Added option to throw when attempting to evaluate a missing feature. (#140)

* Added option to throw for missing features.

* Add default value documentation for feature management options.

Author: jimmyca15

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 that was requested for evaluation was not found.
+        /// </summary>
+        MissingFeature
     }
 }

src/Microsoft.FeatureManagement/FeatureManagementOptions.cs

@@ -11,7 +11,15 @@ public class FeatureManagementOptions
         /// <summary>
         /// Controls the behavior of feature evaluation when dependent feature filters are missing.
         /// 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.
+        /// The default value is false.
         /// </summary>
         public bool IgnoreMissingFeatureFilters { get; set; }
+
+        /// <summary>
+        /// Controls the behavior of feature evaluation when the target feature is missing.
+        /// If missing features are not ignored an exception will be thrown when attempting to evaluate them.
+        /// The default value is true.
+        /// </summary>
+        public bool IgnoreMissingFeatures { get; set; } = true;
     }
 }

src/Microsoft.FeatureManagement/FeatureManager.cs

@@ -141,6 +141,19 @@ private async Task<bool> IsEnabledAsync<TContext>(string feature, TContext appCo
                     }
                 }
             }
+            else
+            {
+                string errorMessage = $"The feature declaration for the feature '{feature}' was not found.";
+
+                if (!_options.IgnoreMissingFeatures)
+                {
+                    throw new FeatureManagementException(FeatureManagementError.MissingFeatureFilter, errorMessage);
+                }
+                else
+                {
+                    _logger.LogWarning(errorMessage);
+                }
+            }
 
             foreach (ISessionManager sessionManager in _sessionManagers)
             {

tests/Tests.FeatureManagement/FeatureManagement.cs

@@ -485,6 +485,31 @@ public async Task SwallowsExceptionForMissingFeatureFilter()
             Assert.False(isEnabled);
         }
 
+        [Fact]
+        public async Task ThrowsForMissingFeatures()
+        {
+            IConfiguration config = new ConfigurationBuilder().AddJsonFile("appsettings.json").Build();
+
+            var services = new ServiceCollection();
+
+            services
+                .Configure<FeatureManagementOptions>(options =>
+                {
+                    options.IgnoreMissingFeatures = false;
+                });
+
+            services
+                .AddSingleton(config)
+                .AddFeatureManagement();
+
+            ServiceProvider serviceProvider = services.BuildServiceProvider();
+
+            IFeatureManager featureManager = serviceProvider.GetRequiredService<IFeatureManager>();
+
+            FeatureManagementException fme = await Assert.ThrowsAsync<FeatureManagementException>(() =>
+                featureManager.IsEnabledAsync("NonExistentFeature"));
+        }
+
         [Fact]
         public async Task CustomFeatureDefinitionProvider()
         {