Integration Testing with TestContainers

When to Use This Skill

Use this skill when:

• Writing integration tests that need real infrastructure (databases, caches, message queues)
• Testing data access layers against actual databases
• Verifying message queue integrations
• Testing Redis caching behavior
• Avoiding mocks for infrastructure components
• Ensuring tests work against production-like environments
• Testing database migrations and schema changes

Reference Files

• database-patterns.md: SQL Server, PostgreSQL, and migration testing examples
• infrastructure-patterns.md: Redis, RabbitMQ, multi-container networks, container reuse, and Respawn

Core Principles

1. Real Infrastructure Over Mocks - Use actual databases/services in containers, not mocks
2. Test Isolation - Each test gets fresh containers or fresh data
3. Automatic Cleanup - TestContainers handles container lifecycle and cleanup
4. Fast Startup - Reuse containers across tests in the same class when appropriate
5. CI/CD Compatible - Works seamlessly in Docker-enabled CI environments
6. Port Randomization - Containers use random ports to avoid conflicts

Why TestContainers Over Mocks?

The Problem with Mocking Infrastructure

// BAD: Mocking a database
public class OrderRepositoryTests
{
    private readonly Mock<IDbConnection> _mockDb = new();

    [Fact]
    public async Task GetOrder_ReturnsOrder()
    {
        // This doesn't test real SQL behavior, constraints, or performance
        _mockDb.Setup(db => db.QueryAsync<Order>(It.IsAny<string>()))
            .ReturnsAsync(new[] { new Order { Id = 1 } });

        var repo = new OrderRepository(_mockDb.Object);
        var order = await repo.GetOrderAsync(1);

        Assert.NotNull(order);
    }
}

Problems: doesn't test actual SQL queries, misses constraints/indexes, gives false confidence, doesn't catch SQL syntax errors.

Better: TestContainers with Real Database

// GOOD: Testing against a real database
public class OrderRepositoryTests : IAsyncLifetime
{
    private readonly TestcontainersContainer _dbContainer;
    private IDbConnection _connection;

    public OrderRepositoryTests()
    {
        _dbContainer = new TestcontainersBuilder<TestcontainersContainer>()
            .WithImage("mcr.microsoft.com/mssql/server:2022-latest")
            .WithEnvironment("ACCEPT_EULA", "Y")
            .WithEnvironment("SA_PASSWORD", "Your_password123")
            .WithPortBinding(1433, true)
            .Build();
    }

    public async Task InitializeAsync()
    {
        await _dbContainer.StartAsync();
        var port = _dbContainer.GetMappedPublicPort(1433);
        var connectionString = $"Server=localhost,{port};Database=TestDb;User Id=sa;Password=Your_password123;TrustServerCertificate=true";
        _connection = new SqlConnection(connectionString);
        await _connection.OpenAsync();
        await RunMigrationsAsync(_connection);
    }

    public async Task DisposeAsync()
    {
        await _connection.DisposeAsync();
        await _dbContainer.DisposeAsync();
    }

    [Fact]
    public async Task GetOrder_WithRealDatabase_ReturnsOrder()
    {
        await _connection.ExecuteAsync(
            "INSERT INTO Orders (Id, CustomerId, Total) VALUES (1, 'CUST1', 100.00)");

        var repo = new OrderRepository(_connection);
        var order = await repo.GetOrderAsync(1);

        Assert.NotNull(order);
        Assert.Equal("CUST1", order.CustomerId);
        Assert.Equal(100.00m, order.Total);
    }
}

See database-patterns.md for complete SQL Server, PostgreSQL, and migration testing examples.

See infrastructure-patterns.md for Redis, RabbitMQ, multi-container networks, container reuse, and Respawn database reset patterns.

Required NuGet Packages

<ItemGroup>
  <PackageReference Include="Testcontainers" Version="*" />
  <PackageReference Include="xunit" Version="*" />
  <PackageReference Include="xunit.runner.visualstudio" Version="*" />

  <!-- Database-specific packages -->
  <PackageReference Include="Microsoft.Data.SqlClient" Version="*" />
  <PackageReference Include="Npgsql" Version="*" /> <!-- For PostgreSQL -->
  <PackageReference Include="MySqlConnector" Version="*" /> <!-- For MySQL -->

  <!-- Other infrastructure -->
  <PackageReference Include="StackExchange.Redis" Version="*" /> <!-- For Redis -->
  <PackageReference Include="RabbitMQ.Client" Version="*" /> <!-- For RabbitMQ -->
</ItemGroup>

Best Practices

1. Always Use IAsyncLifetime - Proper async setup and teardown
2. Wait for Port Availability - Use WaitStrategy to ensure containers are ready