testing-testcontainers

name	testing-testcontainers
description	Write integration tests using TestContainers for .NET with xUnit. Covers infrastructure testing with real databases, message queues, and caches in Docker containers instead of mocks. Use when writing integration tests with Docker dependencies.

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

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?

❌ Problems 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 database constraints, indexes, and performance
• Can give false confidence
• Doesn't catch SQL syntax errors or schema mismatches

✅ 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();

        // Run migrations
        await RunMigrationsAsync(_connection);
    }

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

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

        var repo = new OrderRepository(_connection);

        // Act: Execute against real database
        var order = await repo.GetOrderAsync(1);

        // Assert: Verify actual database behavior
        Assert.NotNull(order);
        Assert.Equal(1, order.Id);
        Assert.Equal("CUST1", order.CustomerId);
        Assert.Equal(100.00m, order.Total);
    }
}

Benefits:

• Tests real SQL queries and database behavior
• Catches constraint violations, index issues, and performance problems
• Verifies migrations work correctly
• Gives true confidence in data access layer

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>

Pattern 1: SQL Server Integration Tests

using Testcontainers;
using Xunit;

public class SqlServerTests : IAsyncLifetime
{
    private readonly TestcontainersContainer _dbContainer;
    private IDbConnection _db;

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

    public async Task InitializeAsync()
    {
        await _dbContainer.StartAsync();

        var port = _dbContainer.GetMappedPublicPort(1433);
        var connectionString = $"Server=localhost,{port};Database=master;User Id=sa;Password=Your_password123;TrustServerCertificate=true";

        _db = new SqlConnection(connectionString);
        await _db.OpenAsync();

        // Create test database
        await _db.ExecuteAsync("CREATE DATABASE TestDb");
        await _db.ExecuteAsync("USE TestDb");

        // Run schema migrations
        await _db.ExecuteAsync(@"
            CREATE TABLE Orders (
                Id INT PRIMARY KEY,
                CustomerId NVARCHAR(50) NOT NULL,
                Total DECIMAL(18,2) NOT NULL,
                CreatedAt DATETIME2 DEFAULT GETUTCDATE()
            )");
    }

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

    [Fact]
    public async Task CanInsertAndRetrieveOrder()
    {
        // Arrange
        await _db.ExecuteAsync(@"
            INSERT INTO Orders (Id, CustomerId, Total)
            VALUES (1, 'CUST001', 99.99)");

        // Act
        var order = await _db.QuerySingleAsync<Order>(
            "SELECT * FROM Orders WHERE Id = @Id",
            new { Id = 1 });

        // Assert
        Assert.Equal(1, order.Id);
        Assert.Equal("CUST001", order.CustomerId);
        Assert.Equal(99.99m, order.Total);
    }
}

Pattern 2: PostgreSQL Integration Tests

public class PostgreSqlTests : IAsyncLifetime
{
    private readonly TestcontainersContainer _dbContainer;
    private NpgsqlConnection _connection;

    public PostgreSqlTests()
    {
        _dbContainer = new TestcontainersBuilder<TestcontainersContainer>()
            .WithImage("postgres:latest")
            .WithEnvironment("POSTGRES_PASSWORD", "postgres")
            .WithEnvironment("POSTGRES_DB", "testdb")
            .WithPortBinding(5432, true)
            .WithWaitStrategy(Wait.ForUnixContainer().UntilPortIsAvailable(5432))
            .Build();
    }

    public async Task InitializeAsync()
    {
        await _dbContainer.StartAsync();

        var port = _dbContainer.GetMappedPublicPort(5432);
        var connectionString = $"Host=localhost;Port={port};Database=testdb;Username=postgres;Password=postgres";

        _connection = new NpgsqlConnection(connectionString);
        await _connection.OpenAsync();

        // Create schema
        await _connection.ExecuteAsync(@"
            CREATE TABLE orders (
                id SERIAL PRIMARY KEY,
                customer_id VARCHAR(50) NOT NULL,
                total NUMERIC(10,2) NOT NULL,
                created_at TIMESTAMP DEFAULT NOW()
            )");
    }

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

    [Fact]
    public async Task PostgreSql_ShouldHandleTransactions()
    {
        using var transaction = await _connection.BeginTransactionAsync();

        await _connection.ExecuteAsync(
            "INSERT INTO orders (customer_id, total) VALUES (@CustomerId, @Total)",
            new { CustomerId = "CUST1", Total = 100.00m },
            transaction);

        await transaction.RollbackAsync();

        var count = await _connection.QuerySingleAsync<int>(
            "SELECT COUNT(*) FROM orders");

        Assert.Equal(0, count); // Rollback should prevent insert
    }
}

Pattern 3: Redis Integration Tests

public class RedisTests : IAsyncLifetime
{
    private readonly TestcontainersContainer _redisContainer;
    private IConnectionMultiplexer _redis;

    public RedisTests()
    {
        _redisContainer = new TestcontainersBuilder<TestcontainersContainer>()
            .WithImage("redis:alpine")
            .WithPortBinding(6379, true)
            .WithWaitStrategy(Wait.ForUnixContainer().UntilPortIsAvailable(6379))
            .Build();
    }

    public async Task InitializeAsync()
    {
        await _redisContainer.StartAsync();

        var port = _redisContainer.GetMappedPublicPort(6379);
        _redis = await ConnectionMultiplexer.ConnectAsync($"localhost:{port}");
    }

    public async Task DisposeAsync()
    {
        await _redis.DisposeAsync();
        await _redisContainer.DisposeAsync();
    }

    [Fact]
    public async Task Redis_ShouldCacheValues()
    {
        var db = _redis.GetDatabase();

        // Set value
        await db.StringSetAsync("key1", "value1");

        // Get value
        var value = await db.StringGetAsync("key1");

        Assert.Equal("value1", value.ToString());
    }

    [Fact]
    public async Task Redis_ShouldExpireKeys()
    {
        var db = _redis.GetDatabase();

        await db.StringSetAsync("temp-key", "temp-value",
            expiry: TimeSpan.FromSeconds(1));

        // Key should exist
        Assert.True(await db.KeyExistsAsync("temp-key"));

        // Wait for expiry
        await Task.Delay(1100);

        // Key should be gone
        Assert.False(await db.KeyExistsAsync("temp-key"));
    }
}

Pattern 4: RabbitMQ Integration Tests

public class RabbitMqTests : IAsyncLifetime
{
    private readonly TestcontainersContainer _rabbitContainer;
    private IConnection _connection;

    public RabbitMqTests()
    {
        _rabbitContainer = new TestcontainersBuilder<TestcontainersContainer>()
            .WithImage("rabbitmq:management-alpine")
            .WithPortBinding(5672, true) // AMQP
            .WithPortBinding(15672, true) // Management UI
            .WithWaitStrategy(Wait.ForUnixContainer().UntilPortIsAvailable(5672))
            .Build();
    }

    public async Task InitializeAsync()
    {
        await _rabbitContainer.StartAsync();

        var port = _rabbitContainer.GetMappedPublicPort(5672);
        var factory = new ConnectionFactory
        {
            HostName = "localhost",
            Port = port,
            UserName = "guest",
            Password = "guest"
        };

        _connection = await factory.CreateConnectionAsync();
    }

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

    [Fact]
    public async Task RabbitMq_ShouldPublishAndConsumeMessage()
    {
        using var channel = await _connection.CreateChannelAsync();

        var queueName = "test-queue";
        await channel.QueueDeclareAsync(queueName, durable: false,
            exclusive: false, autoDelete: true);

        // Publish message
        var message = "Hello, RabbitMQ!";
        var body = Encoding.UTF8.GetBytes(message);
        await channel.BasicPublishAsync(exchange: "",
            routingKey: queueName,
            body: body);

        // Consume message
        var consumer = new EventingBasicConsumer(channel);
        var tcs = new TaskCompletionSource<string>();

        consumer.Received += (model, ea) =>
        {
            var receivedMessage = Encoding.UTF8.GetString(ea.Body.ToArray());
            tcs.SetResult(receivedMessage);
        };

        await channel.BasicConsumeAsync(queueName, autoAck: true,
            consumer: consumer);

        // Wait for message
        var received = await tcs.Task.WaitAsync(TimeSpan.FromSeconds(5));

        Assert.Equal(message, received);
    }
}

Pattern 5: Multi-Container Networks

When you need multiple containers to communicate:

public class MultiContainerTests : IAsyncLifetime
{
    private readonly INetwork _network;
    private readonly TestcontainersContainer _dbContainer;
    private readonly TestcontainersContainer _redisContainer;

    public MultiContainerTests()
    {
        _network = new TestcontainersNetworkBuilder()
            .Build();

        _dbContainer = new TestcontainersBuilder<TestcontainersContainer>()
            .WithImage("postgres:latest")
            .WithNetwork(_network)
            .WithNetworkAliases("db")
            .WithEnvironment("POSTGRES_PASSWORD", "postgres")
            .Build();

        _redisContainer = new TestcontainersBuilder<TestcontainersContainer>()
            .WithImage("redis:alpine")
            .WithNetwork(_network)
            .WithNetworkAliases("redis")
            .Build();
    }

    public async Task InitializeAsync()
    {
        await _network.CreateAsync();
        await Task.WhenAll(
            _dbContainer.StartAsync(),
            _redisContainer.StartAsync());
    }

    public async Task DisposeAsync()
    {
        await Task.WhenAll(
            _dbContainer.DisposeAsync().AsTask(),
            _redisContainer.DisposeAsync().AsTask());
        await _network.DisposeAsync();
    }

    [Fact]
    public async Task Containers_CanCommunicate()
    {
        // Both containers can reach each other via network aliases
        // db -> redis://redis:6379
        // redis -> postgres://db:5432
    }
}

Pattern 6: Reusing Containers Across Tests

For faster test execution, reuse containers across tests in a class:

[Collection("Database collection")]
public class FastDatabaseTests
{
    private readonly DatabaseFixture _fixture;

    public FastDatabaseTests(DatabaseFixture fixture)
    {
        _fixture = fixture;
    }

    [Fact]
    public async Task Test1()
    {
        // Use _fixture.Connection
        // Clean up data after test if needed
    }

    [Fact]
    public async Task Test2()
    {
        // Reuses the same container
    }
}

// Shared fixture
public class DatabaseFixture : IAsyncLifetime
{
    private readonly TestcontainersContainer _container;
    public IDbConnection Connection { get; private set; }

    public DatabaseFixture()
    {
        _container = 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 _container.StartAsync();
        // Setup connection
    }

    public async Task DisposeAsync()
    {
        await Connection.DisposeAsync();
        await _container.DisposeAsync();
    }
}

[CollectionDefinition("Database collection")]
public class DatabaseCollection : ICollectionFixture<DatabaseFixture> { }

Pattern 7: Testing Migrations with Real Databases

public class MigrationTests : IAsyncLifetime
{
    private readonly TestcontainersContainer _container;
    private string _connectionString;

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

        await _container.StartAsync();

        var port = _container.GetMappedPublicPort(1433);
        _connectionString = $"Server=localhost,{port};Database=TestDb;User Id=sa;Password=Your_password123;TrustServerCertificate=true";
    }

    [Fact]
    public async Task Migrations_ShouldRunSuccessfully()
    {
        // Run Entity Framework migrations
        var optionsBuilder = new DbContextOptionsBuilder<AppDbContext>();
        optionsBuilder.UseSqlServer(_connectionString);

        using var context = new AppDbContext(optionsBuilder.Options);

        // Apply migrations
        await context.Database.MigrateAsync();

        // Verify schema
        var canConnect = await context.Database.CanConnectAsync();
        Assert.True(canConnect);

        // Verify tables exist
        var tables = await context.Database.SqlQueryRaw<string>(
            "SELECT TABLE_NAME FROM INFORMATION_SCHEMA.TABLES").ToListAsync();

        Assert.Contains("Orders", tables);
        Assert.Contains("Customers", tables);
    }

    public async Task DisposeAsync()
    {
        await _container.DisposeAsync();
    }
}

Best Practices

1. Always Use IAsyncLifetime - Proper async setup and teardown
2. Wait for Port Availability - Use WaitStrategy to ensure containers are ready
3. Use Random Ports - Let TestContainers assign ports automatically
4. Clean Data Between Tests - Either use fresh containers or truncate tables
5. Reuse Containers When Possible - Faster than creating new ones for each test
6. Test Real Queries - Don't just test mocks; verify actual SQL behavior
7. Verify Constraints - Test foreign keys, unique constraints, indexes
8. Test Transactions - Verify rollback and commit behavior
9. Use Realistic Data - Test with production-like data volumes
10. Handle Cleanup - Always dispose containers in DisposeAsync

Common Issues and Solutions

Issue 1: Container Startup Timeout

Problem: Container takes too long to start

Solution:

_container = new TestcontainersBuilder<TestcontainersContainer>()
    .WithImage("postgres:latest")
    .WithWaitStrategy(Wait.ForUnixContainer()
        .UntilPortIsAvailable(5432)
        .WithTimeout(TimeSpan.FromMinutes(2)))
    .Build();

Issue 2: Port Already in Use

Problem: Tests fail because port is already bound

Solution: Always use random port mapping:

.WithPortBinding(5432, true) // true = assign random public port

Issue 3: Containers Not Cleaning Up

Problem: Containers remain running after tests

Solution: Ensure proper disposal:

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

Issue 4: Tests Fail in CI But Pass Locally

Problem: CI environment doesn't have Docker

Solution: Ensure CI has Docker support:

# GitHub Actions
runs-on: ubuntu-latest # Has Docker pre-installed
services:
  docker:
    image: docker:dind

CI/CD Integration

GitHub Actions

name: testing-testcontainers

on: [push, pull_request]

jobs:
  test:
    runs-on: ubuntu-latest # Has Docker pre-installed

    steps:
    - uses: actions/checkout@v3

    - name: Setup .NET
      uses: actions/setup-dotnet@v3
      with:
        dotnet-version: 9.0.x

    - name: Run Integration Tests
      run: |
        dotnet test tests/YourApp.IntegrationTests \
          --filter Category=Integration \
          --logger trx

    - name: Cleanup Containers
      if: always()
      run: docker container prune -f

Pattern 8: Database Reset with Respawn

When reusing containers across tests, use Respawn to reset database state between tests instead of recreating containers:

<PackageReference Include="Respawn" Version="*" />

Basic Respawn Setup

using Respawn;

public class DatabaseFixture : IAsyncLifetime
{
    private readonly TestcontainersContainer _container;
    private Respawner _respawner = null!;
    public NpgsqlConnection Connection { get; private set; } = null!;
    public string ConnectionString { get; private set; } = null!;

    public async Task InitializeAsync()
    {
        await _container.StartAsync();

        var port = _container.GetMappedPublicPort(5432);
        ConnectionString = $"Host=localhost;Port={port};Database=testdb;Username=postgres;Password=postgres";

        Connection = new NpgsqlConnection(ConnectionString);
        await Connection.OpenAsync();

        // Run migrations first
        await RunMigrationsAsync();

        // Create respawner after schema exists
        _respawner = await Respawner.CreateAsync(ConnectionString, new RespawnerOptions
        {
            TablesToIgnore = new Table[]
            {
                "__EFMigrationsHistory",  // EF Core migrations table
                "AspNetRoles",            // Identity roles (seeded data)
                "schema_version"          // DbUp/Flyway version table
            },
            DbAdapter = DbAdapter.Postgres
        });
    }

    /// <summary>
    /// Reset database to clean state. Call this in test setup or between tests.
    /// </summary>
    public async Task ResetDatabaseAsync()
    {
        await _respawner.ResetAsync(ConnectionString);
    }

    public async Task DisposeAsync()
    {
        await Connection.DisposeAsync();
        await _container.DisposeAsync();
    }
}

Using Respawn in Tests

[Collection("Database collection")]
public class OrderTests : IAsyncLifetime
{
    private readonly DatabaseFixture _fixture;

    public OrderTests(DatabaseFixture fixture)
    {
        _fixture = fixture;
    }

    public async Task InitializeAsync()
    {
        // Reset database before each test
        await _fixture.ResetDatabaseAsync();
    }

    public Task DisposeAsync() => Task.CompletedTask;

    [Fact]
    public async Task CreateOrder_ShouldPersist()
    {
        // Database is clean - no leftover data from other tests
        await _fixture.Connection.ExecuteAsync(
            "INSERT INTO orders (customer_id, total) VALUES (@CustomerId, @Total)",
            new { CustomerId = "CUST1", Total = 100.00m });

        var count = await _fixture.Connection.QuerySingleAsync<int>(
            "SELECT COUNT(*) FROM orders");

        Assert.Equal(1, count);
    }

    [Fact]
    public async Task AnotherTest_StartsWithCleanDatabase()
    {
        // This test also starts with empty tables
        var count = await _fixture.Connection.QuerySingleAsync<int>(
            "SELECT COUNT(*) FROM orders");

        Assert.Equal(0, count); // Clean slate!
    }
}

Respawn Options

var respawner = await Respawner.CreateAsync(connectionString, new RespawnerOptions
{
    // Tables to preserve (reference data, migrations history)
    TablesToIgnore = new Table[]
    {
        "__EFMigrationsHistory",
        new Table("public", "lookup_data"),  // Schema-qualified
    },

    // Schemas to clean (default: all schemas)
    SchemasToInclude = new[] { "public", "app" },

    // Or exclude specific schemas
    SchemasToExclude = new[] { "audit", "logging" },

    // Database adapter
    DbAdapter = DbAdapter.Postgres,  // or SqlServer, MySql

    // Handle circular foreign keys
    WithReseed = true  // Reset identity columns (SQL Server)
});

Why Respawn Over Container Recreation

Approach	Pros	Cons
New container per test	Complete isolation	Slow (10-30s per container)
Respawn	Fast (~50ms), preserves schema/migrations	Requires careful table exclusion
Transaction rollback	Fastest	Can't test commit behavior

Use Respawn when:

• Tests share a container via xUnit collection fixture
• You need to test actual commits (not just rollbacks)
• Container startup time is a bottleneck

Performance Tips

1. Reuse containers - Share fixtures across tests in a collection
2. Use Respawn - Reset data without recreating containers
3. Parallel execution - TestContainers handles port conflicts automatically
4. Use lightweight images - Alpine versions are smaller and faster
5. Cache images - Docker will cache pulled images locally
6. Limit container resources - Set CPU/memory limits if needed:

.WithResourceMapping(new CpuCount(2))
.WithResourceMapping(new MemoryLimit(512 * 1024 * 1024)) // 512MB