Support for the composite pattern.

src/Autofac/Builder/RegistrationBuilder.cs

@@ -224,7 +224,8 @@ public static class RegistrationBuilder
                     data.Ownership,
                     clonedPipelineBuilder,
                     services,
-                    data.Metadata);
+                    data.Metadata,
+                    data.IsServiceOverride);
             }
             else
             {

src/Autofac/Builder/RegistrationData.cs

@@ -137,6 +137,14 @@ public IComponentLifetime Lifetime
         /// </summary>
         public IDictionary<string, object?> Metadata { get; }
 
+        /// <summary>
+        /// Gets or sets a value indicating whether this registration forms a 'Service Override', which forces itself as the default
+        /// registration for all service it provides.
+        /// Registrations with <see cref="IsServiceOverride"/> set to true will not be included when resolving a collection of a service
+        /// to get all implementations.
+        /// </summary>
+        public bool IsServiceOverride { get; set; }
+
         /// <summary>
         /// Gets or sets the callback used to register this component.
         /// </summary>

src/Autofac/Core/IComponentRegistration.cs

@@ -88,6 +88,14 @@ public interface IComponentRegistration : IDisposable
         /// </summary>
         bool IsAdapterForIndividualComponent { get; }
 
+        /// <summary>
+        /// Gets a value indicating whether this registration forms a 'Service Override', which forces itself as the default
+        /// registration for all service it provides.
+        /// Registrations with <see cref="IsServiceOverride"/> set to true will not be included when resolving a collection of a service
+        /// to get all implementations.
+        /// </summary>
+        bool IsServiceOverride { get; }
+
         /// <summary>
         /// Provides an event that will be invoked just before a pipeline is built, and can be used to add additional middleware
         /// at that point.

src/Autofac/Core/Registration/ComponentRegistration.cs

@@ -80,15 +80,17 @@ public class ComponentRegistration : Disposable, IComponentRegistration
         /// <param name="ownership">Whether the component instances are disposed at the end of their lifetimes.</param>
         /// <param name="services">Services the component provides.</param>
         /// <param name="metadata">Data associated with the component.</param>
+        /// <param name="isServiceOverride">Indicates whether this registration is a service override.</param>
         public ComponentRegistration(
            Guid id,
            IInstanceActivator activator,
            IComponentLifetime lifetime,
            InstanceSharing sharing,
            InstanceOwnership ownership,
            IEnumerable<Service> services,
-           IDictionary<string, object?> metadata)
-            : this(id, activator, lifetime, sharing, ownership, new ResolvePipelineBuilder(PipelineType.Registration), services, metadata)
+           IDictionary<string, object?> metadata,
+           bool isServiceOverride = false)
+            : this(id, activator, lifetime, sharing, ownership, new ResolvePipelineBuilder(PipelineType.Registration), services, metadata, isServiceOverride)
         {
         }
 
@@ -103,6 +105,7 @@ public class ComponentRegistration : Disposable, IComponentRegistration
         /// <param name="pipelineBuilder">The resolve pipeline builder for the registration.</param>
         /// <param name="services">Services the component provides.</param>
         /// <param name="metadata">Data associated with the component.</param>
+        /// <param name="isServiceOverride">Indicates whether this registration is a service override.</param>
         public ComponentRegistration(
             Guid id,
             IInstanceActivator activator,
@@ -111,7 +114,8 @@ public class ComponentRegistration : Disposable, IComponentRegistration
             InstanceOwnership ownership,
             IResolvePipelineBuilder pipelineBuilder,
             IEnumerable<Service> services,
-            IDictionary<string, object?> metadata)
+            IDictionary<string, object?> metadata,
+            bool isServiceOverride = false)
         {
             if (activator == null) throw new ArgumentNullException(nameof(activator));
             if (lifetime == null) throw new ArgumentNullException(nameof(lifetime));
@@ -129,6 +133,7 @@ public class ComponentRegistration : Disposable, IComponentRegistration
             Services = Enforce.ArgumentElementNotNull(services, nameof(services));
             Metadata = metadata;
             IsAdapterForIndividualComponent = false;
+            IsServiceOverride = isServiceOverride;
         }
 
         /// <summary>
@@ -160,6 +165,7 @@ public class ComponentRegistration : Disposable, IComponentRegistration
             if (target == null) throw new ArgumentNullException(nameof(target));
             _target = target;
             IsAdapterForIndividualComponent = isAdapterForIndividualComponents;
+            IsServiceOverride = _target.IsServiceOverride;
         }
 
         /// <summary>
@@ -204,6 +210,9 @@ public class ComponentRegistration : Disposable, IComponentRegistration
         /// </summary>
         public IDictionary<string, object?> Metadata { get; }
 
+        /// <inheritdoc/>
+        public bool IsServiceOverride { get; }
+
         /// <inheritdoc />
         public event EventHandler<IResolvePipelineBuilder>? PipelineBuilding;
 

src/Autofac/Core/Registration/ComponentRegistrationLifetimeDecorator.cs

@@ -80,6 +80,9 @@ public ComponentRegistrationLifetimeDecorator(IComponentRegistration inner, ICom
         /// <inheritdoc/>
         public IResolvePipeline ResolvePipeline => _inner.ResolvePipeline;
 
+        /// <inheritdoc/>
+        public bool IsServiceOverride => _inner.IsServiceOverride;
+
         /// <inheritdoc/>
         public event EventHandler<IResolvePipelineBuilder> PipelineBuilding
         {

src/Autofac/Core/Registration/DefaultRegisteredServicesTracker.cs

@@ -183,13 +183,7 @@ public bool TryGetServiceRegistration(Service service, out ServiceRegistration s
             {
                 var info = GetInitializedServiceInfo(service);
 
-                // There is a 'virtual' registration; use it.
-                if (info.RedirectionTargetRegistration is object)
-                {
-                    serviceData = new ServiceRegistration(info.ServicePipeline, info.RedirectionTargetRegistration);
-                    return true;
-                }
-                else if (info.TryGetRegistration(out var registration))
+                if (info.TryGetRegistration(out var registration))
                 {
                     serviceData = new ServiceRegistration(info.ServicePipeline, registration);
                     return true;

src/Autofac/Core/Registration/ServiceRegistrationInfo.cs

@@ -45,6 +45,8 @@ internal class ServiceRegistrationInfo : IResolvePipelineBuilder
         [SuppressMessage("Microsoft.Performance", "CA1823:AvoidUnusedPrivateFields", Justification = "The _service field is useful in debugging and diagnostics.")]
         private readonly Service _service;
 
+        private IComponentRegistration? _serviceOverride = null;
+
         /// <summary>
         ///  List of implicit default service implementations. Overriding default implementations are appended to the end,
         ///  so the enumeration should begin from the end too, and the most default implementation comes last.
@@ -111,7 +113,15 @@ public IEnumerable<IComponentRegistration> Implementations
             {
                 RequiresInitialization();
 
-                return _registeredImplementations!.Value;
+                if (_serviceOverride is object)
+                {
+                    yield return _serviceOverride;
+                }
+
+                foreach (var item in _registeredImplementations!.Value)
+                {
+                    yield return item;
+                }
             }
         }
 
@@ -170,7 +180,11 @@ public bool IsRegistered
         /// <param name="originatedFromSource">Whether the registration originated from a dynamic source.</param>
         public void AddImplementation(IComponentRegistration registration, bool preserveDefaults, bool originatedFromSource)
         {
-            if (preserveDefaults)
+            if (registration.IsServiceOverride)
+            {
+                _serviceOverride = registration;
+            }
+            else if (preserveDefaults)
             {
                 if (originatedFromSource)
                 {
@@ -218,23 +232,6 @@ public void UseServiceMiddleware(IResolveMiddleware middleware, MiddlewareInsert
             }
 
             _customPipelineBuilder.Use(middleware, insertionMode);
-
-            if (middleware is IRedirectingMiddleware redirectMiddleware)
-            {
-                var target = redirectMiddleware.TargetRegistration;
-
-                if (target is null)
-                {
-                    throw new InvalidOperationException(
-                        string.Format(
-                            CultureInfo.CurrentCulture,
-                            ServiceRegistrationInfoResources.RedirectingMiddlewareHasNullRegistration,
-                            redirectMiddleware.ToString()));
-                }
-
-                // This middleware provides a redirect registration for the registration. Store it.
-                RedirectionTargetRegistration = redirectMiddleware.TargetRegistration;
-            }
         }
 
         /// <summary>
@@ -266,7 +263,8 @@ public bool TryGetRegistration([NotNullWhen(returnValue: true)] out IComponentRe
         {
             RequiresInitialization();
 
-            registration = _defaultImplementation ??= _defaultImplementations.LastOrDefault() ??
+            registration = _defaultImplementation ??= _serviceOverride ??
+                                                      _defaultImplementations.LastOrDefault() ??
                                                       _sourceImplementations?.First() ??
                                                       _preserveDefaultImplementations?.First();
 

src/Autofac/Core/Registration/ServiceRegistrationInfoResources.resx

@@ -123,9 +123,6 @@
   <data name="NotInitialized" xml:space="preserve">
     <value>The operation is not valid until the object is initialized.</value>
   </data>
-  <data name="RedirectingMiddlewareHasNullRegistration" xml:space="preserve">
-    <value>Service middleware '{0}' implements IRedirectingMiddleware but returns a null value for TargetRegistration.</value>
-  </data>
   <data name="ServicePipelineCannotBeBuilt" xml:space="preserve">
     <value>The service pipeline cannot be built until the service has finished initialization.</value>
   </data>

src/Autofac/Core/ServiceRegistration.cs

@@ -56,6 +56,12 @@ public ServiceRegistration(IResolvePipeline servicePipeline, IComponentRegistrat
         /// </summary>
         public IComponentRegistration Registration { get; }
 
+        /// <summary>
+        /// Gets a value indicating whether this registration is a service override registrations.
+        /// Service override registrations should generally be skipped when resolving a collection of any type of service.
+        /// </summary>
+        public bool IsServiceOverride => Registration.IsServiceOverride;
+
         /// <summary>
         /// Gets additional data associated with the component.
         /// </summary>

src/Autofac/Features/Collections/CollectionRegistrationSource.cs

@@ -122,6 +122,7 @@ public IEnumerable<IComponentRegistration> RegistrationsFor(Service service, Fun
                 {
                     var itemRegistrations = c.ComponentRegistry
                         .ServiceRegistrationsFor(elementTypeService)
+                        .Where(cr => !cr.IsServiceOverride)
                         .OrderBy(cr => cr.Registration.GetRegistrationOrder())
                         .ToList();
 

src/Autofac/Features/Decorators/DecoratorMiddleware.cs

@@ -64,6 +64,12 @@ public void Execute(ResolveRequestContextBase context, Action<ResolveRequestCont
                 return;
             }
 
+            if (context.Registration.IsServiceOverride)
+            {
+                // We don't decorate service overrides.
+                return;
+            }
+
             // This middleware is only ever added to IServiceWithType pipelines, so this cast will always succeed.
             var swt = (IServiceWithType)context.Service;