Inject existing object into MEF2 (#66364)

src/libraries/System.Composition.TypedParts/src/System.Composition.TypedParts.csproj

@@ -18,6 +18,8 @@ System.Composition.Hosting.ContainerConfiguration</PackageDescription>
     <Compile Include="System\Composition\Debugging\ContainerConfigurationDebuggerProxy.cs" />
     <Compile Include="System\Composition\Debugging\DiscoveredPartDebuggerProxy.cs" />
     <Compile Include="System\Composition\Hosting\ContainerConfiguration.cs" />
+    <Compile Include="System\Composition\Hosting\InstanceExportDescriptorProvider.cs" />
+    <Compile Include="System\Composition\Hosting\SinglePartExportDescriptorProvider.cs" />
     <Compile Include="System\Composition\TypedParts\ActivationFeatures\ActivationFeature.cs" />
     <Compile Include="System\Composition\TypedParts\ActivationFeatures\DisposalFeature.cs" />
     <Compile Include="System\Composition\TypedParts\ActivationFeatures\LifetimeFeature.cs" />

src/libraries/System.Composition.TypedParts/src/System/Composition/Hosting/ContainerConfiguration.cs

@@ -197,6 +197,54 @@ public ContainerConfiguration WithAssemblies(IEnumerable<Assembly> assemblies!!,
             return WithParts(assemblies.SelectMany(a => a.DefinedTypes.Select(dt => dt.AsType())), conventions);
         }
 
+        /// <summary>
+        /// Add a single instance to the container.
+        /// </summary>
+        /// <typeparam name="TExport">The type of the contract of the instance.</typeparam>
+        /// <param name="exportedInstance">The instance to add to the container.</param>
+        /// <returns>A configuration object allowing configuration to continue.</returns>
+        public ContainerConfiguration WithExport<TExport>(TExport exportedInstance!!)
+        {
+            return WithExport(exportedInstance, null, null);
+        }
+
+        /// <summary>
+        /// Add a single instance to the container.
+        /// </summary>
+        /// <typeparam name="TExport">The type of the contract of the instance.</typeparam>
+        /// <param name="exportedInstance">The instance to add to the container.</param>
+        /// <param name="contractName">Optionally, a name that discriminates this contract from others with the same type.</param>
+        /// <param name="metadata">Optionally, a non-empty collection of named constraints that apply to the contract.</param>
+        /// <returns>A configuration object allowing configuration to continue.</returns>
+        public ContainerConfiguration WithExport<TExport>(TExport exportedInstance!!, string contractName = null, IDictionary<string, object> metadata = null)
+        {
+            return WithExport(typeof(TExport), exportedInstance, contractName, metadata);
+        }
+
+        /// <summary>
+        /// Add a single instance to the container.
+        /// </summary>
+        /// <param name="contractType">The type of the contract of the instance.</param>
+        /// <param name="exportedInstance">The instance to add to the container.</param>
+        /// <returns>A configuration object allowing configuration to continue.</returns>
+        public ContainerConfiguration WithExport(Type contractType!!, object exportedInstance!!)
+        {
+            return WithExport(contractType, exportedInstance, null, null);
+        }
+
+        /// <summary>
+        /// Add a single instance to the container.
+        /// </summary>
+        /// <param name="contractType">The type of the contract of the instance.</param>
+        /// <param name="exportedInstance">The instance to add to the container.</param>
+        /// <param name="contractName">Optionally, a name that discriminates this contract from others with the same type.</param>
+        /// <param name="metadata">Optionally, a non-empty collection of named constraints that apply to the contract.</param>
+        /// <returns>A configuration object allowing configuration to continue.</returns>
+        public ContainerConfiguration WithExport(Type contractType!!, object exportedInstance!!, string contractName = null, IDictionary<string, object> metadata = null)
+        {
+            return WithProvider(new InstanceExportDescriptorProvider(exportedInstance, contractType, contractName, metadata));
+        }
+
         internal ExportDescriptorProvider[] DebugGetAddedExportDescriptorProviders()
         {
             return _addedSources.ToArray();

src/libraries/System.Composition.TypedParts/src/System/Composition/Hosting/InstanceExportDescriptorProvider.cs

@@ -0,0 +1,26 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Collections.Generic;
+using System.Composition.Hosting.Core;
+
+namespace System.Composition.Hosting
+{
+    internal class InstanceExportDescriptorProvider : SinglePartExportDescriptorProvider
+    {
+        private readonly object _exportedInstance;
+
+        public InstanceExportDescriptorProvider(object exportedInstance, Type contractType, string contractName, IDictionary<string, object> metadata)
+            : base(contractType, contractName, metadata)
+        {
+            _exportedInstance = exportedInstance;
+        }
+
+        public override IEnumerable<ExportDescriptorPromise> GetExportDescriptors(CompositionContract contract, DependencyAccessor descriptorAccessor)
+        {
+            if (IsSupportedContract(contract))
+                yield return new ExportDescriptorPromise(contract, _exportedInstance.ToString(), true, NoDependencies, _ =>
+                    ExportDescriptor.Create((c, o) => _exportedInstance, Metadata));
+        }
+    }
+}

src/libraries/System.Composition.TypedParts/src/System/Composition/Hosting/SinglePartExportDescriptorProvider.cs

@@ -0,0 +1,43 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Collections.Generic;
+using System.Composition.Hosting.Core;
+using System.Linq;
+
+namespace System.Composition.Hosting
+{
+    internal abstract class SinglePartExportDescriptorProvider : ExportDescriptorProvider
+    {
+        private readonly Type _contractType;
+        private readonly string _contractName;
+
+        protected SinglePartExportDescriptorProvider(Type contractType, string contractName, IDictionary<string, object> metadata)
+        {
+            _contractType = contractType;
+            _contractName = contractName;
+            Metadata = metadata ?? new Dictionary<string, object>();
+        }
+
+        protected bool IsSupportedContract(CompositionContract contract)
+        {
+            if (contract.ContractType != _contractType ||
+                contract.ContractName != _contractName)
+                return false;
+
+            if (contract.MetadataConstraints != null)
+            {
+                var subsetOfConstraints = contract.MetadataConstraints.Where(c => Metadata.ContainsKey(c.Key)).ToDictionary(c => c.Key, c => Metadata[c.Key]);
+                var constrainedSubset = new CompositionContract(contract.ContractType, contract.ContractName,
+                    subsetOfConstraints.Count == 0 ? null : subsetOfConstraints);
+
+                if (!contract.Equals(constrainedSubset))
+                    return false;
+            }
+
+            return true;
+        }
+
+        protected IDictionary<string, object> Metadata { get; }
+    }
+}

src/libraries/System.Composition.TypedParts/tests/ContainerConfigurationTests.cs

@@ -254,6 +254,42 @@ public void WithAssemby_Null_ThrowsNullReferenceExceptionOnCreation()
             Assert.Throws<NullReferenceException>(() => configuration.CreateContainer());
         }
 
+        [Fact]
+        public void WithExport_Base_Success()
+        {
+            var instance = new Base();
+
+            var configuration = new ContainerConfiguration();
+            Assert.Same(configuration, configuration.WithExport<Base>(instance));
+
+            CompositionHost container = configuration.CreateContainer();
+            Assert.Same(instance, container.GetExport<Base>());
+        }
+
+        [Fact]
+        public void WithExport_Derived_Success()
+        {
+            var instance = new Derived();
+
+            var configuration = new ContainerConfiguration();
+            Assert.Same(configuration, configuration.WithExport<Base>(instance));
+
+            CompositionHost container = configuration.CreateContainer();
+            Assert.Same(instance, container.GetExport<Base>());
+        }
+
+        [Fact]
+        public void WithExport_ContractName_Success()
+        {
+            var instance = new Base();
+
+            var configuration = new ContainerConfiguration();
+            Assert.Same(configuration, configuration.WithExport<Base>(instance, "Contract"));
+
+            CompositionHost container = configuration.CreateContainer();
+            Assert.Same(instance, container.GetExport<Base>("Contract"));
+        }
+
         [Fact]
         public void CreateContainer_ExportedSubClass_Success()
         {