Feature-based Injection

src/Microsoft.FeatureManagement/FeatureManagementBuilderExtensions.cs

@@ -39,6 +39,44 @@ public static IFeatureManagementBuilder WithTargeting<T>(this IFeatureManagement
             return builder;
         }
 
+        /// <summary>
+        /// Adds a <see cref="VariantServiceProvider{TService}"/> to the feature management system.
+        /// </summary>
+        /// <param name="builder">The <see cref="IFeatureManagementBuilder"/> used to customize feature management functionality.</param>
+        /// <param name="featureName">The feature flag that should be used to determine which variant of the service should be used. The <see cref="VariantServiceProvider{TService}"/> will return different implementations of TService according to the assigned variant.</param>
+        /// <returns>A <see cref="IFeatureManagementBuilder"/> that can be used to customize feature management functionality.</returns>
+        /// <exception cref="ArgumentNullException">Thrown if feature name parameter is null.</exception>
+        /// <exception cref="InvalidOperationException">Thrown if a variant service of the type has already been added.</exception>
+        public static IFeatureManagementBuilder WithVariantService<TService>(this IFeatureManagementBuilder builder, string featureName) where TService : class
+        {
+            if (string.IsNullOrEmpty(featureName))
+            {
+                throw new ArgumentNullException(nameof(featureName));
+            }
+
+            if (builder.Services.Any(descriptor => descriptor.ServiceType == typeof(IVariantServiceProvider<TService>)))
+            {
+                throw new InvalidOperationException($"A variant service of {typeof(TService).FullName} has already been added.");
+            }
+
+            if (builder.Services.Any(descriptor => descriptor.ServiceType == typeof(IFeatureManager) && descriptor.Lifetime == ServiceLifetime.Scoped))
+            {
+                builder.Services.AddScoped<IVariantServiceProvider<TService>>(sp => new VariantServiceProvider<TService>(
+                    featureName,
+                    sp.GetRequiredService<IVariantFeatureManager>(),
+                    sp.GetRequiredService<IEnumerable<TService>>()));
+            }
+            else
+            {
+                builder.Services.AddSingleton<IVariantServiceProvider<TService>>(sp => new VariantServiceProvider<TService>(
+                    featureName,
+                    sp.GetRequiredService<IVariantFeatureManager>(),
+                    sp.GetRequiredService<IEnumerable<TService>>()));
+            }
+
+            return builder;
+        }
+
         /// <summary>
         /// Adds a telemetry publisher to the feature management system.
         /// </summary>

src/Microsoft.FeatureManagement/IVariantServiceProvider.cs

@@ -0,0 +1,21 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT license.
+//
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace Microsoft.FeatureManagement
+{
+    /// <summary>
+    /// Used to get different implementation variants of TService.
+    /// </summary>
+    public interface IVariantServiceProvider<TService> where TService : class
+    {
+        /// <summary>
+        /// Gets an implementation variant of TService.
+        /// </summary>
+        /// <param name="cancellationToken">The cancellation token to cancel the operation.</param>
+        /// <returns>An implementation of TService.</returns>
+        ValueTask<TService> GetServiceAsync(CancellationToken cancellationToken);
+    }
+}

src/Microsoft.FeatureManagement/VariantServiceAliasAttribute.cs

@@ -0,0 +1,32 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT license.
+//
+using System;
+
+namespace Microsoft.FeatureManagement
+{
+    /// <summary>
+    /// Allows the name of a variant service to be customized to relate to the variant name specified in configuration.
+    /// </summary>
+    public class VariantServiceAliasAttribute : Attribute
+    {
+        /// <summary>
+        /// Creates a variant service alias using the provided alias.
+        /// </summary>
+        /// <param name="alias">The alias of the variant service.</param>
+        public VariantServiceAliasAttribute(string alias)
+        {
+            if (string.IsNullOrEmpty(alias))
+            {
+                throw new ArgumentNullException(nameof(alias));
+            }
+
+            Alias = alias;
+        }
+
+        /// <summary>
+        /// The name that will be used to match variant name specified in the configuration.
+        /// </summary>
+        public string Alias { get; }
+    }
+}

src/Microsoft.FeatureManagement/VariantServiceProvider.cs

@@ -0,0 +1,80 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT license.
+//
+using System;
+using System.Collections.Concurrent;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.Linq;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace Microsoft.FeatureManagement
+{
+    /// <summary>
+    /// Used to get different implementations of TService depending on the assigned variant from a specific variant feature flag.
+    /// </summary>
+    internal class VariantServiceProvider<TService> : IVariantServiceProvider<TService> where TService : class
+    {
+        private readonly IEnumerable<TService> _services;
+        private readonly IVariantFeatureManager _featureManager;
+        private readonly string _featureName;
+        private readonly ConcurrentDictionary<string, TService> _variantServiceCache;
+
+        /// <summary>
+        /// Creates a variant service provider.
+        /// </summary>
+        /// <param name="featureName">The feature flag that should be used to determine which variant of the service should be used.</param>
+        /// <param name="featureManager">The feature manager to get the assigned variant of the feature flag.</param>
+        /// <param name="services">Implementation variants of TService.</param>
+        /// <exception cref="ArgumentNullException">Thrown if <paramref name="featureName"/> is null.</exception>
+        /// <exception cref="ArgumentNullException">Thrown if <paramref name="featureManager"/> is null.</exception>
+        /// <exception cref="ArgumentNullException">Thrown if <paramref name="services"/> is null.</exception>
+        public VariantServiceProvider(string featureName, IVariantFeatureManager featureManager, IEnumerable<TService> services)
+        {
+            _featureName = featureName ?? throw new ArgumentNullException(nameof(featureName));
+            _featureManager = featureManager ?? throw new ArgumentNullException(nameof(featureManager));
+            _services = services ?? throw new ArgumentNullException(nameof(services));
+            _variantServiceCache = new ConcurrentDictionary<string, TService>();
+        }
+
+        /// <summary>
+        /// Gets implementation of TService according to the assigned variant from the feature flag.
+        /// </summary>
+        /// <param name="cancellationToken">The cancellation token to cancel the operation.</param>
+        /// <returns>An implementation matched with the assigned variant. If there is no matched implementation, it will return null.</returns>
+        public async ValueTask<TService> GetServiceAsync(CancellationToken cancellationToken)
+        {
+            Debug.Assert(_featureName != null);
+
+            Variant variant = await _featureManager.GetVariantAsync(_featureName, cancellationToken);
+
+            TService implementation = null;
+
+            if (variant != null)
+            {
+                implementation = _variantServiceCache.GetOrAdd(
+                    variant.Name,
+                    (_) => _services.FirstOrDefault(
+                        service => IsMatchingVariantName(
+                            service.GetType(),
+                            variant.Name))
+                );
+            }
+
+            return implementation;
+        }
+
+        private bool IsMatchingVariantName(Type implementationType, string variantName)
+        {
+            string implementationName = ((VariantServiceAliasAttribute)Attribute.GetCustomAttribute(implementationType, typeof(VariantServiceAliasAttribute)))?.Alias;
+
+            if (implementationName == null)
+            {
+                implementationName = implementationType.Name;
+            }
+
+            return string.Equals(implementationName, variantName, StringComparison.OrdinalIgnoreCase);
+        }
+    }
+}

tests/Tests.FeatureManagement/FeatureManagement.cs

@@ -1459,5 +1459,82 @@ public async Task VariantsInvalidScenarios()
             Assert.Equal(FeatureManagementError.InvalidConfigurationSetting, e.Error);
             Assert.Contains(ConfigurationFields.PercentileAllocationFrom, e.Message);
         }
+
+        [Fact]
+        public async Task VariantBasedInjection()
+        {
+            IConfiguration configuration = new ConfigurationBuilder()
+                .AddJsonFile("appsettings.json")
+                .Build();
+
+            IServiceCollection services = new ServiceCollection();
+
+            services.AddSingleton<IAlgorithm, AlgorithmBeta>();
+            services.AddSingleton<IAlgorithm, AlgorithmSigma>();
+            services.AddSingleton<IAlgorithm>(sp => new AlgorithmOmega("OMEGA"));
+
+            services.AddSingleton(configuration)
+                .AddFeatureManagement()
+                .AddFeatureFilter<TargetingFilter>()
+                .WithVariantService<IAlgorithm>(Features.VariantImplementationFeature);
+
+            var targetingContextAccessor = new OnDemandTargetingContextAccessor();
+
+            services.AddSingleton<ITargetingContextAccessor>(targetingContextAccessor);
+
+            ServiceProvider serviceProvider = services.BuildServiceProvider();
+
+            IVariantFeatureManager featureManager = serviceProvider.GetRequiredService<IVariantFeatureManager>();
+
+            IVariantServiceProvider<IAlgorithm> featuredAlgorithm = serviceProvider.GetRequiredService<IVariantServiceProvider<IAlgorithm>>();
+
+            targetingContextAccessor.Current = new TargetingContext
+            {
+                UserId = "Guest"
+            };
+
+            IAlgorithm algorithm = await featuredAlgorithm.GetServiceAsync(CancellationToken.None);
+
+            Assert.Null(algorithm);
+
+            targetingContextAccessor.Current = new TargetingContext
+            {
+                UserId = "UserSigma"
+            };
+
+            algorithm = await featuredAlgorithm.GetServiceAsync(CancellationToken.None);
+
+            Assert.Null(algorithm);
+
+            targetingContextAccessor.Current = new TargetingContext
+            {
+                UserId = "UserBeta"
+            };
+
+            algorithm = await featuredAlgorithm.GetServiceAsync(CancellationToken.None);
+
+            Assert.NotNull(algorithm);
+            Assert.Equal("Beta", algorithm.Style);
+
+            targetingContextAccessor.Current = new TargetingContext
+            {
+                UserId = "UserOmega"
+            };
+
+            algorithm = await featuredAlgorithm.GetServiceAsync(CancellationToken.None);
+
+            Assert.NotNull(algorithm);
+            Assert.Equal("OMEGA", algorithm.Style);
+
+            services = new ServiceCollection();
+
+            Assert.Throws<InvalidOperationException>(() =>
+                {
+                    services.AddFeatureManagement()
+                        .WithVariantService<IAlgorithm>("DummyFeature1")
+                        .WithVariantService<IAlgorithm>("DummyFeature2");
+                }
+            );
+        }
     }
 }

tests/Tests.FeatureManagement/Features.cs

@@ -30,5 +30,6 @@ static class Features
         public const string VariantFeatureBothConfigurations = "VariantFeatureBothConfigurations";
         public const string VariantFeatureInvalidStatusOverride = "VariantFeatureInvalidStatusOverride";
         public const string VariantFeatureInvalidFromTo = "VariantFeatureInvalidFromTo";
+        public const string VariantImplementationFeature = "VariantImplementationFeature";
     }
 }

tests/Tests.FeatureManagement/VariantServices.cs

@@ -0,0 +1,40 @@
+using Microsoft.FeatureManagement;
+
+namespace Tests.FeatureManagement
+{
+    interface IAlgorithm
+    {
+        public string Style { get; }
+    }
+
+    class AlgorithmBeta : IAlgorithm
+    {
+        public string Style { get; set; }
+
+        public AlgorithmBeta()
+        {
+            Style = "Beta";
+        }
+    }
+
+    class AlgorithmSigma : IAlgorithm
+    {
+        public string Style { get; set; }
+
+        public AlgorithmSigma()
+        {
+            Style = "Sigma";
+        }
+    }
+
+    [VariantServiceAlias("Omega")]
+    class AlgorithmOmega : IAlgorithm
+    {
+        public string Style { get; set; }
+
+        public AlgorithmOmega(string style)
+        {
+            Style = style;
+        }
+    }
+}

tests/Tests.FeatureManagement/appsettings.json

@@ -493,6 +493,54 @@
                     "Name": "On"
                 }
             ]
+        },
+        "VariantImplementationFeature": {
+            "EnabledFor": [
+                {
+                    "Name": "Targeting",
+                    "Parameters": {
+                        "Audience": {
+                            "Users": [
+                                "UserOmega", "UserSigma", "UserBeta"
+                            ]
+                        }
+                    }
+                }
+            ],
+            "Variants": [
+                {
+                    "Name": "AlgorithmBeta"
+                },
+                {
+                    "Name": "Sigma",
+                    "ConfigurationValue": "AlgorithmSigma"
+                },
+                {
+                    "Name": "Omega"
+                }
+            ],
+            "Allocation": {
+                "User": [
+                    {
+                        "Variant": "AlgorithmBeta",
+                        "Users": [
+                            "UserBeta"
+                        ]
+                    },
+                    {
+                        "Variant": "Omega",
+                        "Users": [
+                            "UserOmega"
+                        ]
+                    },
+                    {
+                        "Variant": "Sigma",
+                        "Users": [
+                            "UserSigma"
+                        ]
+                    }
+                ]
+            }
         }
     }
 }