Creating an Application Using Entity Framework 4.1 Code First Approach

Introduction

In my previous article I discussed developing a simple basic multilayered architecture for a .Net application. However there were a few points that I skipped since the article was for beginners. When we talk about an application architecture there are certain points that need to be clarified before proceeding; they are:

• Is the architecture loosely coupled?
• Is it to be service-based?
• Where should the entities reside?
• What should be the Mode of Communication with the database?
• Does it require design patterns? If yes, What patterns should be implemented?
• Should it be based on a Separation of Concerns?

To answer these types of questions, .Net 4 has come up with a generic solution, making use of the 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 the 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 Separation 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 explain all these points, and we'll stick to this architecture throughout this article thus implementing a pragmatic understanding.

I have created a very simple application to add student details to a database. To edit it and to delete the data, my first page shows a list of all students added.

The architecture is service-based; the following are the components and pictorial representation (Figure1) of it. We'll discuss each of the following components in detail:

1. CodeFirstData layer.
2. CodeFirstEntities layer.
3. CodeFirstServices layer.
4. MVC Portal layer.

Figure1



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 the database. It contains context classes and a factory pattern implementation to interact with the database. The layer contains the repository for each entity to map with the database, thus making a complete ORM (Object Resource Model) solution. The class library references the EntityFramework DLL to implement the dbcontext classes.
   
2. CodeFirstEntities layer : The entity layer acts as a model for the MVC application, and is also responsible for the creation of the DataBase objects when the dbset is first executed. It contains Entity classes in POCO form, having relations and data annotations (Rules/Constraints to be specified for Database table/columns).
   
   The properties of the class results in a column name in the database and the name of the class in the Database table. The primary key is either defined by the property named Id or "Classname[Id]", in our case "StudentId"; this is the default protocol set by the Entity Framework to keep in mind when creating entities. Since this application is code first, we need to create the entities first.
   
3. CodeFirstServices layer : The layer contains services that use repositories to fetch data from the database. The interaction between Services and Repositories is kept loosely coupled thus implementing Inversion of Control using Dependency Injection. Its constructor is based on dependency injection and does not allow the service to make a direct instance of our repositories. The service layer acts as an interface between controllers and repositories and es controller requests to repositories.
   
4. MVCPortal layer : MVCPortal layer is our UI layer; it contains Model/ViewModels, Views and Controllers. I am not going into details of MVC since its not our primary target. I assume you already know how to create and run a MVC application; let's have a quick review of MVC:
   
   
   1. Model / Viewmodel : Model classes are responsible for holding and manipulating data. We bind model/viewmodel classes to views to display model-specific data. Model classes are pre-populated by controllers to show data with the help of views.
      
   2. Views : Views hold together our UI templates. We have controller methods defined for every view, which populates the View Design with data, and requests a particular type of view to be rendered at the user's end as required.
      
   3. Controllers : Web-based MVC frameworks map URLs to server code in a slightly 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 access service instances. The IOC is achieved using the global.asax file using the UnityFramework library of Microsoft.
      
5. DLLs : I have created a DLLs folder and given the output build path for every class library to that folder, for ease of access to DLLs for adding a 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 the Model layer with Dependency Injection. In addition, the IoC container manages the lifetime of service objects. For example the Context object. I set the lifetime type to PerThreadLifetimeManager in the 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 that ASP.NET MVC3 has its own way to provide Dependency Injection for a controller via implementing the 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 the container. The Resolve<T>() method instead of the "new" keyword.
   
   IService service = unityContainer.Resolve< Iservice>();
   
   I have added the following references to do that:
   
   Microsoft.Practices.Unity
   Microsoft.Practices.Unity.Configuration
   
   The code for IOC is implemented in the global.asax file.

The step-by-step process to create the application using EntityFramework Architecture follows.

Application Overview : We'll try to develop a simple studentDetails application by discussing the architecture. The application's main module will be to create a new student, edit an existing student, delete an existing student, and to show a list of all students. Looking into a wider/generic perspective, the application performs CRUD operations on the database with the help of the EF.

Note that we'll not create databases on our own; we just write classes and define the connection configuration of our database server in the web.config file and let the EntityFramework do the remainder of the job of creating the database, managing the database and mapping the database with our entities.

Step 1 : Creating a solution and blank projects/separating them logically/physically as layers

Our first initiative is to prepare our initial-level architecture, thereby creating a 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 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 to Figure 2 for implementing the first step.

(Figure 2)



Step 2 : Creating Entities

Add the Student.cs class to the CodeFirstEntities project. This class will contain the student details, specific properties that will become the database table and columns. The class makes use of a DataAnnotation DLL to put data annotations (rules/constraints) over the properties that will be reflected in the database table. The constraints like max length and required parameters are provided as attributes over the properties of the class as shown in Figure 3 and Figure 4:

(Figure 3)


(Figure 4)



The above entities are very simple POCO (Plain Old CLR Object) classes 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 the domain model which will enable Domain-Driven Development for applications. The EF Code First support is currently enabled with a separate API that 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.

1. Create Context Class for Entity Framework
   
   We have already prepared our domain model. Now let's create a class for working with the Entity Framework Code First. I have added a reference to the EntitFramework.dll CTP 4.1 to this class library project. We create two folders, DBInteractions and EntityRepositories, to segregate our classes as in Figure 5; we'll come to that later. First let me explain the context class to you.
   
   (Figure 5)
   
   
   (Figure 6)
   
   
   
   The above class CodeFirstContext in Figure 6 is derived from DbContext that can connect your model classes to a database. The CodeFirstContext class is mapping our Student class to the database table Student using DbSet<TEntity> where TEntity is any POCO class. When we run the application the first time, it will automatically create the database. The EF Code First looks for a connection string in the web.config or app.config that has the same name as the dbcontext class. If it does not find a connection string matching the convention, it will automatically create a database in the local SQL Express database by default and the name of the database will be the same name as the dbcontext class. You can also define the name of the database in the constructor of the dbcontext class. The model classes of the Code First work 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 '<classname>Id' as I discussed before. If the primary key properties are detected with type 'int', 'long' or 'short', they will automatically be 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 is a namespace and it automatically enforces validation rules when a model object is updated or saved.
   
   I ) DBInteractions
   
2. Generic Repository for EntityFramework Code First
   
   We have now created a model class and a dbcontext class. Now we create a generic repository pattern for data persistence with EF Code First. Let's create a generic repository to work with DbContext and DbSet as follows. The following classes will be added to the DBInteractions folder for ease of understanding the logic.
   
   (Figure 7)
   
   
   
3. DataBase Factory
   
   We have our database factory class defined as follows:
   
   (Figure 8)
   
   
   
   (Figure 9)
   
   
   
   Where IDBFactory is the interface implemented by our factory class DBFactory. The factory class is inherited from the Disposable class as shown below in Figure 10, responsible for releasing and disposing database resources.
   
   (Figure 10)
   
   
   
4. 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, but for now, considering our application, these are enough for the proper understanding of how it works.
   
5. 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 the Unit of Work pattern:
   
   Interface
   
   
   
   Class
   
   
   
   The Commit method is written in the UnitOfWork which will call the commit method of our Context class and it will execute the SaveChanges method of the DbContext class.
   
   II ) Repository
   
   In this article, we will be primarily focusing on the persistence against the Student entity. Let's create a repository for handling CRUD operations for Student
   using derive from a generic Repository EntityRepositoryBase<T>.
   
   Repository class for Student
   
   Interface
   
   
   
   Class
   
   
   

Step 4 : Creating a Service layer

The 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 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 the corresponding student service for our application business logic. Since we are targeting CRUD operations, so the methods are quite simple in implementation.

Interface



As we can see in the above interface the methods are to Get Student Details, List, Update and Delete Student.

Class

using System.Collections.Generic;
using CodeFirstData.DBInteractions;
using CodeFirstData.EntityRepositories;
using CodeFirstEntities;

using CodeFirstServices.Interfaces;
namespace CodeFirstServices.Services
{
public class StudentService : IStudentService
{
private readonly IStudentRepository _studentRepository;
private readonly IUnitOfWork _unitOfWork;
public StudentService(IStudentRepository studentRepository, IUnitOfWork unitOfWork)
{
this._studentRepository = studentRepository;
this._unitOfWork = unitOfWork;
}
#region IStudentService Members
public IEnumerable<Student> GetStudents()
{
var students = _studentRepository.GetAll();
return students;
}
public Student GetStudentById(int id)
{
var student = _studentRepository.GetById(id);
return student;
}
public void CreateStudent(Student student)
{
_studentRepository.Add(student);
_unitOfWork.Commit();
}

public void DeleteStudent(int id)
{
var student = _studentRepository.GetById(id);
_studentRepository.Delete(student);
_unitOfWork.Commit();
}

public void UpdateStudent(Student student)
{
_studentRepository.Update(student);
_unitOfWork.Commit();
}
public void SaveStudent()
{
_unitOfWork.Commit();
}
#endregion
}
}

The class uses references of Repositories and Entities, and Dependency of Repositories is resolved in the Constructor of the service itself.

Step 5 : The MVC Portal

Before we start the MVC portal, let's clarify our logic for IOC and DI. I have already discussed IOC in the article. For resolving dependencies we create certain classes as discussed below, to get an independent service request. We also create a custom lifetime manager for Unity to store the container in the current ____

public class HttpContextLifetimeManager<T> : LifetimeManager, IDisposable
{
public override object GetValue()
{
var assemblyQualifiedName = typeof (T).AssemblyQualifiedName;
if (assemblyQualifiedName != null)
return HttpContext.Current.Items[assemblyQualifiedName];
return null;
}

public override void RemoveValue()
{
var assemblyQualifiedName = typeof (T).AssemblyQualifiedName;
if (assemblyQualifiedName != null)
HttpContext.Current.Items.Remove(assemblyQualifiedName);
}

public override void SetValue(object newValue)
{
var assemblyQualifiedName = typeof (T).AssemblyQualifiedName;
if (assemblyQualifiedName != null)
HttpContext.Current.Items[assemblyQualifiedName] = newValue;
}
public void Dispose()
{
RemoveValue();
}
}
}