Understanding and Implementing Repository and Unit of Work Pattern in ASP.NET MVC Application

In this article we will try to see what is Repository and Unit of Work Pattern in an ASP.NET MVC application. We will also implement a small rudimentary sample application to understand the same.

Background

Possibility of using ORMs in our application saves us from a lot of code that needs to be written in order to create our entities and data access logic. But using ORMs like entity framework sometimes lead to scattered data access logic/predicates in various place in code.

Repository and Unit of work pattern provides a clean way to access data and at the same time maintain the test-ablility of the application. Let us try to understand this by implementing a simple ASP.NET MVC application.

Using the code

Let us first try to create a simple database on which we will be performing CRUD operations. We will define a simple tables in the database as:

DbDesign

Now with the database/table in created, we will go ahead and generate the ADO.NET entity data Modelfor these tables in our application. The generated entities will look like:

entities

Performing Simple Data Access

Now we have the entity framework ready to be used in our application. We can very well use the Context class in each controller to perform database operations. Let us try to see this by trying to retrieve the data in our Index action of HomeController.

And when we try to run this application, we will see that it is getting the data from the database as:

simpleRunning

Note: We will not be doing other CRUD operations here because they can be done on same lines very easily.

To visualize the above implementation:

cdsimple

Now there is nothing wrong from the code and functionality perspective in doing this. But there are two problems in this approach.

  1. The Data access code is scattered across the application and this is a maintenance nightmare.
  2. The Actionin the Controlleris creating the Contextinside itself. This makes this function non testable using dummy data and we can never be able to verify the results unless we use test data.

Note: If the second point is not clear then it is recommended to read about Test Driven Development using MVC. We cannot discuss it in this article otherwise the article will become digressing.

Creating a Repository

Now how can we solve the problem. We can solve the problem by moving all the data access code of entity framework in one place. So let us define a class that will contain all the data access logic for Bookstable.

But before creating this class, let us also think about the second problem for an instance. If we create a simple interface defining the contract for accessing the books data and then implement this interface in our proposed class, we will have one benefit. We can then have another class implementing the same interface but playing around with the dummy data. Now as long as the controller is using the Interface our test projects can pass the dummy data class and our controller will not complain.

So let us first define the contract for accessing books data.

And the implementation of this class will contain the actual logic to perform the CRUD operations on the Bookstable.

Now let us create a simple Controller in which we will have the reference to this class being used perform the CRUD operations on Books table.

Now here in this above code when the application runs the default parameter-less constructor will run which will create a BooksRepositoryobject and it will be used in the class. The result of which is that the application will be able to work with actual data from the database.

Now from our test project we will call the parameterized constructor with an object of the dummy class containing dummy data. The benefit of which is that we should be able to test and verify the controller classes using the dummy data.

Lets run the application to see the output

RepoRunning

Note: We will not be doing other CRUD operations here because they can be done on same lines very easily.

Lets try to visualize this version of implementation

cdRepo

Having Multiple Repositories

Now imagine the scenario where we have multiple tables in the database. Then we need to create multiple repositories in order to map the domain model to the data model. Now having multiple repository classes poses on problem.

The problem is regarding the ObjectContextobject. If we create multiple repositories, should they contain their ObjectContextseparately? We know that using multiple instances of ObjectContextobject simultaneously can be a problem so should we really allow each repository to contain their own instances?

To solve this problem. Why to let each Repository class instance have its own instance of the ObjectContext. Why not create the instance of ObjectContextin some central location and then pass this instance to the repository classes whenever they are being instantiated. Now this new class will be called as UnitOfWorkand this class will be responsible for creating the ObjectContext nstance and handing over all the repository instances to the controllers.

Unit Of Work

So let us create a separate Repository to which will be used via UnitOfWorkclass and the ObjectContextwill be passed to this class from outside.

Now this Repository class is taking the ObjectContextobject from outside(whenever it is being created). Also, we don’t need to implement IDisposablehere because this class is not creating the instance and so its not this class’s responsibility to dispose it.

Now if we have to create multiple repositories, we can simply have all the repositories take the ObjectContextobject at the time of construction. Now let us see how the UnitOfWorkclass creates the repository and passes it on to the Controller.

Now we have a parameter-less constructor which will be called from controller default constructor i.e. whenever our page runs. We also have a parameterized constructor which will be created from test project and passed on to the controllers parameterized constructors.

The dispose pattern is now implemented by the UnitOfWorkclass because now it is responsible for creating the ObjectContextso it should be the one disposing it.

Let us look at the implementation of the Controller class now.

Now the test-ablity of this controller is still maintained by having the combination of default and parameterized constructor. Also, the data access code is now centralized in one place with the possibility of having multiple repository classes being instantiated at the same time. Let us run the application.

UowRepo

Note: We will not be doing other CRUD operations here because they can be done on same lines very easily.

And finally let us visualize our implementation with Unit of Work in place.

cdUow

Point of interest

In this article we saw what is Repository and Unit of work pattern. We have also seen a rudimentary implementation for the same in an ASP.NET MVC application. The next step to the project would be to convert all the repository classes into one generic repository so that we don’t need to create multiple repository classes. I hope this has been informative.

Download sample code for this article: RepoUowSample

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