InstanceDecorator.cs

// This software is part of the Autofac IoC container
// Copyright © 2018 Autofac Contributors
// https://autofac.org
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using System.Collections.Generic;
using System.Linq;
using Autofac.Core;
using Autofac.Core.Registration;

namespace Autofac.Features.Decorators
{
    internal static class InstanceDecorator
    {
        internal static object TryDecorateRegistration(
            Service service,
            IComponentRegistration registration,
            object instance,
            IComponentContext context,
            IEnumerable<Parameter> parameters)
        {
            if (service is DecoratorService
                || !(service is IServiceWithType serviceWithType)
                || registration is ExternalComponentRegistration) return instance;

            var decoratorRegistrations = context.ComponentRegistry.DecoratorsFor(serviceWithType);
            if (decoratorRegistrations.Count == 0) return instance;

            var serviceType = serviceWithType.ServiceType;
            var resolveParameters = parameters as Parameter[] ?? parameters.ToArray();

            var instanceType = instance.GetType();
            var decoratorContext = DecoratorContext.Create(instanceType, serviceType, instance);
            var decoratorCount = decoratorRegistrations.Count;

            for (var index = 0; index < decoratorCount; index++)
            {
                var decoratorRegistration = decoratorRegistrations[index];
                var decoratorService = decoratorRegistration.Services.OfType<DecoratorService>().First();
                if (!decoratorService.Condition(decoratorContext)) continue;

                var serviceParameter = new TypedParameter(serviceType, instance);
                var contextParameter = new TypedParameter(typeof(IDecoratorContext), decoratorContext);

                var invokeParameters = new Parameter[resolveParameters.Length + 2];
                for (var i = 0; i < resolveParameters.Length; i++)
                    invokeParameters[i] = resolveParameters[i];

                invokeParameters[invokeParameters.Length - 2] = serviceParameter;
                invokeParameters[invokeParameters.Length - 1] = contextParameter;

                var resolveRequest = new ResolveRequest(decoratorService, decoratorRegistration, invokeParameters, registration);
                instance = context.ResolveComponent(resolveRequest);

                if (index < decoratorCount - 1)
                    decoratorContext = decoratorContext.UpdateContext(instance);
            }

            return instance;
        }
    }
}