Enable missing feature switch

Author: josephwoodward

src/Microsoft.FeatureManagement/FeatureManagementError.cs

@@ -16,6 +16,11 @@ public enum FeatureManagementError
         /// <summary>
         /// A feature filter configured for the feature being evaluated is an ambiguous reference to multiple registered feature filters.
         /// </summary>
-        AmbiguousFeatureFilter
+        AmbiguousFeatureFilter,
+
+        /// <summary>
+        /// A feature filter is missing configuration
+        /// </summary>
+        MissingFeature
     }
 }

src/Microsoft.FeatureManagement/FeatureManagementOptions.cs

@@ -13,5 +13,11 @@ public class FeatureManagementOptions
         /// If missing feature filters are not ignored an exception will be thrown when attempting to evaluate a feature that depends on a missing feature filter.
         /// </summary>
         public bool IgnoreMissingFeatureFilters { get; set; }
+
+        /// <summary>
+        /// Controls the behavior of feature evaluation when the dependent feature declaration is missing.
+        /// If missing feature declaration is not ignored an exception will be thrown when attempting to evaluate a feature that depends on a missing feature declaration.
+        /// </summary>
+        public bool IgnoreMissingFeatures { get; set; }
     }
 }

src/Microsoft.FeatureManagement/FeatureManager.cs

@@ -140,6 +140,9 @@ private async Task<bool> IsEnabledAsync<TContext>(string feature, TContext appCo
                         }
                     }
                 }
+            } else if (!_options.IgnoreMissingFeatures)
+            {
+                throw new FeatureManagementException(FeatureManagementError.MissingFeature, $"The feature declaration for specified feature '{feature}' was not found.");
             }
 
             foreach (ISessionManager sessionManager in _sessionManagers)

tests/Tests.FeatureManagement/FeatureManagement.cs

@@ -14,6 +14,7 @@
 using System.Linq;
 using System.Net;
 using System.Net.Http;
+using System.Reflection;
 using System.Threading.Tasks;
 using Xunit;
 
@@ -25,6 +26,7 @@ public class FeatureManagement
         private const string OffFeature = "OffFeature";
         private const string ConditionalFeature = "ConditionalFeature";
         private const string ContextualFeature = "ContextualFeature";
+        private const string MissingFeature = "MissingFeature";
 
         [Fact]
         public async Task ReadsConfiguration()
@@ -33,6 +35,12 @@ public async Task ReadsConfiguration()
 
             var services = new ServiceCollection();
 
+            services
+                .Configure<FeatureManagementOptions>(options =>
+                {
+                    options.IgnoreMissingFeatures = true;
+                });
+            
             services
                 .AddSingleton(config)
                 .AddFeatureManagement()
@@ -483,5 +491,59 @@ public async Task SwallowsExceptionForMissingFeatureFilter()
 
             Assert.False(isEnabled);
         }
+
+        [Fact]
+        public async Task ThrowsOnMissingFeature()
+        {
+            IConfiguration config = new ConfigurationBuilder().AddJsonFile("appsettings.json").Build();
+
+            var services = new ServiceCollection();
+
+            services
+                .Configure<FeatureManagementOptions>(options =>
+                {
+                    options.IgnoreMissingFeatures = false;
+                });
+
+            services
+                .AddSingleton(config)
+                .AddFeatureManagement()
+                .AddFeatureFilter<TestFilter>();
+
+            ServiceProvider serviceProvider = services.BuildServiceProvider();
+
+            IFeatureManager featureManager = serviceProvider.GetRequiredService<IFeatureManager>();
+
+            FeatureManagementException e = await Assert.ThrowsAsync<FeatureManagementException>(async () => await featureManager.IsEnabledAsync(MissingFeature));
+
+            Assert.Equal(FeatureManagementError.MissingFeature, e.Error);
+        }
+
+        [Fact]
+        public async Task SwallowsOnMissingFeature()
+        {
+            IConfiguration config = new ConfigurationBuilder().AddJsonFile("appsettings.json").Build();
+
+            var services = new ServiceCollection();
+
+            services
+                .Configure<FeatureManagementOptions>(options =>
+                {
+                    options.IgnoreMissingFeatures = true;
+                });
+
+            services
+                .AddSingleton(config)
+                .AddFeatureManagement()
+                .AddFeatureFilter<TestFilter>();
+
+            ServiceProvider serviceProvider = services.BuildServiceProvider();
+
+            IFeatureManager featureManager = serviceProvider.GetRequiredService<IFeatureManager>();
+
+            var isEnabled = await featureManager.IsEnabledAsync(ConditionalFeature);
+
+            Assert.False(isEnabled);
+        } 
     }
 }