Removed the TypeActivator from TagHelper creation.

- We now rely on Activator.CreateInstance to instantiate TagHelpers.
- Added test to validate throwing.
- Removed existing tests that expected the constructor injection behavior.

#1303

Author: N. Taylor Mullen

src/Microsoft.AspNet.Mvc.Razor/Properties/Resources.Designer.cs

@@ -25,7 +25,6 @@ public abstract class RazorPage : IRazorPage
         private readonly Stack<TextWriter> _writerScopes;
         private TextWriter _originalWriter;
         private IUrlHelper _urlHelper;
-        private ITypeActivator _typeActivator;
         private ITagHelperActivator _tagHelperActivator;
         private bool _renderedBody;
 
@@ -114,19 +113,6 @@ public dynamic ViewBag
         /// <inheritdoc />
         public abstract Task ExecuteAsync();
 
-        private ITypeActivator TypeActivator
-        {
-            get
-            {
-                if(_typeActivator == null)
-                {
-                    _typeActivator = ViewContext.HttpContext.RequestServices.GetService<ITypeActivator>();
-                }
-
-                return _typeActivator;
-            }
-        }
-
         private ITagHelperActivator TagHelperActivator
         {
             get
@@ -139,19 +125,30 @@ private ITagHelperActivator TagHelperActivator
                 return _tagHelperActivator;
             }
         }
-
         /// <summary>
         /// Creates and activates a <see cref="ITagHelper"/>.
         /// </summary>
         /// <typeparam name="TTagHelper">A <see cref="ITagHelper"/> type.</typeparam>
         /// <returns>The activated <see cref="ITagHelper"/>.</returns>
         /// <remarks>
-        /// If the <see cref= "ITagHelper" /> implements <see cref="ICanHasViewContext"/> the 
-        /// <see cref="ICanHasViewContext.Contextualize(ViewContext)"/> method is called with <see cref="ViewContext"/>.
+        /// <typeparamref name="TTagHelper"/> must have a parameterless constructor.
         /// </remarks>
         public TTagHelper CreateTagHelper<TTagHelper>() where TTagHelper : ITagHelper
         {
-            var tagHelper = TypeActivator.CreateInstance<TTagHelper>(ViewContext.HttpContext.RequestServices);
+            TTagHelper tagHelper;
+
+            try
+            {
+                tagHelper = Activator.CreateInstance<TTagHelper>();
+            }
+            catch
+            {
+                // Activator.CreateInstance will throw a MissingMethodException if there is no parameterless 
+                // constructor. Rethrow a more informative error.
+                throw new MissingMethodException(
+                    Resources.FormatCreateTagHelper_TagHelpersMustHaveParameterlessConstructors(
+                        typeof(TTagHelper).FullName));
+            }
 
             TagHelperActivator.Activate(tagHelper, ViewContext);
 

src/Microsoft.AspNet.Mvc.Razor/RazorPage.cs

@@ -186,4 +186,7 @@
   <data name="View_MethodCannotBeCalled" xml:space="preserve">
     <value>The method '{0}' cannot be invoked by this view.</value>
   </data>
+  <data name="CreateTagHelper_TagHelpersMustHaveParameterlessConstructors" xml:space="preserve">
+    <value>Tag helper `{0}` must have a public paramterless constructor.</value>
+  </data>
 </root>

src/Microsoft.AspNet.Mvc.Razor/Resources.resx

@@ -2,6 +2,7 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
+using System.Globalization;
 using System.IO;
 using System.Threading.Tasks;
 using Microsoft.AspNet.Http;
@@ -18,56 +19,48 @@ namespace Microsoft.AspNet.Mvc.Razor
     public class RazorPageCreateTagHelperTest
     {
         [Fact]
-        public void CreateTagHelper_CreatesProvidedTagHelperType()
-        {
-            // Arrange
-            var instance = CreateTestRazorPage();
-
-            // Act
-            var tagHelper = instance.CreateTagHelper<NoServiceTagHelper>();
-
-            // Assert
-            Assert.NotNull(tagHelper);
-        }
-
-        [Fact]
-        public void CreateTagHelper_ActivatesProvidedTagHelperType_Constructor()
+        public void CreateTagHelper_ThrowsIfNoParameterlessConstructor()
         {
             // Arrange
             var instance = CreateTestRazorPage();
+            var expectedErrorMessage = string.Format(
+                CultureInfo.InvariantCulture,
+                "Tag helper `{0}` must have a public paramterless constructor.",
+                typeof(NoParameterlessConstructor).FullName);
 
-            // Act
-            var tagHelper = instance.CreateTagHelper<ConstructorServiceTagHelper>();
+            // Act & Assert
+            var ex = Assert.Throws<MissingMethodException>(() =>
+            {
+                instance.CreateTagHelper<NoParameterlessConstructor>();
+            });
 
-            // Assert
-            Assert.NotNull(tagHelper.PassedInService);
+            Assert.Equal(expectedErrorMessage, ex.Message);
         }
 
         [Fact]
-        public void CreateTagHelper_ActivatesProvidedTagHelperType_Property()
+        public void CreateTagHelper_CreatesProvidedTagHelperType()
         {
             // Arrange
             var instance = CreateTestRazorPage();
 
             // Act
-            var tagHelper = instance.CreateTagHelper<ActivateAttributeServiceTagHelper>();
+            var tagHelper = instance.CreateTagHelper<NoServiceTagHelper>();
 
             // Assert
-            Assert.NotNull(tagHelper.ActivatedService);
+            Assert.NotNull(tagHelper);
         }
 
         [Fact]
-        public void CreateTagHelper_ActivatesProvidedTagHelperType_PropertyAndConstructor()
+        public void CreateTagHelper_ActivatesProvidedTagHelperType()
         {
             // Arrange
             var instance = CreateTestRazorPage();
 
             // Act
-            var tagHelper = instance.CreateTagHelper<AttributeConstructorServiceTagHelper>();
+            var tagHelper = instance.CreateTagHelper<ServiceTagHelper>();
 
             // Assert
             Assert.NotNull(tagHelper.ActivatedService);
-            Assert.NotNull(tagHelper.PassedInService);
         }
 
         [Fact]
@@ -94,7 +87,7 @@ public void CreateTagHelper_ProvidesTagHelperTypeWithViewContextAndActivates()
 
             // Assert
             Assert.NotNull(tagHelper.ViewContext);
-            Assert.NotNull(tagHelper.PassedInService);
+            Assert.NotNull(tagHelper.ActivatedService);
         }
 
         private static TestRazorPage CreateTestRazorPage()
@@ -138,33 +131,10 @@ private class NoServiceTagHelper : TagHelper
         {
         }
 
-        private class ConstructorServiceTagHelper : TagHelper
-        {
-            public MyService PassedInService { get; set; }
-
-            public ConstructorServiceTagHelper(MyService service)
-            {
-                PassedInService = service;
-            }
-        }
-
-        private class ActivateAttributeServiceTagHelper : TagHelper
-        {
-            [Activate]
-            public MyService ActivatedService { get; set; }
-        }
-
-        private class AttributeConstructorServiceTagHelper : TagHelper
+        private class ServiceTagHelper : TagHelper
         {
             [Activate]
             public MyService ActivatedService { get; set; }
-
-            public MyService PassedInService { get; set; }
-
-            public AttributeConstructorServiceTagHelper(MyService service)
-            {
-                PassedInService = service;
-            }
         }
 
         private class ViewContextTagHelper : TagHelper
@@ -175,11 +145,14 @@ private class ViewContextTagHelper : TagHelper
 
         private class ViewContextServiceTagHelper : ViewContextTagHelper
         {
-            public MyService PassedInService { get; set; }
+            [Activate]
+            public MyService ActivatedService { get; set; }
+        }
 
-            public ViewContextServiceTagHelper(MyService service)
+        private class NoParameterlessConstructor : TagHelper
+        {
+            public NoParameterlessConstructor(string a)
             {
-                PassedInService = service;
             }
         }