Throw an exception if BeginScope returns null instead of falling back…

… to defaults silently

src/System.Web.Http/Dispatcher/DefaultHttpControllerActivator.cs

@@ -4,7 +4,6 @@
 using System.Net.Http;
 using System.Threading;
 using System.Web.Http.Controllers;
-using System.Web.Http.Dependencies;
 using System.Web.Http.Internal;
 using System.Web.Http.Properties;
 
@@ -112,16 +111,12 @@ private static IHttpController GetInstanceOrActivator(HttpRequestMessage request
             Contract.Assert(request != null);
             Contract.Assert(controllerType != null);
 
-            IDependencyScope scope = request.GetDependencyScope();
-            if (scope != null)
+            // If dependency resolver returns controller object then use it.
+            IHttpController instance = (IHttpController)request.GetDependencyScope().GetService(controllerType);
+            if (instance != null)
             {
-                // If dependency resolver returns controller object then use it.
-                IHttpController instance = (IHttpController)scope.GetService(controllerType);
-                if (instance != null)
-                {
-                    activator = null;
-                    return instance;
-                }
+                activator = null;
+                return instance;
             }
 
             // Otherwise create a delegate for creating a new instance of the type

src/System.Web.Http/HttpRequestMessageExtensions.cs

@@ -55,7 +55,12 @@ public static IDependencyScope GetDependencyScope(this HttpRequestMessage reques
             IDependencyScope result;
             if (!request.Properties.TryGetValue<IDependencyScope>(HttpPropertyKeys.DependencyScope, out result))
             {
-                result = request.GetConfiguration().DependencyResolver.BeginScope();
+                IDependencyResolver dependencyResolver = request.GetConfiguration().DependencyResolver;
+                result = dependencyResolver.BeginScope();
+                if (result == null)
+                {
+                    throw Error.InvalidOperation(SRResources.DependencyResolver_BeginScopeReturnsNull, dependencyResolver.GetType().Name);
+                }
                 request.Properties[HttpPropertyKeys.DependencyScope] = result;
                 request.RegisterForDispose(result);
             }

src/System.Web.Http/Properties/SRResources.resx

@@ -413,4 +413,7 @@ The request for '{0}' has found the following matching controllers:{2}</value>
   <data name="ValidationAttributeOnNonPublicProperty" xml:space="preserve">
     <value>Non-public property '{0}' on type '{1}' is attributed with one or more validation attributes. Validation attributes on non-public properties are not supported. Consider using a public property for validation instead.</value>
   </data>
+  <data name="DependencyResolver_BeginScopeReturnsNull" xml:space="preserve">
+    <value>A dependency resolver of type '{0}' returned an invalid value of null from its BeginScope method. If the container does not have a concept of scope, consider returning a scope that resolves in the root of the container instead.</value>
+  </data>
 </root>

test/System.Web.Http.Test/Dispatcher/DefaultHttpControllerActivatorTest.cs

@@ -132,7 +132,7 @@ public void Create_MixupInstanceCreationAndDependencyScope()
         }
 
         [Fact]
-        public void Create_MakesInstanceOfControllerForNullDependencyScope()
+        public void Create_ThrowsForNullDependencyScope()
         {
             // Arrange
             var config = new HttpConfiguration();
@@ -144,12 +144,18 @@ public void Create_MakesInstanceOfControllerForNullDependencyScope()
             var descriptorSimpleController = new HttpControllerDescriptor(config, "Simple", typeof(SimpleController));
             var activator = new DefaultHttpControllerActivator();
 
-            // Act
-            IHttpController simpleController = activator.Create(request, descriptorSimpleController, typeof(SimpleController));
+            // Act & Assert
+            InvalidOperationException exception = Assert.Throws<InvalidOperationException>(
+                () => activator.Create(request, descriptorSimpleController, typeof(SimpleController)));
+
+            Assert.Equal(
+                "An error occurred when trying to create a controller of type 'SimpleController'. Make sure that the controller has a parameterless public constructor.",
+                exception.Message);
+            Assert.NotNull(exception.InnerException);
+            Assert.Equal(
+                "A dependency resolver of type 'IDependencyResolverProxy' returned an invalid value of null from its BeginScope method. If the container does not have a concept of scope, consider returning a scope that resolves in the root of the container instead.",
+                exception.InnerException.Message);
 
-            // Assert
-            Assert.NotNull(simpleController);
-            Assert.IsType<SimpleController>(simpleController);
             mockResolver.Verify();
         }