Single Registration Protections

test.cs

using Autofac;
using Autofac.Core;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Net.Http;
using Xunit;

namespace PreventDuplicateSingletons
{
    public class SingleRegistrationServices
    {
        private static readonly HashSet<Type> servicesThatOnlyAllowOneRegistration = new HashSet<Type>
        {
            typeof(HttpClient)
        };

        private readonly HashSet<Type> servicesAlreadyRegistered = new HashSet<Type>();

        public void ServiceRegistered(object sender, ComponentRegisteredEventArgs e)
        {
            // Get the typed services in the registration.
            var typedServices = e.ComponentRegistration.Services.OfType<TypedService>();

            foreach (var service in typedServices)
            {
                // Is this service type in our list of 'protected' types?
                if(servicesThatOnlyAllowOneRegistration.Contains(service.ServiceType))
                {
                    // If we cannot add it to the set we've already registered,
                    // it means we have already seen it.
                    if(!servicesAlreadyRegistered.Add(service.ServiceType))
                    {
                        throw new InvalidOperationException($"Detected invalid double-registration of {service.ServiceType.Name}");
                    }
                }
            }
        }
    }

    public class PreventDuplicateServicesTests
    {

        [Fact]
        public void ThrowsOnDuplicateService()
        {
            var builder = new ContainerBuilder();
            var singleAssertions = new SingleRegistrationServices();
            builder.RegisterCallback(regBuilder => regBuilder.Registered += singleAssertions.ServiceRegistered);

            var httpClient1 = new HttpClient();
            var httpClient2 = new HttpClient();

            builder.RegisterInstance(httpClient1);
            builder.RegisterInstance(httpClient2);

            Assert.Throws<InvalidOperationException>(() =>
            {
                builder.Build();
            });
        }
    }
}