What is Repository pattern and why should we use it?

With the Repository pattern, we create an abstraction layer between the data access and the business logic layer of an application. By using it, we are promoting a more loosely coupled approach to access our data from the database. Also, the code is cleaner and easier to maintain and reuse. Data access logic is in a separate class, or sets of classes called a repository, with the responsibility of persisting the application’s business model.

Implementing the repository pattern is our topic for this post. Additionally, this article has a strong relationship with EF Core, so we strongly recommend reading our EF Core tutorial to get familiar or just a better understanding of that topic.

So let’s start.

If you want to see all the basic instructions and complete navigation for this series, please follow the following link: Introduction page for this tutorial.

For the previous part check out: Creating .NET Core WebApi project – Custom logging in .NET Core

The source code is available for download at .NET Core, Angular and MySQL. Part 4 – Source Code

This post is divided into several sections:

Creating a Models

Let’s begin by creating a new Class Library (.NET Core) project named Entities and inside it a new folder with the name Models, which will contain all the model classes. Model classes will represent the tables inside the database and will serve you to map the data from the database to the .NET Core. After that, reference this project to the main project.

In the Models folder create two classes and add this code:

As you can see, there are two models decorated with the attribute Table(“tableName”). This attribute will configure the corresponding table name in the database. Also, every primary key has the decoration attribute Key that configures the primary key of the table. All the mandatory fields have the attribute [Required] and if you want to constrain the strings, you can use the attribute [StringLength]

Notice that these models are clean, they have only those properties that match the columns in the tables. I like to keep models as clean as possible and this is the way to do it. Later on, we will create DTO classes with the properties to connect one owner with all of its accounts or to connect a single account with its owner.

Context Class and the Database Connection

Now, let us create the context class, which will be a middleware component for the communication with the database. It has DbSet properties which contain the table data from the database. For a better understanding of the Context class and DbSet properties and how they work with EF Core overall, you can read Getting Started with EF Core article.

In the root of Entities project create the RepositoryContext class and modify it:

To enable communication between the .NET core part and the MySQL database, you’ll need to install a third-party library named Pomelo.EntityFrameworkCore.MySql. In the main project, you may install it with the NuGet package manager or Package manager console.

After the installation, open the appsettings.json file and add DB connection settings inside:

In the ServiceExtensions class, you are going to write the code for configuring the MySQL context.

First, add the using directives and then add the method ConfigureMySqlContext:

With the help of the IConfiguration config parameter, you can access the appsettings.json file and take all the data you need from it. Afterward, in the Startup class in the ConfigureServices method, add the context service to the IOC right above the services.AddMvc():

Repository Pattern Logic

After establishing a connection with the database, it is time to create the generic repository that will serve us all the CRUD methods. As a result, all the methods can be called upon any repository class in your project.

Furthermore, creating the generic repository and repository classes which use that generic repository is not going to be the final step. You will go a  step further and create a wrapper around repository classes and inject it as a service. Consequently,  you can instantiate this wrapper once and then call any repository class you need inside any of your controllers. You will understand the advantages of this wrapper when we use it in the project.

First, let’s create an interface for the repository inside the Contracts project:

Right after the interface creation, you are going to create a new Class Library (.NET Core) project with the name Repository (add the reference from Contracts to this project), and inside the Repository project create the abstract class RepositoryBase which will implement the interface IRepositoryBase.

Reference this project to the main project too. 

Tip: If you have problems with referencing EntityFrameworkCore in your Assemblies, for this new Repository project, you need to right-click on the Repository project, click on the Unload Project. When the project unloads, right click on it and choose Edit Repository.csproj. Add this code to the opened file:referencing entity framework core for repository pattern

Finally, save the file and right-click on the Repository project and click the Reload Project.

Add the following code to the RepositoryBase class:

This abstract class, as well as IRepositoryBase interface, use generic type T to work with. This type T gives even more reusability to the RepositoryBase class. That means you don’t have to specify exact model (class) right now for the RepositoryBase to work with, you’ll do that later on.

Repository User Classes

Now that you have the RepositoryBase class, create the user classes that will inherit this abstract class. Every user class will have its own interface, for additional model specific methods. Furthermore, by inheriting from the RepositoryBase class they will have access to all the methods from the RepositoryBase. This way, we are separating the logic, that is common for all our repository user classes and also specific for every user class itself.

Let’s create the interfaces in the Contracts project for your Owner and Account classes.

Don’t forget to add a reference from the Entities project. As soon as we do this, we can delete the Entities reference from the main project because it is now provided through the Repository project which already has the Contracts project referenced with the Entities reference inside.

Now create a repository user classes in the Repository project:

After these steps, you are finished with creating the repository and repository user classes. But there are still more things to do.

Creating a Repository Wrapper

Imagine if inside a controller you need to collect all the Owners and to collect only the certain Accounts (for example Domestic ones). You would need to instantiate OwnerRepository and AccountRepository classes and then call the FindAll and FindByCondition methods.

Maybe it’s not a problem when you have only two classes, but what if you need logic from 5 different classes or even more. Having that in mind, let’s create a wrapper around your repository user classes. Then place it into the IOC and finally inject it inside the controller’s constructor. Now, with that wrappers instance, you may call any repository class you need.

Create a new interface in the Contract project:

Add a new class to the Repository project:

As you can see, we are creating properties that will expose the concrete repositories and also we have the Save() method to be used after all the modifications are finished on a certain object. This is a good practice because now we can, for example, add two owners, modify two accounts and delete one owner, all in one method, and then just call the Save method once. All the changes will be applied or if something fails, all the changes will be reverted:

In the ServiceExtensions class add this code:

And in the Startup class inside the ConfigureServices method, above the services.AddMvc() line, add this code:


All you need to do is to test this code the same way you did with your custom logger in the part3 of this series.

Inject the RepositoryWrapper service inside the Values controller and call any method from the RepositoryBase class:

Place the breakpoint inside the Get() method and you’ll see the data returned from the database.

We have created our Repository Pattern synchronously but it could be done asynchronously as well. If you want to learn how to do that you can visit Implementing Async Repository in .NET Core. Although we strongly recommend finishing all the parts from this series for easier understanding of the project’s business logic.


The Repository pattern increases the level of abstraction in your code. This may make the code more difficult to understand for developers who are unfamiliar with the pattern. But once you are familiar with it, it will reduce the amount of redundant code and make the logic much easier to maintain.

In this post you have learned:

  • What is repository pattern
  • How to create models and model attributes
  • How to create context class and database connection
  • The right way to create repository logic
  • And the way to create a wrapper around your repository classes

Thank you all for reading this post and I hope you read some useful information in it.

See you soon in the next article, where we will use repository logic to create HTTP requests.

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