterns should be implemented?
  • Should it be based on Seperation of Concerns?
To answer these type of questions, .Net 4 has come up with a generic solution,making use of Entity Framework.
My effort in this article would be to put some light on building a generic multilayered architecture using Entity Framework 4.1 and MVC3 razor view engine.We’ll use inversion of control to resolve dependency of layers.The points to be covered in this article would be as follows,
  • Use of Seperation of Concerns,
  • Use of Code First approach,
  • POCO objects,
  • Repository Pattern,
  • Dependency Injection and Inversion of Control.
Architecture Overview:
I have created an architecture to explaing all the above mentioned points, and we’ll stick to this architecture till the end of article thus implementing practically our understanding.
I have created a very simple application to add student details in database, To edit it and to delete it,My first page shows list of all added student.
The arcitecture is service based, following are the components and pictorial representation (fig1) of it, we’ll discuss each component in detail,
  1. CodeFirstData layer.
  2. CodeFirstEntities layer.
  3. CodeFirstServices layer.
  4. MVC Portal layer.


    Architecture components:

    Let’s discuss each and every component of the defined architecture in detail,
    1. CodeFirstData layer :
    The data layer is the class library defining a layer which is responsible for interacting with database, contains context classes and a factory pattern implementation to interact with database.The layer contains the repository for each entity to map with database,thus making a complete ORM (Object Resource Model) solution.The class library references EntityFramework dll to implement the dbcontext classes.
    1. CodeFirstEntities layer:
    Entity layer acts as a model to MVC application,and also responsible for creation of DataBase objects when the dbset is first executed.It contains Entity classes in POCO form, having relations and data annotations(Rules/Constraints to be put on Database table/columns).
    The properties of the class results in column name in database and name osf the class in Dtaabase table.Primary key is either defined by the property named Id or “Classname[Id]”,in our case “StudentId”,this is default protocol set by entityframework to keep in mind while creating entities.Since this application is codefirest,we need to create entities first.
    1. CodeFirstServices layer:
    The layer contains services which uses repositories to fetch data from database. The interaction between Services and Repositories is kept loosely coupled thus implementing Inversion of Control using Dependency Injection.Its constructor based dependency injection and do not allow service to make direct instance of our repositories.Service layer acts as an interface between controllers and repositories,passes request of controller to repositories.
    1. MVCPortal layer:
    MVCPortal layer is our UI layer, it contains Model/ViewModels,Views and Controllers.I am not going into details of MVC as its not our primary target,I assume that you already know how to create and run MVC application,Lets have a quick revision of MVC,
      1. Model / Viewmodel:
    Model classea are responsible for holding up and manipulating data,we bind model/viewmodel classes to views to display model specific data.model classes are prepopulated by controllers to show data with the lehp of views.
      1. Views :
    Views holds up our UI templates, We have controller methods defined for every view, which populates View Design with data, and requests particular type of view to be rendered at user’s end as per requirement.
      1. Controllers :
    Web-based MVC frameworks map URLs to server code in a bit different fashion. Instead of mapping incoming URLs to html/aspx files, they instead map URLs to methods on classes. These classes are called “Controllers” and they are responsible for processing incoming HTTP requests, handling user input, retrieving and saving data, and determining the response to send back to the client (display HTML, download a file, redirect to a different URL, etc.).
    The layer makes use of Dependency injection to achieve Inversion of Control for services, thus not allowing controllers to directly acces services instances,The IOC is acheived using global.asax file using UnityFramework library of Microsoft.
    5. Dlls : I have created DLLs folder and given the output build path for every class library to that folder, for ease of access to Dll’s to add reference.Now each Dll will be created in the Dlls folder and we can access the desired Dll from the same folder. Also keep the EntityFramework,UnityConfiguration Dlls into it.
    IOC and DI :
    Dependency Injection is an important component in my application . All the services are required to be late bound with Model layer with Dependency Injection. In addition, the IoC container manages the lifetime of service objects. For example the Context object. I set lifetime type as PerThreadLifetimeManager in Unity configuration. This makes one and only one context object created in a single request and the different request has a different context object. Another thing I want to mention is ASP.NET MVC3 has its own way to provide Dependency Inject for controller via implementing DependencyResolver interface. The IoC container I used in the demo is Unity.
    Container :
    The “Container” or “IOC Container” is the main object that is used to create objects and inject dependencies into them. Whenever you want an object to be open to IoC, you have to use the container to create the instance using container.Resolve() method instead of the “new” keyword.
    IService service = unityContainer.Resolve();
    I have added following references to acheive the same,
    Microsoft.Practices.Unity
    Microsoft.Practices.Unity.Configuration
    The code for IOC is implemented in global.asax file.
    Step by Step Process to create the Application using EntityFramework Architecture :
    Application Overview :
    We’ll try to develop a simple studentDetails application with the help of discusses architecture,The application’s main module will be to Create a new student, Edit existing student,Delete existing student, and to show list of all students.Looking into a wider/generic perspective, the application performs CRUD operations on database with the help of EF .
    Note that we’ll not create databse by our own,we just write classes and defing connection configuration of our database server in web.config file, and let EntityFramework do rest of the job of creating database, managing database and Mapping database with our entities.
    Step1. Creating a solution and blank projects/seperating them logically/physically as layers :
    Our first initiative is to prepare our initial level architecture there by creating solution and adding project files to it, We create a solution named CodeFirstEntities and add three class libraries to it thus defining our three layers which are Entity,Data and Service, the names of the class libraries which i chose are CodeFirstEntities,CodeFirstData and CodeFirstServices respectively.
    Now add an MVC3 application to the solution called MvcPortal, that will act as a UI layer for our application. Refer fig2 for implementing the first step.


                                                                       ( fig2 )

    Step 2. Creating Entities :
    Add Student.cs class to CodeFirstEntities project , this class will contain the student details specific properties that will take the shape of database table and columns . The class makes use of DataAnnotation dll to put data annotations (rules/constraints) over the properties that will be reflected in database table.The constraints like max length and required parameter are provided as attributes over the properties of the class as shown in fig3 and fig4,

                                                                                ( fig3 )

                                                                             ( fig4)
    The above entities is very simple POCO (Plain Old CLR Object) class and the entity Student is decorated with validation attributes in the System.ComponentModel.DataAnnotations namespace. Now we want to use these entities for defining model objects for the Entity Framework 4. Using the Code First approach of Entity Framework, we can first define the entities by simply writing POCO classes without any coupling with any API or database library. This approach lets you focus on domain model which will enable Domain-Driven Development for applications. EF code first support is currently enabled with a separate API that is runs on top of the Entity Framework 4.
    Step 3. Creating Repositories/Contexts :
    Our next step is to create the contexts/repositories for our application and classes that will interact with the database.
    A. Create Context Class for Entity Framework :
    We have already prepared our domain model now let’s create a class in order to working with Entity Framework Code First. I have added reference to EntitFramework.dll CTP 4.1 to this class library project. We create two folders DBInteractions and EntityRepositories to segregate our classes as in fig5, we’ll come to that later, first let me explain you the context class.
                                                                (fig 5)


                                                                                   (fig 6)

    The above class CodeFirstContext in fig 6 is derived from DbContext that can connect your model classes to a database. The CodeFirstContext class is mapping our Student class to
    database tables Student use DbSet where TEntity is any POCO class. When we arerunning the application at first time, it will automatically create the database. EF code-first look for a connection string in web.config or app.config that has the same name as the dbcontext class. If it is not find any connection string with the convention, it will automatically create database in local SQL Express database by default and the name of the database will be same name as the dbcontext class. You can also define the name of database in constructor of the the dbcontext class. The model classes of Code First are working on the basis of conventions and we can also use a fluent API to refine our model. The convention for primary key is ‘Id’ or ‘Id’ as I discussed before. If primary key properties are detected with type ‘int’, ‘long’ or ‘short’, they will automatically registered as identity columns in the database by default. Primary key detection is not case sensitive. We can define our model classes with validation attributes in the System.ComponentModel.DataAnnotations namespace and it automatically enforces validation rules when a model object is updated or saved.
    I ) DBInteractions :
    B. Generic Repository for EntityFramework Code First:
    We have now created model class and dbcontext class. Now we create a generic repository pattern for data persistence with EF code first. Let’s create a generic repository to working with DbContext and DbSet as follows,The following classes will be added to DBInteractions folder for ease of understanding logic.


                            
                                                                            (fig 7)

    1. DataBase Factory:
    We have our database factory class defined as follows,


                                                                (fig 8)

                                                                     (fig 9)
    Where IDBFactory is the interface implemented by our factory class DBFactory.The factory class is inherited from Disposable class as shown below in fig 10,Responsible for releasing disposing database resources.
    (fig 10)
    1. RepositoryBase – The Generic Entity Repository base class :


      Above is the repository base class that contains all the methods to be implemented for CRUD DB operations, we can define more of our generic methods here, for now considering our application, these are eough for proper understanding of the working.

      E. Unit of Work
      The Unit of Work pattern maintains a list of objects affected by a business transaction and coordinates the writing out of changes and the resolution of concurrency problems.We create a class for handling Unit of Work pattern,
      Interface :


      Class :



      The Commit method written in of the UnitOfWork will call the commit method of our Context class and it will execute the SaveChanges method of DbContext class.

      II ) Repository :
      In this article, we will be primarily focus on the persistence against Student entity . Let’s create a repository for handling CRUD operations for Student
      using derive from a generic Repository EntityRepositoryBase.
      Repository class for Student :
      Interface :

      Class :

       

    Step 4. Creating a Service layer :
    Service Layer defines an application’s scope and its available set of operations from the perspective of interfacing client layers. It encapsulates the application’s business logic,
    controlling transactions and coordinating responses in the implementation of its operations. Controller classes should be made light and do not put much of business logic onto it. We can use
    the service layer as the business logic layer and can encapsulate the rules of the application.
    We define interfaces and corresponding student service for our application business logic,Since we are targetting CRUD operations, so the methods are quite simple in implementation,
    Interface :

     
    As we can see in abve interface the methods are to Get Student Details,List,Update and Delete Student.
    Class :
    usingSystem.Collections.Generic;
    usingCodeFirstData.DBInteractions;
    usingCodeFirstData.EntityRepositories;
    usingCodeFirstEntities;
    usingCodeFirstServices.Interfaces;
    namespaceCodeFirstServices.Services
    {
    publicclassStudentService: IStudentService
    {
    privatereadonlyIStudentRepository_studentRepository;
    privatereadonlyIUnitOfWork_unitOfWork;
    publicStudentService(IStudentRepositorystudentRepository, IUnitOfWorkunitOfWork)
    {
    this._studentRepository = studentRepository;
    this._unitOfWork = unitOfWork;
    }
    #regionIStudentService Members
    publicIEnumerable<Student> GetStudents()
    {
    varstudents = _studentRepository.GetAll();
    returnstudents;
    }
    publicStudentGetStudentById(intid)
    {
    varstudent = _studentRepository.GetById(id);
    returnstudent;
    }
    publicvoidCreateStudent(Studentstudent)
    {
    _studentRepository.Add(student);
    _unitOfWork.Commit();
    }
    publicvoidDeleteStudent(intid)
    {
    varstudent = _studentRepository.GetById(id);
    _studentRepository.Delete(student);
    _unitOfWork.Commit();
    }
    publicvoidUpdateStudent(Studentstudent)
    {
    _studentRepository.Update(student);
    _unitOfWork.Commit();
    }
    publicvoidSaveStudent()
    {
    _unitOfWork.Commit();
    }
    #endregion
    }
    }
    Class uses refernces of Repositories and Entities, and Dependency of Repositories is resolved in Constructor of the service itself.
    Step 5. The MVC Portal :
    Before we start MVC portal,Lets clear our logic for IOC and DI.I have already discussed IOC in the article.For resolving Dependency we create certain classes as discussed below, to get independent service request,
    We also create a custom lifetime manager for Unity to store container in the current HttpContext.
    publicclassHttpContextLifetimeManager : LifetimeManager, IDisposable
    {
    publicoverrideobjectGetValue()
    {
    varassemblyQualifiedName = typeof(T).AssemblyQualifiedName;
    if(assemblyQualifiedName != null)
    returnHttpContext.Current.Items[assemblyQualifiedName];
    returnnull;
    }
    publicoverridevoidRemoveValue()
    {
    varassemblyQualifiedName = typeof(T).AssemblyQualifiedName;
    if(assemblyQualifiedName != null)
    HttpContext.Current.Items.Remove(assemblyQualifiedName);
    }
    publicoverridevoidSetValue(objectnewValue)
    {
    varassemblyQualifiedName = typeof(T).AssemblyQualifiedName;
    if(assemblyQualifiedName != null)
    HttpContext.Current.Items[assemblyQualifiedName] = newValue;
    }
    publicvoidDispose()
    {
    RemoveValue();
    }
    }
    }
    We create a dependency resolver for resolving service dependency as follows,
    ASP.NET MVC 3 has introduced a new interface IControllerActivator which lets you activate controllers with custom behavior and can be use it for dependency injection purpose.The
    IControllerActivator interface is discoverable using the dependency resolver. Let’s create a custom controller activator class by deriving from IControllerActivator intreface
    usingSystem;
    usingSystem.Web.Mvc;
    namespaceCodeFirstPortal.IoC
    {
    publicclassCustomControllerActivator: IControllerActivator
    {
    IControllerIControllerActivator.Create(
    System.Web.Routing.RequestContextrequestContext,
    TypecontrollerType){
    returnDependencyResolver.Current
    .GetService(controllerType) asIController;
    }
    }
    }
    We also create a UnityController Factory and Configure contract and concrete types of unity in global.asax file.
    ASP.NET MVC 3 has also introduced a new interface IDependencyResolver which exposes two methods – GetService and GetServices.The GetService method resolves singly registered services
    that support arbitrary object creation and the GetServices resolves multiply registered services. Implementations of the IDependencyResolver interface should delegate to the underlying
    dependency injection container to provide the registered service for the requested type. When there are no registered services of the requested type, the ASP.NET MVC framework expects
    implementations of this interface to return null from GetService and to return an empty collection from GetServices. Let’s create a custom dependency resolver class by deriving from
    IDependencyResolver intreface in order to working with Unity to providing dependency injection.
    usingSystem;
    usingSystem.Web;
    usingSystem.Web.Mvc;
    usingSystem.Web.Routing;
    usingMicrosoft.Practices.Unity;
    namespaceCodeFirstPortal.IoC
    {
    publicclassUnityControllerFactory: DefaultControllerFactory
    {
    IUnityContainercontainer;
    publicUnityControllerFactory(IUnityContainercontainer)
    {
    this.container = container;
    }
    protectedoverrideIControllerGetControllerInstance(RequestContextreqContext, TypecontrollerType)
    {
    IControllercontroller;
    if(controllerType == null)
    thrownewHttpException(
    404, String.Format(
    “The controller for ‘{0}‘ could not be found”+ “or it does not implement IController.”,
    reqContext.HttpContext.Request.Path));
    if(!typeof(IController).IsAssignableFrom(controllerType))
    thrownewArgumentException(
    string.Format(
    “Requested type is not a controller: {0}“,
    controllerType.Name),
    “controllerType”);
    try
    {
    controller= container.Resolve(controllerType) asIController;
    }
    catch(Exceptionex)
    {
    thrownewInvalidOperationException(String.Format(
    “Error resolving the controller {0}“,
    controllerType.Name), ex);
    }
    returncontroller;
    }
    }
    Resolving Service Dependency :
    usingSystem;
    usingSystem.Collections.Generic;
    usingSystem.Web.Mvc;
    usingMicrosoft.Practices.Unity;
    namespaceCodeFirstPortal.IoC
    {
    publicclassUnityDependencyResolver: IDependencyResolver
    {
    IUnityContainercontainer;
    publicUnityDependencyResolver(IUnityContainercontainer)
    {
    this.container = container;
    }
    publicobjectGetService(TypeserviceType)
    {
    try
    {
    returncontainer.Resolve(serviceType);
    }
    catch
    {
    returnnull;
    }
    }
    publicIEnumerable<object> GetServices(TypeserviceType)
    {
    try
    {
    returncontainer.ResolveAll(serviceType);
    }
    catch
    {
    returnnewList<object>();
    }
    }
    }
    }
    global.asax :
    Add service project reference , entity project reference and data project reference to the portal.
    The SetResolver method of DependencyResolver class provides a registration point for dependency injection containers. In this method, we configure the UnityDependencyResolver class for
    providing dependency injection with Unity 2.0. The SetResolver method will be working with any dependency injection container.If you want to use StructureMap as the dependency injection
    container, you can create a dependency resolver class in order to working with StructureMap by deriving IDependencyResolver intreface and later you can configure this class with SetResolver
    method. The ASP.NET MVC 3 is providing a good support for working with dependency injection containers.
    usingSystem.Web.Mvc;
    usingSystem.Web.Routing;
    usingCodeFirstData.DBInteractions;
    usingCodeFirstData.EntityRepositories;
    usingCodeFirstPortal.IoC;
    usingCodeFirstServices.Interfaces;
    usingCodeFirstServices.Services;
    usingMicrosoft.Practices.Unity;
    namespaceMvcPortal
    {
    publicclassMvcApplication: System.Web.HttpApplication
    {
    publicstaticvoidRegisterGlobalFilters(GlobalFilterCollectionfilters)
    {
    filters.Add(newHandleErrorAttribute());
    }
    publicstaticvoidRegisterRoutes(RouteCollectionroutes)
    {
    routes.IgnoreRoute(“{resource}.axd/{*pathInfo}”);
    routes.MapRoute(
    “Default”, // Route name
    “{controller}/{action}/{id}”, // URL with parameters
    new{ controller = “Home”, action = “Index”, id = UrlParameter.Optional } // Parameter defaults
    );
    }
    protectedvoidApplication_Start()
    {
    AreaRegistration.RegisterAllAreas();
    RegisterGlobalFilters(GlobalFilters.Filters);
    RegisterRoutes(RouteTable.Routes);
    IUnityContainercontainer = GetUnityContainer();
    DependencyResolver.SetResolver(newUnityDependencyResolver(container));
    }
    privateIUnityContainerGetUnityContainer()
    {
    //Create UnityContainer
    IUnityContainercontainer = newUnityContainer()
    .RegisterType<IDBFactory, DBFactory>(newHttpContextLifetimeManager<IDBFactory>())
    .RegisterType<IUnitOfWork, UnitOfWork>(newHttpContextLifetimeManager<IUnitOfWork>())
    .RegisterType<IStudentService, StudentService>(newHttpContextLifetimeManager<IStudentService>())
    .RegisterType<IStudentRepository, StudentRepository>(newHttpContextLifetimeManager<IStudentRepository>());
    returncontainer;
    }
    }
    }
    To Start with MVC our first step is to define our connection string as follows in the web.config file.
    Steps to Create Controllers and Views :
    1. We start with home Controller, as it is the default controller set in global.asax to be called first.
      When the Index action of the controller is called we redirect it to our Student Controller,which straight away returns view to show listy of students if exist,since its first time we are creating the controller,it shows an empty list, the controller method of Student fetches student list from database , but first it creates the database, thus acheiving our objective.
    1. We create Student Controller, We define actions in the controller for each operation we want to perform as follows,
    usingSystem;
    usingSystem.Linq;
    usingSystem.Web.Mvc;
    usingCodeFirstEntities;
    usingCodeFirstServices.Interfaces;
    namespaceCodeFirstPortal.Controllers
    {
    publicclassStudentController: Controller
    {
    privatereadonlyIStudentService_studentService;
    publicStudentController(IStudentServicestudentService)
    {
    this._studentService = studentService;
    }
    [HttpGet]
    publicActionResultDetails(int? id)
    {
    varstudentDetails = _studentService.GetStudentById((int) id);
    if(studentDetails == null) thrownewArgumentNullException(“Not Found”);
    returnView(studentDetails);
    }
    [HttpGet]
    publicActionResultDelete(int? id)
    {
    varstudentDetails = _studentService.GetStudentById((int) id);
    if(studentDetails == null) thrownewArgumentNullException(“Not Found”);
    returnView(studentDetails);
    }
    [HttpPost]
    publicActionResultDelete(Studentstudent)
    {
    _studentService.DeleteStudent(student.StudentId);
    returnRedirectToAction(“List”, “Student”);
    }
    [HttpGet]
    publicActionResultEdit(int? id)
    {
    varstudentDetails = _studentService.GetStudentById((int) id);
    if(studentDetails == null) thrownewArgumentNullException(“Not Found”);
    returnView(studentDetails);
    }
    [HttpPost]
    publicActionResultEdit(Studentstudent)
    {
    _studentService.UpdateStudent(student);
    returnRedirectToAction(“List”, “Student”);
    }
    [HttpGet]
    publicActionResultCreate()
    {
    returnView();
    }
    [HttpPost]
    publicActionResultCreate(Studentstudent)
    {
    varstudentModel = newStudent()
    {
    Address = student.Address,
    Country = student.Country,
    Name = student.Name,
    Age = student.Age,
    Email = student.Email
    };
    _studentService.CreateStudent(studentModel);
    returnRedirectToAction(“List”, “Student”);
    }
    [HttpGet]
    publicActionResultList()
    {
    varstudents = _studentService.GetStudents();
    if(students.Any())
    {
    returnView(“List”, students);
    }
    returnView(“List”);
    }
    }
    }
    There are get and corresponding Posts for each method responsible for Data Updations. The Constructor of the controller initializes the Service,we can see it do not create direct instance.
    1. We Create Views for every Action,Its easy if we right click the controller action and create view,It automatically create a view with a Default folder named in the name of Constructor,So we can create all the views for our operations, and our solution looks like,
      4. Now we are ready with our application to be executed,keep your fingers crossed and see the magic, I have used _layout as master page to give some meaningful look and feel to my application,U can customize the same.When u run the application by pressing F5, We get redirected to the Index view of Student Controller,which shows list of students,since we are running it first time,we dont have existing list, and our application shows,



      1. No wonder , just have a look at your DataBase, My case I am using Sql Server 2008,


        I got a Database created automativcally with the name CodeFirstApp having Student Table, Now you can cross verify the table with your entity,its the same.So we have hit the target and our database is successfully created.

        1. Further Operations :
        You can now test the application and perform further CRUD operations in the application and see the DataBase getting updated.e.g. Create Student,


        after Submit by pressing Create, we get,


        therefore one student created.

        We can edit the same student by pressing edit link , we get the view,


        Likewise we can see the details of already created student,


        and delete the student will redirect to ,


        I had created 3 students in the same manner, You can create more and play with the application.

        Risk Factor : There is also a Risk Factor in implementing EntityFramework, If we do any change in the entity, like deleting,changing property,The context will drop and recreate the database, which may clearly lead to loss of your existing data.To avoid this critical situation, we code some more, we call SetInitializer method of DataBase class and set it to null in our dataBase Factory Class.

        usingSystem.Data.Entity;
        namespaceCodeFirstData.DBInteractions
        {
        publicclassDBFactory: Disposable, IDBFactory
        {
        publicDBFactory()
        {
        Database.SetInitializer<CodeFirstContext>(null);
        }
        privateCodeFirstContextdataContext;
        publicCodeFirstContextGet()
        {
        returndataContext ?? (dataContext = newCodeFirstContext());
        }
        protectedoverridevoidDisposeCore()
        {
        if(dataContext != null)
        dataContext.Dispose();
        }
        }
        }
        Database class is from namespace System.Data.Entity, whcih provides such feature.
        Conclusion :
        In this article we discussed about creating an application using EntityFramework 4.1 Code First Approach, We used Inversion of control and Dependency Injection to Resolve Dependency between layers.We used MVC 3 razor view engine as our UI and integrated the layers to acheive our objective.The application was a description of simple CRUD operations on Database.We learned how to use Repository Pattern,Unit of Work pattern and Unity Framework.You can download the codebase and further enhance/extend it according to your need.Happy Coding.

        Articles : http://www.codeproject.com/script/Articles/BlogFeedList.aspx?amid=7869570

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Understanding Multilayered Architecture in .Net


     Introduction :
     This article focussses on understanding a basic multilayered architecture in C#.Net,
The article starts with introduction to one tier, two tier and n-tier architectures,their pros and cons,  and later describes how we can acheive a simple basic multilayered architecture in .Net.
My effort in this article would be to focus on next level programming in .Net for developing an enterprise application.
     Layered Architecture :
When the different components in a system are organized systematically we call it a system architecture. The architecture is the enterprise-scale division of a system into layers or tiers, each having responsibility for a major part of the system and with as little direct influence
on other layers.
One, two, three and n-tier applications :
There are plenty of ways where a system can be split into no. of  logical tiers.
  One-tier applications
Single-tier applications are basically simple standalone programs.
It’s not necessary to take network communication and the risk of network failure
into consideration in these type of cases as they do not have the network access.
Since all the data resides within the same application,so these programs do not focus on synchronization of data. When separating the tiers physically the application get slower due to the  fact that communication over network will result in a loss of performance, therefore one-tier applications certainly have high performance.
Two-tier applications
A two-tier application, in comparison to the one-tier application as described, does not
combine all functions into a single process but separate different functions. For example
a chat application. This kind of application contains two separated tiers, client and a server.
The client has the responsibility of capturing user input and displaying the actual messages.
The server is responsible of the communication between the people that uses the chat client.
Three-tier applications
A three-tier application, adds another tier to the previous mentioned chat application, this
could be in the form of a database. One could think of a three-tier application as a dynamic
web application, which has a user interface, business logic,services and a database each placed in different tiers, as illustrated in Figure 1. As mentioned in the previous section a two-tier architecture   separates the user interface from the business logic, in the same way the three tier architecture separates the  database from the business logic.
N-tier applications
A logical n-tier application is an application where all logical parts are separated into discrete classes. In a typical business application, this generally involves a presentation layer, business logic layer and a data access layer. This separation makes the application easier to maintain. The advantages of this architecture are that all business rules are centralized which make them easy to create, use and re-use. The data access is also centralized, which has the same advantage as the centralization of the business rules. Centralizing the data access routines are also good when it comes to maintenance since changes only has to be implemented at one location. There are really not that many disadvantages of this kind of architecture, however, it
takes a bit longer to get up and running since several separate components has to be
developed, which also might make the code a bit more complicated to grasp for less
experienced developers.
A Practical Approach :
Lets start developing a simple multilayered architecture in .Net, I take c# as my programming language,however programming language is not a bar at all , one can choose the same as per comfort of programming.The architecture which we are to implement has the following design as mentioned in Figure 2.We will start creating the architecture for a web application , later it could be converted into a windows application too.
We will make use of class libraries to physically seperate our layers.There for one Web Application/Project, one Business logic layer class library , one Data access layer class library and one Common layer class library can be included in the solution.
Lets follow the implementation Step by Step,

 

 
Step 1 : Add a Web Project (Presentation layer).
             Open your Visual Studio and add a simple website to the solution, name it Presentation Layer.
 

 

Your Development Environment may look like,
 
 
Our Presentation Layer is the Web application, that will be exposed to the end user,
The Web application includes Web Forms i.e. aspx pages, User Controls i.e. Ascx pages,
Scrips (client side java scripts),Styles (css and custom styles for styling the page),Master Page(.master extension, for providing common functionality to group of desired pages,Configuration Files like web.config and app.config etc.).
Lets setup our next projects and define them in seperate layers, Add three folders to your solution,Folders named, BusinessLogicLayer, DataAccessLayer and CommonLayer.Your solution will look like as below,
 
 
   Step 2 : Add a Business Logic layer, Data Access Layer and Common Layer :
                Right click the Business Logic Layer folder and add a c# class library project to it, call it BLL.csproj,
 
 
  Doing this will ad a c# class library project to our solution in the Businee Logic Layer Folder.
  Like wise add two more projects to Common Layer and DataAccessLayer folder respectively and call them Common.csproj and DAL.csproj projects.
  The Data Access Layer Project Contains the entities classes and objects to communicate with database, where as our common project contains properties, helper classes to communicate with all the three layers commonly.It contains the objects to be passed to and fro from presentation layer to data access layer commonly, and also acts as a mode of message passing between the layers.

   Now our solution would look like as below,

 
Step 3 : Create a database with a sample table and put some default values in it, for example i have created a database named “EkuBase” and created a siple Student table with following fields, StudentId,Name,Email,Address,Age,Country.The Create script of the table is as follows,
<!–[if gte vml 1]>

     USE [EkuBase]

GO

 

/****** Object:  Table [dbo].[Student]    Script Date: 12/24/2011 14:53:14 ******/

SET ANSI_NULLS ON

GO

 

SET QUOTED_IDENTIFIER ON

GO

 

CREATE TABLE [dbo].[Student](

   [StudentID] [int] NOT NULL,

   [Name] [nvarchar](50) NULL,

   [Email] [nvarchar](50) NULL,

   [Address] [nvarchar](50) NULL,

   [Age] [int] NULL,

   [Country] [nvarchar](50) NULL,

 CONSTRAINT [PK_Student] PRIMARY KEY CLUSTERED

(

   [StudentID] ASC

)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]

) ON [PRIMARY]

 

GO

<![endif]–>

      USE [EkuBase]
GO
/****** Object:  Table [dbo].[Student]    Script Date: 12/24/2011 14:53:14 ******/
SET ANSI_NULLSON
GO
SET QUOTED_IDENTIFIERON
GO
CREATE TABLE[dbo].[Student](
   [StudentID] [int] NOT NULL,
   [Name] [nvarchar](50) NULL,
   [Email] [nvarchar](50) NULL,
   [Address] [nvarchar](50) NULL,
   [Age] [int] NULL,
   [Country] [nvarchar](50) NULL,
 CONSTRAINT[PK_Student] PRIMARY KEYCLUSTERED
(
   [StudentID] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON[PRIMARY]
     Therefore making studentid as primary key.
   Step 4: Lets add classes to our projetcs,Add StudentBLL.cs,StudentDAL.cs,StudentEntity.cs to BLL,DAl and Common Layer respectively,make sure you qualify them with logical namespaces,so that its easy to recognize and use them.
      BLL:            
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace BLL
{
    public class StudentBLL
    {
    }
}
     DAL:                
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace DAL
{
    public class StudentDAL
    {
    }
}
  Common:        
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace Common
{
    public class StudentEntity
    {
    }
}
  Step 5: Add Connection String in web.config and a Helper class to DAL to interact with DataBase,In my case i am using SQL helper for ease.
         
  Web.Config :
                <connectionStrings>
<add name=“Connections” connectionString=“Data Source=69B5ZR1\SQLEXPRESS;Initial Catalog=EkuBase;Persist Security Info=True;User ID=akhil;Password=akhil” providerName=“System.Data.SqlClient“/>
                </connectionStrings>
  and define the Connection String in your Sql Helper so that you dnt have to again and again create a connection and pass it to your method when you interact with database.
            publicstatic readonlystring CONN_STRING = ConfigurationManager.ConnectionStrings[“Connections”].ConnectionString;
        add reference of System.Configuration to DAL project before reading the above connection string.
    Step 6:Configure the solution and add dll’s to dependent layers,

            Since we need to seperate the business logic,presentation and data access,and we do not want presentation layer to directly interact with database nor the business logic,we add reference of business logic layer to our presentation layer and data access layer to the business logic layer and common layer to all the three layers,To acheive the same add a commonn DLL folder to the physical location of the Solution and give build path of all the three class projects to that DLL folder,doing this we can directly get access to DLL’s of all the layers to the layers needed that DLL.We’ll get the dll’s in that folder once we complite our project,   

 
 
 
 
Do this for DAL and Common Layer as shown in Figure 8.Aftre compling all the projects we get dll’s in DLL folder created.

 

 

 

 
Now add reference of Common.dll,BLL.dll to Presentation Layer, Common.dll,DAL.dll to BLL Layer,
Common.dll to DAL layer and compile your solution.
Now the code of BLL is accessible to Presentation Layer,and DAL is accessible to BLL, and Common is accessible to all three layers.
Step 7: Write methods to get the flow.
             Now we need to write some code to our layers to get the feel of flow between all the layers, Let’s create a scenarion, Suppose we need to get the details of all the students whose student id is less than 5.For that add some sample data to your Student table,about 7 rows would work(Figure 11),and add a gridview to Default.aspx in presentation layer to show the data.
 
 
                                                             Figure 11
           
Default.aspx:
        <%@ PageTitle=”Home Page” Language=”C#” MasterPageFile=”~/Site.master” AutoEventWireup=”true”
    CodeFile=”Default.aspx.cs” Inherits=”_Default” %>
<asp:Content ID=”HeaderContent”runat=”server”ContentPlaceHolderID=”HeadContent”>
</asp:Content>
<asp:Content ID=”BodyContent”runat=”server”ContentPlaceHolderID=”MainContent”>
    <h2>
        Welcome to ASP.NET!
    </h2>
    <p>
        To learn more about ASP.NET visit <ahref=”http://www.asp.net&#8221;title=”ASP.NET Website”>http://www.asp.net</a>.
    </p>
    <p>
        You can also find <a href=”http://go.microsoft.com/fwlink/?LinkID=152368&clcid=0x409&#8243;
            title=”MSDN ASP.NET Docs”>documentation on ASP.NET at MSDN</a>.
    </p>
    <div>
    <asp:GridView runat =”server” ID=”grdStudent”></asp:GridView>
    </div>
    <div>
                   <asp:Label runat=”server” ID=”lblError” Font-Bold=true ForeColor=red ></asp:Label>
                  </div>
   
</asp:Content>
Now decorate your StudentEntity Class in Common layer with following code, to make properties for each column we are going to access from Student table,
using System;
usingSystem.Collections.Generic;
using System.Linq;
using System.Text;
namespace Common
{
    publicclass StudentEntity
    {
        int studentID;
        public int StudentID
        {
            get { return studentID; }
            set { studentID = value; }
        }
        string name;
        public string Name
        {
            get { return name; }
            set { name = value; }
        }
        string email;
        public string Email
        {
            get { return email; }
            set { email = value; }
        }
        string address;
        public string Address
        {
            get { return address; }
            set { address = value; }
        }
        int age;
        public int Age
        {
            get { return age; }
            set { age = value; }
        }
        string country;
        public string Country
        {
            get { return country; }
            set { country = value; }
        }
    }
}
and StudentDAL with following code to call the data from database,We always write data interaction code in this layer only,making it as a protocol.
using System;
usingSystem.Collections.Generic;
using System.Linq;
using System.Text;
using System.Data;
namespace DAL
{
    publicclass StudentDAL
    {
        public DataSetFetchSelectedStudents()
        {
            string sqlCommand = “select * from Student where Studentid<5”;
            DataSet dataSet = SqlHelper.ExecuteDataset(SqlHelper.CONN_STRING, CommandType.Text, sqlCommand);
            return dataSet;
        }
    }
}
  Here we simply make a call to database to get studutents having id less than 5.
   We write following code to BLL class,where we perform validation check for the id whether it is less or greater that 5, and correspondigly throw error is its greater than 5,which we show at our default.aspx page by setting the message to error label text.BLL makes call to DAL to fetch the students,and passes to Presentation Layer, where data is shown in GridView.
using System;
usingSystem.Collections.Generic;
using System.Linq;
using System.Text;
using DAL;
using System.Data;
using Common;
namespace BLL
{
    publicclass StudentBLL
    {
        public DataTable GetStudentBelow5(StudentEntity student)
        {
            StudentDAL studentDAL = newStudentDAL();
            if (ValidateID(student.StudentID))
            {
                return studentDAL.FetchSelectedStudents().Tables[0];
            }
            return null;
        }
        private bool ValidateID(int studentID)
        {
            if (studentID > 5)
            {
                throw new ApplicationException(“Id Should be less than 5”);
            }
            return true;
        }
    }
}
  and in Presentation layer we write code to bind our gridview else show error message in case of error returned from BLL.
  Default.aspx.cs:
using System;
usingSystem.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
usingSystem.Web.UI.WebControls;
using System.Data;
using BLL;
using Common;
public partial class _Default : System.Web.UI.Page
{
    protectedvoid Page_Load(objectsender, EventArgs e)
    {
        StudentBLL studentBLL = new StudentBLL();
        StudentEntity studentEntity=newStudentEntity();
        studentEntity.StudentID=6;
        DataTable dTable = new DataTable();
        try
        {
            dTable = studentBLL.GetStudentBelow5(studentEntity);
            grdStudent.DataSource = dTable;
            grdStudent.DataBind();
        }
        catch (ApplicationExceptionapplicationException)
        {
            lblError.Text = applicationException.Message;
        }
    }
}
  In the above code we specify student id as 6 in Student Entity and pass it to BLL,when we run the code we get the following page with our error label set with the error message,
 
 
It clearly states that id should be less than 5.Note that we do not get to DAL before the data is validated.
Now change the student id to 5 and see the result,We get the following page,
 
Thus we get the clear result.
Here we have seen how we communicated through different layers performing different roles to fetch the data.
General advantages of layered applications:
There are various advantages of developing applications that are split up into different tiers or
layers. Most importantly this form of architecture helps to enforce the principles of high
cohesion within system components and low coupling between different components. A
system built on these principles will be more robust, easier to adjust, easier to maintain and
understand and it allows different software components to be developed in parallel.
The key point is that a system should be split into different smaller parts that are as
cohesive and self-governing as possible. Each part has distinct responsibilities and interacts
with other parts of the system to accomplish its tasks and responsibilities. This also ensures
that the systems can corporate across different platforms and communication protocols and
makes it easier to reuse existing solutions to problems often encountered.
All in all these advantages are desirable because they work in the direction of low
coupling and high cohesion. The hard part is, however, to implement this in practice and to
know when and how it should be implemented. What objects should have responsibility for
the different tasks and how do they interact.
Summary :
In this article I discussed about what are layered applications,different types of layered applications and how to create a multilayered application in .Net.We can handle the Exceptions more intelligently at DAL and BLL,however that was not the scope of the article so that part is skipped and i’ll surely discuss this in my forthcoming articles.The article was a focus on development for beginners, who face challenges to create an architecture before starting development.Happy Coding 🙂 .
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