set feature name in constructor

Author: zhiyuanliang-ms

src/Microsoft.FeatureManagement/FeatureManagementBuilderExtensions.cs

@@ -56,26 +56,22 @@ public static IFeatureManagementBuilder WithVariantService<TService>(this IFeatu
 
             if (builder.Services.Any(descriptor => descriptor.ServiceType == typeof(IVariantServiceProvider<TService>)))
             {
-                throw new InvalidOperationException($"Variant services of {typeof(TService)} has been added.");
+                throw new InvalidOperationException($"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>(
-                    sp.GetRequiredService<IEnumerable<TService>>(),
-                    sp.GetRequiredService<IVariantFeatureManager>())
-                {
-                    FeatureName = featureName,
-                });
+                    featureName,
+                    sp.GetRequiredService<IVariantFeatureManager>(),
+                    sp.GetRequiredService<IEnumerable<TService>>()));
             }
             else
             {
                 builder.Services.AddSingleton<IVariantServiceProvider<TService>>(sp => new VariantServiceProvider<TService>(
-                    sp.GetRequiredService<IEnumerable<TService>>(),
-                    sp.GetRequiredService<IVariantFeatureManager>())
-                {
-                    FeatureName = featureName,
-                });
+                    featureName,
+                    sp.GetRequiredService<IVariantFeatureManager>(),
+                    sp.GetRequiredService<IEnumerable<TService>>()));
             }
 
             return builder;

src/Microsoft.FeatureManagement/VariantServiceProvider.cs

@@ -24,30 +24,20 @@ internal class VariantServiceProvider<TService> : IVariantServiceProvider<TServi
         /// <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>
-        /// <param name="featureManager">Feature manager to get the assigned variant of the variant feature flag.</param>
-        /// <exception cref="ArgumentNullException">Thrown if <paramref name="services"/> is null.</exception>
+        /// <exception cref="ArgumentNullException">Thrown if <paramref name="featureName"/> is null.</exception>
         /// <exception cref="ArgumentNullException">Thrown if <paramref name="featureManager"/> is null.</exception>
-        public VariantServiceProvider(IEnumerable<TService> services, IVariantFeatureManager featureManager)
+        /// <exception cref="ArgumentNullException">Thrown if <paramref name="services"/> is null.</exception>
+        public VariantServiceProvider(string featureName, IVariantFeatureManager featureManager, IEnumerable<TService> services)
         {
-            _services = services ?? throw new ArgumentNullException(nameof(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>
-        /// The variant feature flag used to assign variants.
-        /// </summary>
-        public string FeatureName
-        {
-            get => _featureName;
-
-            init
-            {
-                _featureName = value ?? throw new ArgumentNullException(nameof(value));
-            }
-        }
-
         /// <summary>
         /// Gets implementation of TService according to the assigned variant from the feature flag.
         /// </summary>