Writing a test attribute

An attribute that sets a test up is a class implementing one of five interfaces. The runner calls it at the point in the test's construction the interface names, for every test that carries it.

public sealed class SeededDatabaseAttribute : Attribute, IHardenedTestStartupAttribute {

    public int Order => 100;

    public async Task Startup(
        AttributeCollection attributes, MethodInfo method,
        IHardenedEnvironment environment, IServiceProvider provider) {
        await provider.GetRequiredService<ISeeder>().Seed();
    }
}

[HardenedTest]
[SeededDatabase]
public async Task FindsASeededOrder(IOrderRepository orders) {
    Assert.NotNull(await orders.Get("ORDER#1"));
}

The five interfaces

In the order the runner calls them:

Interface	Called	To
IHardenedTestEnvironmentAttribute	before the modules are applied, with the environment name and its values	add or change environment values
IHardenedTestDependencyRegistrationAttribute	after the application's modules have registered, with the IServiceCollection	register or replace a service
IHardenedParameterProviderAttribute	with the collection, and later once per parameter	supply a parameter the container does not hold
IHardenedTestConfigurationAttribute	with an IAppConfig	amend a configuration model for the test
IHardenedTestStartupAttribute	after the provider is built and the application's startup services have run	start a container, create a table, seed data

All five extend IHardenedOrderedAttribute. Order sorts the startup attributes, lowest first.

Every interface is read from the method, its class and the assembly. AttributeCollection holds the three sets, and GetAttribute<T>() finds one of a kind across them, which is how an attribute reads a [EnvironmentName] or a setting of its own declared beside it.

A registration attribute

public sealed class InMemoryStoreAttribute : Attribute, IHardenedTestDependencyRegistrationAttribute {

    public int Order => 0;

    public void RegisterDependencies(
        AttributeCollection attributes, MethodInfo method,
        IHardenedEnvironment environment, IServiceCollection services) {
        services.AddSingleton<IOrderStore, InMemoryOrderStore>();
    }
}

It runs after the application's modules, so the registration replaces the application's own. Substituting services has a TimeProvider registered this way.

An environment attribute

public sealed class FeatureFlagsOnAttribute : Attribute, IHardenedTestEnvironmentAttribute {

    public int Order => 0;

    public void ConfigureEnvironment(
        AttributeCollection attributes, MethodInfo method,
        string environmentName, IDictionary<string, object> environment) {
        environment["FEATURE_X"] = "on";
        environment["FEATURE_Y"] = "on";
    }
}

[EnvironmentValue] sets one value. An environment attribute sets a set of them, once, under one name.

A configuration attribute

public sealed class ShortRetriesAttribute : Attribute, IHardenedTestConfigurationAttribute {

    public int Order => 0;

    public void Configure(
        AttributeCollection attributes, MethodInfo method,
        IHardenedEnvironment environment, IAppConfig appConfig) {
        appConfig.Amend((RetryConfiguration retry) => retry.MaxAttempts = 1);
    }
}

The IAppConfig is registered as the test's IConfigurationPackage, so the amender runs the first time the model is resolved. See Amending configuration.

Deriving from a shipped attribute

[LocalDynamoDb] is a startup attribute meant to be derived from. Override DdbSetup to create the tables the test needs; see DynamoDB Local.

Next

• Substituting services: [Mock] for the one-line case
• Environments in tests: the shipped environment attributes
• Testing AWS handlers: the Lambda and DynamoDB Local attributes