使用Autofac，如何解决服务集合并忽略无法解决的服务？

Question

Summary

I'm trying to see if there's a way I can ask Autofac to resolve all implementations of a particular type ( IEnumerable<TService> ), returning all implementations that can be successfully resolved while silently ignoring those that fail .

Details

In one of my libraries I define an ICommunicationService interface which I implement in various other places. I've created a "test bed" ASP.NET Core web service that I can use to pick up all implementations of this interface among its dependencies and allow me to test them.

When I try to access the selection page in the browser, the web service uses the following query to find all candidates:

var query = from service in container.Resolve<IEnumerable<ICommunicationService>>()
            let serviceType = service.GetType()
            let serviceAssembly = serviceType.GetTypeInfo().Assembly
            let assemblyName = serviceAssembly.GetName().Name
            let version = serviceAssembly.GetName().Version.ToString()
            select new { service, assemblyName, version };

However, recently I added a new kind of "restartable" communication service that doesn't handle any communication itself but delegates to other services. This makes it unsuitable as a candidate for testing. However, since it's a public concrete type, Autofac tries (and fails) to resolve it:

DependencyResolutionException: None of the constructors found with 'Autofac.Core.Activators.Reflection.DefaultConstructorFinder' on type 'MyCompany.Communications.UsbDevice.RestartableUsbCommunicationService' can be invoked with the available services and parameters: Cannot resolve parameter 'Int32 vendorId' of constructor 'Void .ctor(Int32, Int32, DelegateServiceFactory, LibUsbDotNet.DeviceNotify.IDeviceNotifier, Serilog.ILogger)'.

I make the assumption that any service that can be resolved is one I want to test (one directly involved with communication with a device), whereas ones that fail to resolve are types like this, that just wrap other services.

Answer 1

I've fixed this by writing the method below, which iterates over every registration that exposes the desired type of service and tries to resolve them in turn. The ones that resolve are returned and the ones that fail to are ignored.

private static IEnumerable<T> TryResolveCollection<T>(IComponentContext context)
{
    bool RegistrationProvidesService(IComponentRegistration registration) =>
        registration.Services.OfType<IServiceWithType>().Any(it => it.ServiceType.Name == typeof(T).Name);

    var registrations = context.ComponentRegistry.Registrations.Where(RegistrationProvidesService);
    var resolvedServices = new HashSet<Type>();

    foreach (var registration in registrations)
    {
        T service;
        try
        {
            service = (T)context.ResolveComponent(registration, Enumerable.Empty<Parameter>());
        }
        catch (DependencyResolutionException)
        {
            continue;
        }

        if (resolvedServices.Add(service.GetType()))
        {
            yield return service;
        }
    }
}

This could be turned into an extension method on IComponentContext pretty easily, but since I only need it in one place, I just defined it as a private method and use it like this:

var query = from service in TryResolveCollection<ICommunicationService>(container)
            let serviceType = service.GetType()
            let serviceAssembly = serviceType.GetTypeInfo().Assembly
            let assemblyName = serviceAssembly.GetName().Name
            let version = serviceAssembly.GetName().Version.ToString()
            select new { service, assemblyName, version };

What's interesting is that when I run this, it still manages to resolve a RestartableUsbCommunicationService , even though I have no idea what will happen if I try to run that (I don't know which underlying communication service it's wrapping).