Discover Implementations

Wire convention-based discovery for a set of implementations behind an abstraction.

The pattern in one line

Inject IInstancesOf<TInterface> from Cratis.Types. Mark implementations with [Singleton]. Delete any services.AddSingleton<TInterface, Impl>() lines that registered them.

When this skill applies

The consumer of an abstraction needs to iterate, filter, or fan out to every registered implementation. Common shapes:

• *Handlers, *Filters, *Validators, *Formatters, *Strategies, *Providers, *Resolvers — anything plural that delegates to a set.
• A dispatcher that picks the right implementation by calling CanHandle(...) and forwarding to the matching one.
• A composite that fans a single input out to all implementations and aggregates results.

When it does NOT apply

• The consumer needs one implementation chosen at composition time → constructor-inject the concrete interface and let normal DI resolve IFoo → Foo.
• The method returns a sequence of values to a caller → IEnumerable<T> (or IReadOnlyList<T>, etc.) is still the right return type. The rule is only about injecting implementations of an abstraction.

Step 1 — Confirm the implementations are discoverable

IInstancesOf<T> discovers types by convention from loaded assemblies. The only requirements:

• Each implementation is a non-abstract public class.
• It implements the interface directly (not via another layer that hides it).

No assembly attribute is needed — the Cratis framework's type discovery picks them up automatically.

Step 2 — Mark implementations as singletons (default)

[Singleton]
public class EventResultHandler(IEventTypes eventTypes) : IReactorSideEffectHandler
{
    public bool CanHandle(ReactorContext context, object value) => /* ... */;
    public Task Handle(ReactorContext context, object value) => /* ... */;
}

Add using Cratis; if [Singleton] is unresolved.

Skip [Singleton] only when the implementation must be transient — i.e. it holds per-call state that cannot be shared. The convention IFoo → Foo still applies for transients; do not register them explicitly.

Step 3 — Inject IInstancesOf<T> in the consumer

using Cratis.Types;

[Singleton]
public class ReactorSideEffectHandlers(IInstancesOf<IReactorSideEffectHandler> handlers) : IReactorSideEffectHandlers
{
    public bool CanHandle(ReactorContext context, object value) =>
        handlers.Any(h => h.CanHandle(context, value));

    public Task Handle(ReactorContext context, object value) =>
        handlers.First(h => h.CanHandle(context, value)).Handle(context, value);
}

IInstancesOf<T> implements IEnumerable<T> — LINQ works directly on it. Materialize with .ToArray() only if you need a stable snapshot (rare).

Step 4 — Delete the dead registrations

Find every line in composition roots and service-collection extensions that registered the implementations or the consumer, and remove them:

// Delete these — IInstancesOf<T> discovers them, [Singleton] registers them
services.AddSingleton<IReactorSideEffectHandler, EventResultHandler>();
services.AddSingleton<IReactorSideEffectHandler, EventsResultHandler>();
services.AddSingleton<IReactorSideEffectHandlers, ReactorSideEffectHandlers>();

Use the codebase search tools to find every reference and verify nothing else relies on these registrations.

Step 5 — Verify

1. dotnet build — zero warnings, zero errors.
2. Run the relevant specs/integration tests for the affected feature.
3. If a System.MissingMethodException: Cannot dynamically create an instance of type '...'. Reason: Cannot create an instance of an interface. appears at runtime, an implementation is missing [Singleton] or the interface signature changed — re-check Step 2.

Why this matters

The cost of services.AddSingleton<TInterface, Impl>() looks zero at the registration line. The hidden cost shows up later:

• Adding a new implementation in a different folder silently does nothing until someone remembers to register it.
• Removing an implementation leaves a stale registration that fails at startup.
• Spec setups have to duplicate the same registrations to mirror production.
• The composition root grows linearly with the number of implementations — high churn, high merge conflict.

IInstancesOf<T> and [Singleton] push that knowledge into the implementation itself. Adding or removing an implementation is a single-file change.