Modern software applications rarely work in isolation. On the contrary, a typical application will talk to several external systems like databases, messaging systems, cache providers, and many 3rd party services. And it's up to you to ensure everything functions correctly.

Hopefully, I don't have to convince you about the value of writing tests.

You should be writing tests. Period.

However, I do want to discuss the value of integration testing.

Unit tests are helpful to test business logic in isolation, without any external services. They are easy to write and provide almost instant feedback.

But you can't be fully confident in your application without integration tests.

So, in this week's newsletter, I'll show you how to use Docker for integration testing.

Here's what we will use to write integration tests :

Testcontainers
Docker
xUnit

Let's dive in!

What is Testcontainers?

Testcontainers is a library for writing tests with throwaway Docker containers.

Why should you use it?

Integration testing is considered "difficult" because you have to maintain testing infrastructure. Before running tests, you need to make sure the database is up and running. You also have to seed any data required for the tests. If you have tests running in parallel on the same database, they could interfere with each other.

A possible solution could be using in-memory variations of the required services. But this isn't much different from using mocks. In-memory services might not have all the features of the production service.

Testcontainers solves this by using Docker to spin up real services for integration testing.

Here's an example of creating a SQL Server container:

MsSqlContainer dbContainer = new MsSqlBuilder()
    .WithImage("mcr.microsoft.com/mssql/server:2022-latest")
    .WithPassword("Strong_password_123!")
    .Build();

You can then use the MsSqlContainer instance to get a connection string for the database running inside the container.

Do you see how this is valuable for writing integration tests?

No more need for mocks or fake in-memory databases. Instead, you can use the real deal.

I won't do a deep dive into this library here, so refer to the documentation for more information.

Implementing a Custom WebApplicationFactory

ASP.NET Core provides an in-memory test server that we can use to spin up an application instance for running tests. The Microsoft.AspNetCore.Mvc.Testing package provides the WebApplicationFactory class that we will use as the base for our implementation.

WebApplicationFactory<TEntryPoint> is used to create a TestServer for the integration tests.

The custom IntegrationTestWebAppFactory will do a few things:

Create and configure a MsSqlContainer instance
Call ConfigureTestServices to set up EF Core with the container database
Start and stop the container instance with IAsyncLifetime

MsSqlContainer has a GetConnectionString method to grab the connection string for the current container. Note that this can change between tests, as each test class will create a separate container instance. Test cases inside the same test class will use the same container instance. So keep that in mind if you need to do a cleanup between tests.

Another thing to keep in mind is database migrations. You will have to run them manually before every test to create the required database structure.

Starting the container instance is done asynchronously using IAsyncLifetime. The container is started inside StartAsync before any of the tests run. And it's stopped inside StopAsync.

Here's the complete code for IntegrationTestWebAppFactory:

public class IntegrationTestWebAppFactory
    : WebApplicationFactory<Program>,
      IAsyncLifetime
{
    private readonly MsSqlContainer _dbContainer = new MsSqlBuilder()
        .WithImage("mcr.microsoft.com/mssql/server:2022-latest")
        .WithPassword("Strong_password_123!")
        .Build();

    protected override void ConfigureWebHost(IWebHostBuilder builder)
    {
        builder.ConfigureTestServices(services =>
        {
            var descriptorType =
                typeof(DbContextOptions<ApplicationDbContext>);

            var descriptor = services
                .SingleOrDefault(s => s.ServiceType == descriptorType);

            if (descriptor is not null)
            {
                services.Remove(descriptor);
            }

            services.AddDbContext<ApplicationDbContext>(options =>
                options.UseSqlServer(_dbContainer.GetConnectionString()));
        });
    }

    public Task InitializeAsync()
    {
        return _dbContainer.StartAsync();
    }

    public new Task DisposeAsync()
    {
        return _dbContainer.StopAsync();
    }
}

Creating The Base Test Class

The base test class will implement a class fixture interface IClassFixture. It indicates the class contains tests and provides shared object instances across the test cases inside. This is a good place to instantiate any services that are required for most tests.

For example, I'm creating an IServiceScope for resolving scoped services inside the tests.

ISender for sending commands and queries
ApplicationDbContext for database setup or verifying results

public abstract class BaseIntegrationTest
    : IClassFixture<IntegrationTestWebAppFactory>,
      IDisposable
{
    private readonly IServiceScope _scope;
    protected readonly ISender Sender;
    protected readonly ApplicationDbContext DbContext;

    protected BaseIntegrationTest(IntegrationTestWebAppFactory factory)
    {
        _scope = factory.Services.CreateScope();

        Sender = _scope.ServiceProvider.GetRequiredService<ISender>();

        DbContext = _scope.ServiceProvider
            .GetRequiredService<ApplicationDbContext>();
    }

    public void Dispose()
    {
        _scope?.Dispose();
        DbContext?.Dispose();
    }
}

With all the infrastructure in place, we're finally ready to write the tests.

Putting It All Together - Writing Integration Tests

Here's a ProductTests class with an integration test inside.

I use the Arrange-Act-Assert pattern to structure tests:

Arrange - create the CreateProduct.Command instance
Act - send the command using ISender and store the result
Assert - use the result from the Act step to verify the database state

The value of writing integration tests like this is that you can use the complete MediatR request pipeline. If you have any IPipelineBehavior wrapping the request, it will also be executed.

The same applies if you write your business logic inside service classes. Instead of resolving the ISender, you would resolve the specific services you want to test.

Most importantly, this test uses a real database instance running inside a Docker container.

public class ProductTests : BaseIntegrationTest
{
    public ProductTests(IntegrationTestWebAppFactory factory)
        : base(factory)
    {
    }

    [Fact]
    public async Task Create_ShouldCreateProduct()
    {
        // Arrange
        var command = new CreateProduct.Command
        {
            Name = "AMD Ryzen 7 7700X",
            Category = "CPU",
            Price = 223.99m
        };

        // Act
        var productId = await Sender.Send(command);

        // Assert
        var product = DbContext
            .Products
            .FirstOrDefault(p => p.Id == productId);

        Assert.NotNull(product);
    }
}

Running Integration Tests In CI/CD Pipelines

You can also run integration tests with Testcontainers inside CI/CD pipelines. The only requirement is that it supports Docker.

GitHub Actions does support Docker. If you are hosting your project there, integration tests will work out of the box.

You can learn more about building a CI/CD pipeline with GitHub Actions here.

And if you want a plug-in solution, here's a GitHub Actions workflow you can use:

name: Run Tests 🚀

on:
  workflow_dispatch:
  push:
    branches:
      - main

jobs:
  run-tests:
    runs-on: ubuntu-latest

    steps:
      - uses: actions/checkout@v3

      - name: Setup .NET
        uses: actions/setup-dotnet@v3
        with:
          dotnet-version: '7.0.x'

      - name: Restore
        run: dotnet restore ./Products.Api.sln

      - name: Build
        run: dotnet build ./Products.Api.sln --no-restore

      - name: Test
        run: dotnet test ./Products.Api.sln --no-build

Takeaway

Testcontainers is an excellent solution for writing integration tests with Docker. You can spin up and configure any Docker image and use it from your application. This is far better than using mocks or in-memory variations because they lack many features.

If you have a CI/CD pipeline that supports Docker, Testcontainers will work out of the box.

Only a few integration tests will drastically improve your confidence in the system.

You can grab the source code for this newsletter on my GitHub.

It's completely free, so what are you waiting for?

And if you prefer video, here's a quick tutorial on integration testing with Testcontainers.

Hope this was valuable.

Stay awesome!

P.S. Whenever you're ready, there are 2 ways I can help you:

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Testcontainers - Integration Testing Using Docker In .NET