LazyWithMetadataRegistrationSource.cs

// This software is part of the Autofac IoC container
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// http://autofac.org
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using System;
using System.Collections.Generic;
using System.Linq;
using System.Reflection;
using Autofac.Builder;
using Autofac.Core;
using Autofac.Features.Metadata;
using Autofac.Util;

namespace Autofac.Features.LazyDependencies
{
    /// <summary>
    /// Support the <c>System.Lazy&lt;T, TMetadata&gt;</c>
    /// types automatically whenever type T is registered with the container.
    /// Metadata values come from the component registration's metadata.
    /// When a dependency of a lazy type is used, the instantiation of the underlying
    /// component will be delayed until the Value property is first accessed.
    /// </summary>
    internal class LazyWithMetadataRegistrationSource : IRegistrationSource
    {
        private static readonly MethodInfo CreateLazyRegistrationMethod = typeof(LazyWithMetadataRegistrationSource).GetTypeInfo().GetDeclaredMethod("CreateLazyRegistration");

        private delegate IComponentRegistration RegistrationCreator(Service service, IComponentRegistration valueRegistration);

        public IEnumerable<IComponentRegistration> RegistrationsFor(Service service, Func<Service, IEnumerable<IComponentRegistration>> registrationAccessor)
        {
            if (registrationAccessor == null)
                throw new ArgumentNullException(nameof(registrationAccessor));

            var swt = service as IServiceWithType;
#if NET45
            var lazyType = GetLazyType(swt);
            if (swt == null || lazyType == null || !swt.ServiceType.IsGenericTypeDefinedBy(lazyType))
                return Enumerable.Empty<IComponentRegistration>();
#else
            var lazyType = typeof(Lazy<,>);
            if (swt == null || !swt.ServiceType.IsGenericTypeDefinedBy(lazyType))
                return Enumerable.Empty<IComponentRegistration>();
#endif

            var genericTypeArguments = swt.ServiceType.GetTypeInfo().GenericTypeArguments.ToArray();
            var valueType = genericTypeArguments[0];
            var metaType = genericTypeArguments[1];

            if (!metaType.GetTypeInfo().IsClass)
                return Enumerable.Empty<IComponentRegistration>();

            var valueService = swt.ChangeType(valueType);

            var registrationCreator = (RegistrationCreator)CreateLazyRegistrationMethod.MakeGenericMethod(valueType, metaType).CreateDelegate(typeof(RegistrationCreator));

            return registrationAccessor(valueService)
                .Select(v => registrationCreator(service, v));
        }

        public bool IsAdapterForIndividualComponents => true;

        public override string ToString()
        {
            return LazyWithMetadataRegistrationSourceResources.LazyWithMetadataRegistrationSourceDescription;
        }

        private static IComponentRegistration CreateLazyRegistration<T, TMeta>(Service providedService, IComponentRegistration valueRegistration)
        {
            var metadataProvider = MetadataViewProvider.GetMetadataViewProvider<TMeta>();
            var metadata = metadataProvider(valueRegistration.Target.Metadata);

            var rb = RegistrationBuilder.ForDelegate(
                (c, p) =>
                {
                    var context = c.Resolve<IComponentContext>();
                    var lazyType = ((IServiceWithType)providedService).ServiceType;
                    var valueFactory = new Func<T>(() => (T)context.ResolveComponent(valueRegistration, p));
                    return Activator.CreateInstance(lazyType, valueFactory, metadata);
                })
                .As(providedService)
                .Targeting(valueRegistration);

            return rb.CreateRegistration();
        }

#if NET45
        private static Type GetLazyType(IServiceWithType serviceWithType)
        {
            return serviceWithType != null
                   && serviceWithType.ServiceType.GetTypeInfo().IsGenericType
                   && serviceWithType.ServiceType.GetGenericTypeDefinition().FullName == "System.Lazy`2"
                       ? serviceWithType.ServiceType.GetGenericTypeDefinition()
                       : null;
        }
#endif
    }
}