MediatR and StructureMap–Using Generic NotificationHandlers.

One of our application is using a CQRS approach in combination with MediatR and StructureMap. Part of the application is rather straightforward and to avoid a lot of boilerplate code we would like to use a NotificationHandler that could handle generic events. Here is the code we had written:

	public class CreatedEntityEvent<TEntity> : EntityEvent<TEntity> where TEntity : Entity
	{
	public override string ToString() => $"{nameof(TEntity)} with id '{Id}' created.";
	}
	public class UpdatedEntityEvent<TEntity> : EntityEvent<TEntity> where TEntity : Entity
	{
	public override string ToString() => $"{nameof(TEntity)} with id '{Id}' updated.";
	}
	public class DeletedEntityEvent<TEntity> : EntityEvent<TEntity> where TEntity : Entity
	{
	public override string ToString() => $"{nameof(TEntity)} with id '{Id}' deleted.";
	}

view raw CRUDEntityEvents.cs hosted with ❤ by GitHub

	public abstract class EntityEvent : INotification
	{
	public Guid Id { get; set; }
	}
	public abstract class EntityEvent<T> : EntityEvent
	{
	}

view raw EntityEvent.cs hosted with ❤ by GitHub

	public class NotificationHandler<TEntity> : INotificationHandler<EntityEvent<TEntity>>
	{
	public Task Handle(EntityEvent<TEntity> notification, CancellationToken cancellationToken)
	{
	//Implementation removed
	}
	}

view raw NotificationHandler.cs hosted with ❤ by GitHub

However when we tried to register this using StructureMap nothing happened resulting in no event that was picked up by the NotificationHandler. Here is the registration code we had:

	var registry = new Registry();
	registry
	.For<IMediator>().Use<Mediator>();
	registry.For<SingleInstanceFactory>().Use<SingleInstanceFactory>(ctx => t => ctx.GetInstance(t));
	registry.For<MultiInstanceFactory>().Use<MultiInstanceFactory>(ctx => (t) => ctx.GetAllInstances(t));
	registry.Scan(scanner => {
	scanner.AssemblyContainingType(this.GetType());
	scanner.ConnectImplementationsToTypesClosing(typeof(IRequestHandler<>)); // Handlers with no response
	scanner.ConnectImplementationsToTypesClosing(typeof(IRequestHandler<,>)); // Handlers with a response
	scanner.ConnectImplementationsToTypesClosing(typeof(INotificationHandler<>));
	});

view raw StructureMapConfiguration.cs hosted with ❤ by GitHub

The problem is that it is one level of generic too deep to handle by StructureMap, so StructureMap has no clue on how to close this open generic. To solve this problem we can create our own registry convention:

	public class AddRequestHandlersWithGenericParametersToRegistry : IRegistrationConvention
	{
	public void ScanTypes(TypeSet types, Registry registry)
	{
	foreach (var concreteClass in types.FindTypes(TypeClassification.Concretes))
	{
	if (concreteClass.BaseType.Equals(typeof(Entity)))
	{
	RegisterType(registry, concreteClass, typeof(CreatedEntityEvent<>));
	RegisterType(registry, concreteClass, typeof(UpdatedEntityEvent<>));
	RegisterType(registry, concreteClass, typeof(DeletedEntityEvent<>));
	}
	}
	}
	private static void RegisterType(Registry registry, Type concreteClass, Type eventType)
	{
	var genericNotification = eventType.MakeGenericType(concreteClass);
	var interfaceHandlerType = typeof(INotificationHandler<>).MakeGenericType(genericNotification);
	var concreteHandlerType = typeof(NotificationHandler<>).MakeGenericType(concreteClass);
	registry.For(interfaceHandlerType).Use(concreteHandlerType);
	}
	}

view raw AddRequestHandlersWithGenericParametersToRegistry.cs hosted with ❤ by GitHub

As a last step we have to apply this convention when configuring StructureMap:

	registry.Scan(scanner => {
	scanner.AssemblyContainingType(this.GetType());
	scanner.ConnectImplementationsToTypesClosing(typeof(IRequestHandler<>)); // Handlers with no response
	scanner.ConnectImplementationsToTypesClosing(typeof(IRequestHandler<,>)); // Handlers with a response
	scanner.ConnectImplementationsToTypesClosing(typeof(INotificationHandler<>));
	scanner.With(new AddRequestHandlersWithGenericParametersToRegistry());
	});

view raw RegisterConvention.cs hosted with ❤ by GitHub