Fix WebFx-169 and #118

- move `DynamicObject` derivation up to new `DynamicViewData` class, fixing [WebFx-169](http://projectk-tc:8080/browse/WEBFX-169)
- avoid direct `_data` lookup in previous `TryGetMember()`, fixing [#118](https://github.com/aspnet/WebFx/issues/118)
- rename ViewData -> ViewDataDictionary

Also
- flesh out `IDictionary<string, object>` implementation in `ViewData`
- provide `ViewData` copy constructor that allows TModel to change
- remove `TryGetIndex()` and `TrySetIndex()` implementations; use `ViewData[]` instead
- restore `ViewContext.ViewBag` from legacy MVC

samples/MvcSample.Web/Views/Home/Create.cshtml

@@ -1,24 +1,23 @@
 @using MvcSample.Web.Models
 @model User
 @{
-    string nullValue = null;
     ViewBag.Title = (Model == null) ? "Create Page" : "Edit Page";
-    if (ViewData["Gift"] == null)
+    if (ViewBag.Gift == null)
     {
         ViewBag.Gift = "nothing";
     }
 }
 
 <div class="row">
-    <h2 title="@ViewBag.Title" class="@nullValue">@ViewBag.Title</h2>
-    <h3 title="Thanks" class="@nullValue">Thanks for @ViewBag.Gift</h3>
+    <h2 title="@ViewBag.Title" class="@ViewBag.NullValue">@ViewBag.Title</h2>
+    <h3 title="Thanks" class="@ViewBag.NullValue">Thanks for @ViewBag.Gift</h3>
     @if (Model == null)
     {
-        <h4 title ="Null Model" class="@nullValue">Howdy, your model is null.</h4>
+        <h4 title ="Null Model" class="@ViewBag.NullValue">Howdy, your model is null.</h4>
     }
     else
     {
-        <h4 title="@Model.Name" class="@nullValue">Hello @Model.Name! Happy @Model.Age birthday.</h4>
+        <h4 title="@Model.Name" class="@ViewBag.NullValue">Hello @Model.Name! Happy @(Model.Age)th birthday.</h4>
     }
 
     @{

src/Microsoft.AspNet.Mvc.Core/ActionResultHelper.cs

@@ -36,7 +36,7 @@ namespace Microsoft.AspNet.Mvc
             return new JsonResult(value);
         }
 
-        public IActionResult View(string view, ViewData viewData)
+        public IActionResult View(string view, ViewDataDictionary viewData)
         {
             return new ViewResult(_serviceProvider, _viewEngine)
             {

src/Microsoft.AspNet.Mvc.Core/ActionResults/ViewResult.cs

@@ -21,7 +21,7 @@ namespace Microsoft.AspNet.Mvc
 
         public string ViewName {get; set; }
 
-        public ViewData ViewData { get; set; }
+        public ViewDataDictionary ViewData { get; set; }
 
         public async Task ExecuteResultAsync([NotNull] ActionContext context)
         {

src/Microsoft.AspNet.Mvc.Core/Controller.cs

@@ -6,6 +6,8 @@ namespace Microsoft.AspNet.Mvc
 {
     public class Controller
     {
+        private DynamicViewData _viewBag;
+
         public void Initialize(IActionResultHelper actionResultHelper)
         {
             Result = actionResultHelper;
@@ -33,11 +35,19 @@ namespace Microsoft.AspNet.Mvc
 
         public IUrlHelper Url { get; set; }
 
-        public ViewData<object> ViewData { get; set; }
+        public ViewDataDictionary<object> ViewData { get; set; }
 
         public dynamic ViewBag
         {
-            get { return ViewData; }
+            get
+            {
+                if (_viewBag == null)
+                {
+                    _viewBag = new DynamicViewData(() => ViewData);
+                }
+
+                return _viewBag;
+            }
         }
 
         public IActionResult View()

src/Microsoft.AspNet.Mvc.Core/DefaultControllerFactory.cs

@@ -52,15 +52,19 @@ namespace Microsoft.AspNet.Mvc
 
             foreach (var prop in controllerType.GetRuntimeProperties())
             {
-                if(prop.Name == "ActionContext" && prop.PropertyType.GetTypeInfo().IsAssignableFrom(typeof(ActionContext).GetTypeInfo()))
+                if(prop.Name == "ActionContext" &&
+                    prop.PropertyType.GetTypeInfo().IsAssignableFrom(typeof(ActionContext).GetTypeInfo()))
                 {
                     prop.SetValue(controller, actionContext);
                 }
-                else if (prop.Name == "ViewData" && prop.PropertyType.GetTypeInfo().IsAssignableFrom(typeof(ViewData<object>).GetTypeInfo()))
+                else if (prop.Name == "ViewData" &&
+                    prop.PropertyType.GetTypeInfo().IsAssignableFrom(typeof(ViewDataDictionary<object>).GetTypeInfo()))
                 {
-                    prop.SetValue(controller, new ViewData<object>(_serviceProvider.GetService<IModelMetadataProvider>(), actionContext.ModelState));
+                    prop.SetValue(controller, new ViewDataDictionary<object>(
+                        _serviceProvider.GetService<IModelMetadataProvider>(), actionContext.ModelState));
                 }
-                else if (prop.Name == "Url" && prop.PropertyType.GetTypeInfo().IsAssignableFrom(typeof(IUrlHelper).GetTypeInfo()))
+                else if (prop.Name == "Url" &&
+                    prop.PropertyType.GetTypeInfo().IsAssignableFrom(typeof(IUrlHelper).GetTypeInfo()))
                 {
                     var urlHelper = new UrlHelper(
                         actionContext.HttpContext,

src/Microsoft.AspNet.Mvc.Core/IActionResultHelper.cs

@@ -7,6 +7,6 @@ namespace Microsoft.AspNet.Mvc
         IActionResult Content(string value);
         IActionResult Content(string value, string contentType);
         IJsonResult Json(object value);
-        IActionResult View(string view, ViewData viewData);
+        IActionResult View(string view, ViewDataDictionary viewData);
     }
 }

src/Microsoft.AspNet.Mvc.Core/ViewComponents/DefaultViewComponentInvoker.cs

@@ -83,10 +83,10 @@ namespace Microsoft.AspNet.Mvc
                     prop.SetValue(component, context);
                 }
                 else if (prop.Name == "ViewData" && 
-                    typeof(ViewData).GetTypeInfo().IsAssignableFrom(prop.PropertyType.GetTypeInfo()))
+                    typeof(ViewDataDictionary).GetTypeInfo().IsAssignableFrom(prop.PropertyType.GetTypeInfo()))
                 {
                     // We're flowing the viewbag across, but the concept of model doesn't really apply here
-                    var viewData = new ViewData(context.ViewData);
+                    var viewData = new ViewDataDictionary(context.ViewData);
                     viewData.Model = null;
 
                     prop.SetValue(component, viewData);

src/Microsoft.AspNet.Mvc.Core/ViewComponents/DefaultViewComponentResultHelper.cs

@@ -23,7 +23,7 @@ namespace Microsoft.AspNet.Mvc
             return new JsonViewComponentResult(value);
         }
 
-        public IViewComponentResult View([NotNull] string viewName, [NotNull] ViewData viewData)
+        public IViewComponentResult View([NotNull] string viewName, [NotNull] ViewDataDictionary viewData)
         {
             return new ViewViewComponentResult(_viewEngine, viewName, viewData);
         }

src/Microsoft.AspNet.Mvc.Core/ViewComponents/IViewComponentResultHelper.cs

@@ -9,6 +9,6 @@ namespace Microsoft.AspNet.Mvc
 
         IViewComponentResult Json([NotNull] object value);
 
-        IViewComponentResult View([NotNull] string viewName, [NotNull] ViewData viewData);
+        IViewComponentResult View([NotNull] string viewName, [NotNull] ViewDataDictionary viewData);
     }
 }

src/Microsoft.AspNet.Mvc.Core/ViewComponents/ViewComponent.cs

@@ -16,7 +16,7 @@ namespace Microsoft.AspNet.Mvc
 
         public ViewContext ViewContext { get; set; }
 
-        public ViewData ViewData { get; set; }
+        public ViewDataDictionary ViewData { get; set; }
 
         public void Initialize(IViewComponentResultHelper result)
         {
@@ -40,7 +40,7 @@ namespace Microsoft.AspNet.Mvc
 
         public IViewComponentResult View<TModel>(string viewName, TModel model)
         {
-            var viewData = new ViewData<TModel>(ViewData);
+            var viewData = new ViewDataDictionary<TModel>(ViewData);
             if (model != null)
             {
                 viewData.Model = model;

src/Microsoft.AspNet.Mvc.Core/ViewComponents/ViewViewComponentResult.cs

@@ -14,9 +14,10 @@ namespace Microsoft.AspNet.Mvc
 
         private readonly IViewEngine _viewEngine;
         private readonly string _viewName;
-        private readonly ViewData _viewData;
+        private readonly ViewDataDictionary _viewData;
 
-        public ViewViewComponentResult([NotNull] IViewEngine viewEngine, [NotNull] string viewName, ViewData viewData)
+        public ViewViewComponentResult([NotNull] IViewEngine viewEngine, [NotNull] string viewName,
+            ViewDataDictionary viewData)
         {
             _viewEngine = viewEngine;
             _viewName = viewName;

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

@@ -26,6 +26,14 @@ namespace Microsoft.AspNet.Mvc.Razor
             get { return Context == null ? null : Context.Url; }
         }
 
+        public dynamic ViewBag
+        {
+            get
+            {
+                return (Context == null) ? null : Context.ViewBag;
+            }
+        }
+
         private string BodyContent { get; set; }
 
         public virtual async Task RenderAsync([NotNull] ViewContext context)

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

@@ -16,31 +16,26 @@ namespace Microsoft.AspNet.Mvc.Razor
             }
         }
 
-        public dynamic ViewBag
-        {
-            get { return ViewData; }
-        }
-
-        public ViewData<TModel> ViewData { get; private set; }
+        public ViewDataDictionary<TModel> ViewData { get; private set; }
 
         public HtmlHelper<TModel> Html { get; set; }
 
         public override Task RenderAsync([NotNull] ViewContext context)
         {
-            ViewData = context.ViewData as ViewData<TModel>;
+            ViewData = context.ViewData as ViewDataDictionary<TModel>;
             if (ViewData == null)
             {
                 if (context.ViewData != null)
                 {
-                    ViewData = new ViewData<TModel>(context.ViewData);
+                    ViewData = new ViewDataDictionary<TModel>(context.ViewData);
                 }
                 else
                 {
                     var metadataProvider = context.ServiceProvider.GetService<IModelMetadataProvider>();
-                    ViewData = new ViewData<TModel>(metadataProvider);
+                    ViewData = new ViewDataDictionary<TModel>(metadataProvider);
                 }
 
-                // Have new ViewData; make sure it's visible everywhere.
+                // Have new ViewDataDictionary; make sure it's visible everywhere.
                 context.ViewData = ViewData;
             }
 

src/Microsoft.AspNet.Mvc.Rendering/Html/HtmlHelper.cs

@@ -17,7 +17,7 @@ namespace Microsoft.AspNet.Mvc.Rendering
         public static readonly string ValidationSummaryCssClassName = "validation-summary-errors";
         public static readonly string ValidationSummaryValidCssClassName = "validation-summary-valid";
 
-        public HtmlHelper([NotNull] HttpContext httpContext, ViewData viewData)
+        public HtmlHelper([NotNull] HttpContext httpContext, ViewDataDictionary viewData)
         {
             HttpContext = httpContext;
             ViewData = viewData;
@@ -30,7 +30,7 @@ namespace Microsoft.AspNet.Mvc.Rendering
 
         public HttpContext HttpContext { get; private set; }
 
-        public ViewData ViewData
+        public ViewDataDictionary ViewData
         {
             get;
             private set;

src/Microsoft.AspNet.Mvc.Rendering/Html/HtmlHelperOfT.cs

@@ -4,13 +4,13 @@ namespace Microsoft.AspNet.Mvc.Rendering
 {
     public class HtmlHelper<TModel> : HtmlHelper
     {
-        public HtmlHelper([NotNull]HttpContext httpContext, ViewData<TModel> viewData)
+        public HtmlHelper([NotNull]HttpContext httpContext, ViewDataDictionary<TModel> viewData)
             : base(httpContext, viewData)
         {
             ViewData = viewData;
         }
 
-        public new ViewData<TModel> ViewData
+        public new ViewDataDictionary<TModel> ViewData
         {
             get; private set;
         }

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

@@ -42,6 +42,22 @@ namespace Microsoft.AspNet.Mvc.Rendering
             return string.Format(CultureInfo.CurrentCulture, GetString("Common_PartialViewNotFound"), p0, p1);
         }
 
+        /// <summary>
+        /// ViewData value must not be null.
+        /// </summary>
+        internal static string DynamicViewData_ViewDataNull
+        {
+            get { return GetString("DynamicViewData_ViewDataNull"); }
+        }
+
+        /// <summary>
+        /// ViewData value must not be null.
+        /// </summary>
+        internal static string FormatDynamicViewData_ViewDataNull()
+        {
+            return GetString("DynamicViewData_ViewDataNull");
+        }
+
         /// <summary>
         /// The model item passed is null, but this ViewData instance requires a non-null model item of type '{0}'.
         /// </summary>

src/Microsoft.AspNet.Mvc.Rendering/Properties/Resources.resx

@@ -123,11 +123,14 @@
   <data name="Common_PartialViewNotFound" xml:space="preserve">
     <value>The partial view '{0}' was not found or no view engine supports the searched locations. The following locations were searched:{1}</value>
   </data>
+  <data name="DynamicViewData_ViewDataNull" xml:space="preserve">
+    <value>ViewData value must not be null.</value>
+  </data>
   <data name="ViewData_ModelCannotBeNull" xml:space="preserve">
-    <value>The model item passed is null, but this ViewData instance requires a non-null model item of type '{0}'.</value>
+    <value>The model item passed is null, but this ViewDataDictionary instance requires a non-null model item of type '{0}'.</value>
   </data>
   <data name="ViewData_WrongTModelType" xml:space="preserve">
-    <value>The model item passed into the ViewData is of type '{0}', but this ViewData instance requires a model item of type '{1}'.</value>
+    <value>The model item passed into the ViewDataDictionary is of type '{0}', but this ViewDataDictionary instance requires a model item of type '{1}'.</value>
   </data>
   <data name="ViewEngine_ViewNotFound" xml:space="preserve">
     <value>The view '{0}' was not found. The following locations were searched:{1}.</value>

src/Microsoft.AspNet.Mvc.Rendering/View/DynamicViewData.cs

@@ -0,0 +1,55 @@
+using System;
+using System.Collections.Generic;
+using System.Dynamic;
+
+namespace Microsoft.AspNet.Mvc.Rendering
+{
+    public class DynamicViewData : DynamicObject
+    {
+        private readonly Func<ViewDataDictionary> _viewDataFunc;
+
+        public DynamicViewData([NotNull] Func<ViewDataDictionary> viewDataFunc)
+        {
+            _viewDataFunc = viewDataFunc;
+        }
+
+        private ViewDataDictionary ViewData
+        {
+            get
+            {
+                ViewDataDictionary viewData = _viewDataFunc();
+                if (viewData == null)
+                {
+                    throw new InvalidOperationException(Resources.DynamicViewData_ViewDataNull);
+                }
+
+                return viewData;
+            }
+        }
+
+        // Implementing this function extends the ViewBag contract, supporting or improving some scenarios. For example
+        // having this method improves the debugging experience as it provides the debugger with the list of all
+        // properties currently defined on the object.
+        public override IEnumerable<string> GetDynamicMemberNames()
+        {
+            return ViewData.Keys;
+        }
+
+        public override bool TryGetMember([NotNull] GetMemberBinder binder, out object result)
+        {
+            result = ViewData[binder.Name];
+
+            // ViewDataDictionary[key] will never throw a KeyNotFoundException.
+            // Similarly, return true so caller does not throw.
+            return true;
+        }
+
+        public override bool TrySetMember([NotNull] SetMemberBinder binder, object value)
+        {
+            ViewData[binder.Name] = value;
+
+            // Can always add / update a ViewDataDictionary value.
+            return true;
+        }
+    }
+}

src/Microsoft.AspNet.Mvc.Rendering/View/ViewContext.cs

@@ -7,6 +7,8 @@ namespace Microsoft.AspNet.Mvc.Rendering
 {
     public class ViewContext
     {
+        private DynamicViewData _viewBag;
+
         public ViewContext(IServiceProvider serviceProvider, HttpContext httpContext, IDictionary<string, object> viewEngineContext)
         {
             ServiceProvider = serviceProvider;
@@ -22,7 +24,20 @@ namespace Microsoft.AspNet.Mvc.Rendering
 
         public IUrlHelper Url { get; set; }
 
-        public ViewData ViewData { get; set; }
+        public dynamic ViewBag
+        {
+            get
+            {
+                if (_viewBag == null)
+                {
+                    _viewBag = new DynamicViewData(() => ViewData);
+                }
+
+                return _viewBag;
+            }
+        }
+
+        public ViewDataDictionary ViewData { get; set; }
 
         public IDictionary<string, object> ViewEngineContext { get; private set; }
 

src/Microsoft.AspNet.Mvc.Rendering/View/ViewData.cs

@@ -1,155 +0,0 @@
-using System;
-using System.Collections.Generic;
-using System.Dynamic;
-using Microsoft.AspNet.Mvc.ModelBinding;
-
-namespace Microsoft.AspNet.Mvc.Rendering
-{
-    public class ViewData : DynamicObject
-    {
-        private readonly Dictionary<object, dynamic> _data;
-        private object _model;
-        private ModelMetadata _modelMetadata;
-        private IModelMetadataProvider _metadataProvider;
-
-        public ViewData([NotNull] IModelMetadataProvider metadataProvider)
-            : this(metadataProvider, new ModelStateDictionary())
-        {
-        }
-
-        public ViewData([NotNull] IModelMetadataProvider metadataProvider, [NotNull] ModelStateDictionary modelState)
-        {
-            ModelState = modelState;
-            _data = new Dictionary<object, dynamic>();
-            _metadataProvider = metadataProvider;
-        }
-
-        public ViewData([NotNull] ViewData source)
-            : this(source.MetadataProvider)
-        {
-            _modelMetadata = source.ModelMetadata;
-
-            foreach (var entry in source.ModelState)
-            {
-                ModelState.Add(entry.Key, entry.Value);
-            }
-
-            foreach (var entry in source)
-            {
-                _data.Add(entry.Key, entry.Value);
-            }
-
-            SetModel(source.Model);
-        }
-
-        public object Model
-        {
-            get { return _model; }
-            set { SetModel(value); }
-        }
-
-        public ModelStateDictionary ModelState { get; private set; }
-
-        public dynamic this[string index]
-        {
-            get
-            {
-                dynamic result;
-
-                _data.TryGetValue(index, out result);
-
-                return result;
-            }
-            set
-            {
-                _data[index] = value;
-            }
-        }
-
-        public virtual ModelMetadata ModelMetadata
-        {
-            get
-            {
-                return _modelMetadata;
-            }
-            set
-            {
-                _modelMetadata = value;
-            }
-        }
-
-        /// <summary>
-        /// Provider for subclasses that need it to override <see cref="ModelMetadata"/>.
-        /// </summary>
-        protected IModelMetadataProvider MetadataProvider
-        {
-            get { return _metadataProvider; }
-        }
-        
-        public Dictionary<object, dynamic>.Enumerator GetEnumerator()
-        {
-            return _data.GetEnumerator();
-        }
-
-        public override bool TryGetMember(GetMemberBinder binder, out object result)
-        {
-            result = _data[binder.Name];
-
-            // We return true here because ViewDataDictionary returns null if the key is not
-            // in the dictionary, so we simply pass on the returned value.
-            return true;
-        }
-
-        public override bool TrySetMember(SetMemberBinder binder, object value)
-        {
-            // This cast should always succeed.
-            dynamic v = value;
-            _data[binder.Name] = v;
-            return true;
-        }
-
-        public override bool TryGetIndex(GetIndexBinder binder, object[] indexes, out object result)
-        {
-            if (indexes == null || indexes.Length != 1)
-            {
-                throw new ArgumentException("Invalid number of indexes");
-            }
-
-            object index = indexes[0];
-            result = this[(string)index];
-            return true;
-        }
-
-        public override bool TrySetIndex(SetIndexBinder binder, object[] indexes, object value)
-        {
-            if (indexes == null || indexes.Length != 1)
-            {
-                throw new ArgumentException("Invalid number of indexes");
-            }
-
-            object index = indexes[0];
-
-            // This cast should always succeed.
-            this[(string)index] = value;
-            return true;
-        }
-
-        // This method will execute before the derived type's instance constructor executes. Derived types must
-        // be aware of this and should plan accordingly. For example, the logic in SetModel() should be simple
-        // enough so as not to depend on the "this" pointer referencing a fully constructed object.
-        protected virtual void SetModel(object value)
-        {
-            _model = value;
-            if (value == null)
-            {
-                // Unable to determine model metadata.
-                _modelMetadata = null;
-            }
-            else if (_modelMetadata == null || value.GetType() != ModelMetadata.ModelType)
-            {
-                // Reset or override model metadata based on new value type.
-                _modelMetadata = _metadataProvider.GetMetadataForType(() => value, value.GetType());
-            }
-        }
-    }
-}

src/Microsoft.AspNet.Mvc.Rendering/View/ViewDataDictionary.cs

@@ -0,0 +1,198 @@
+using System;
+using System.Collections;
+using System.Collections.Generic;
+using Microsoft.AspNet.Mvc.ModelBinding;
+
+namespace Microsoft.AspNet.Mvc.Rendering
+{
+    public class ViewDataDictionary : IDictionary<string, object>
+    {
+        private readonly IDictionary<string, object> _data;
+        private object _model;
+        private ModelMetadata _modelMetadata;
+        private IModelMetadataProvider _metadataProvider;
+
+        public ViewDataDictionary([NotNull] IModelMetadataProvider metadataProvider)
+            : this(metadataProvider, new ModelStateDictionary())
+        {
+        }
+
+        public ViewDataDictionary([NotNull] IModelMetadataProvider metadataProvider,
+            [NotNull] ModelStateDictionary modelState)
+        {
+            ModelState = modelState;
+            _data = new Dictionary<string, object>(StringComparer.OrdinalIgnoreCase);
+            _metadataProvider = metadataProvider;
+        }
+
+        /// <summary>
+        /// <see cref="ViewDataDictionary"/> copy constructor for use when model type does not change.
+        /// </summary>
+        public ViewDataDictionary([NotNull] ViewDataDictionary source)
+            : this(source, source.Model)
+        {
+        }
+
+        /// <summary>
+        /// <see cref="ViewDataDictionary"/> copy constructor for use when model type may change. This avoids
+        /// exceptions a derived class may throw when <see cref="SetModel()"/> is called.
+        /// </summary>
+        public ViewDataDictionary([NotNull] ViewDataDictionary source, object model)
+            : this(source.MetadataProvider)
+        {
+            _modelMetadata = source.ModelMetadata;
+
+            foreach (var entry in source.ModelState)
+            {
+                ModelState.Add(entry.Key, entry.Value);
+            }
+
+            foreach (var entry in source)
+            {
+                _data.Add(entry.Key, entry.Value);
+            }
+
+            SetModel(model);
+        }
+
+        public object Model
+        {
+            get { return _model; }
+            set { SetModel(value); }
+        }
+
+        public ModelStateDictionary ModelState { get; private set; }
+
+        public virtual ModelMetadata ModelMetadata
+        {
+            get
+            {
+                return _modelMetadata;
+            }
+            set
+            {
+                _modelMetadata = value;
+            }
+        }
+
+        /// <summary>
+        /// Provider for subclasses that need it to override <see cref="ModelMetadata"/>.
+        /// </summary>
+        protected IModelMetadataProvider MetadataProvider
+        {
+            get { return _metadataProvider; }
+        }
+
+        #region IDictionary properties
+        // Do not just pass through to _data: Indexer should not throw a KeyNotFoundException.
+        public object this[string index]
+        {
+            get
+            {
+                object result;
+                _data.TryGetValue(index, out result);
+                return result;
+            }
+            set
+            {
+                _data[index] = value;
+            }
+        }
+
+        public int Count
+        {
+            get { return _data.Count; }
+        }
+
+        public bool IsReadOnly
+        {
+            get { return _data.IsReadOnly; }
+        }
+
+        public ICollection<string> Keys
+        {
+            get { return _data.Keys; }
+        }
+
+        public ICollection<object> Values
+        {
+            get { return _data.Values; }
+        }
+        #endregion
+
+        // This method will execute before the derived type's instance constructor executes. Derived types must
+        // be aware of this and should plan accordingly. For example, the logic in SetModel() should be simple
+        // enough so as not to depend on the "this" pointer referencing a fully constructed object.
+        protected virtual void SetModel(object value)
+        {
+            _model = value;
+            if (value == null)
+            {
+                // Unable to determine model metadata.
+                _modelMetadata = null;
+            }
+            else if (_modelMetadata == null || value.GetType() != ModelMetadata.ModelType)
+            {
+                // Reset or override model metadata based on new value type.
+                _modelMetadata = _metadataProvider.GetMetadataForType(() => value, value.GetType());
+            }
+        }
+
+        #region IDictionary methods
+        public void Add([NotNull] string key, object value)
+        {
+            _data.Add(key, value);
+        }
+
+        public bool ContainsKey([NotNull] string key)
+        {
+            return _data.ContainsKey(key);
+        }
+
+        public bool Remove([NotNull] string key)
+        {
+            return _data.Remove(key);
+        }
+
+        public bool TryGetValue([NotNull] string key, out object value)
+        {
+            return _data.TryGetValue(key, out value);
+        }
+
+        public void Add([NotNull] KeyValuePair<string, object> item)
+        {
+            _data.Add(item);
+        }
+
+        public void Clear()
+        {
+            _data.Clear();
+        }
+
+        public bool Contains([NotNull] KeyValuePair<string, object> item)
+        {
+            return _data.Contains(item);
+        }
+
+        public void CopyTo([NotNull] KeyValuePair<string, object>[] array, int arrayIndex)
+        {
+            _data.CopyTo(array, arrayIndex);
+        }
+
+        public bool Remove([NotNull] KeyValuePair<string, object> item)
+        {
+            return _data.Remove(item);
+        }
+
+        IEnumerator<KeyValuePair<string, object>> IEnumerable<KeyValuePair<string, object>>.GetEnumerator()
+        {
+            return _data.GetEnumerator();
+        }
+
+        IEnumerator IEnumerable.GetEnumerator()
+        {
+            return _data.GetEnumerator();
+        }
+        #endregion
+    }
+}

src/Microsoft.AspNet.Mvc.Rendering/View/ViewDataDictionaryOfT.cs

@@ -3,27 +3,35 @@ using Microsoft.AspNet.Mvc.ModelBinding;
 
 namespace Microsoft.AspNet.Mvc.Rendering
 {
-    public class ViewData<TModel> : ViewData
+    public class ViewDataDictionary<TModel> : ViewDataDictionary
     {
-        // Fallback ModelMetadata based on TModel. Used when Model is null and base ViewData class is unable to
-        // determine the correct metadata.
+        // Fallback ModelMetadata based on TModel. Used when Model is null and base ViewDataDictionary class is unable
+        // to determine the correct metadata.
         private readonly ModelMetadata _defaultModelMetadata;
 
-        public ViewData([NotNull] IModelMetadataProvider metadataProvider)
+        public ViewDataDictionary([NotNull] IModelMetadataProvider metadataProvider)
             : base(metadataProvider)
         {
             _defaultModelMetadata = MetadataProvider.GetMetadataForType(null, typeof(TModel));
         }
         
-        public ViewData([NotNull] IModelMetadataProvider metadataProvider, [NotNull] ModelStateDictionary modelState)
+        public ViewDataDictionary([NotNull] IModelMetadataProvider metadataProvider,
+            [NotNull] ModelStateDictionary modelState)
             : base(metadataProvider, modelState)
         {
         }
 
-        public ViewData(ViewData source)
-            : base(source)
+        /// <inheritdoc />
+        public ViewDataDictionary([NotNull] ViewDataDictionary source)
+            : this(source, source.Model)
         {
-            var original = source as ViewData<TModel>;
+        }
+
+        /// <inheritdoc />
+        public ViewDataDictionary([NotNull] ViewDataDictionary source, object model)
+            : base(source, model)
+        {
+            var original = source as ViewDataDictionary<TModel>;
             if (original != null)
             {
                 _defaultModelMetadata = original._defaultModelMetadata;

test/Microsoft.AspNet.Mvc.Core.Test/ControllerTests.cs

@@ -0,0 +1,31 @@
+using Microsoft.AspNet.Mvc.ModelBinding;
+using Microsoft.AspNet.Mvc.Rendering;
+using Xunit;
+
+namespace Microsoft.AspNet.Mvc.Core
+{
+    public class ControllerTests
+    {
+        [Fact]
+        public void SettingViewData_AlsoUpdatesViewBag()
+        {
+            // Arrange
+            var metadataProvider = new DataAnnotationsModelMetadataProvider();
+            var controller = new Controller();
+            var originalViewData = controller.ViewData = new ViewDataDictionary<object>(metadataProvider);
+            var replacementViewData = new ViewDataDictionary<object>(metadataProvider);
+
+            // Act
+            controller.ViewBag.Hello = "goodbye";
+            controller.ViewData = replacementViewData;
+            controller.ViewBag.Another = "property";
+
+            // Assert
+            Assert.NotSame(originalViewData, controller.ViewData);
+            Assert.Same(replacementViewData, controller.ViewData);
+            Assert.Null(controller.ViewBag.Hello);
+            Assert.Equal("property", controller.ViewBag.Another);
+            Assert.Equal("property", controller.ViewData["Another"]);
+        }
+    }
+}

test/Microsoft.AspNet.Mvc.Core.Test/project.json

@@ -4,6 +4,8 @@
     "Microsoft.AspNet.Mvc.Core" : "",
     "Microsoft.AspNet.Mvc" : "",
     "Microsoft.AspNet.Testing": "0.1-alpha-*",
+    "Microsoft.AspNet.Mvc.ModelBinding": "",
+    "Microsoft.AspNet.Mvc.Rendering": "",
     "Moq": "4.2.1312.1622",
     "Xunit.KRunner": "0.1-alpha-*",
     "xunit.abstractions": "2.0.0-aspnet-*",

test/Microsoft.AspNet.Mvc.Rendering.Test/ViewContextTests.cs

@@ -0,0 +1,28 @@
+using Xunit;
+
+namespace Microsoft.AspNet.Mvc.Rendering
+{
+    public class ViewContextTests
+    {
+        [Fact]
+        public void SettingViewData_AlsoUpdatesViewBag()
+        {
+            // Arrange (eventually passing null to these consturctors will throw)
+            var context = new ViewContext(serviceProvider: null, httpContext: null, viewEngineContext: null);
+            var originalViewData = context.ViewData = new ViewDataDictionary(metadataProvider: null);
+            var replacementViewData = new ViewDataDictionary(metadataProvider: null);
+
+            // Act
+            context.ViewBag.Hello = "goodbye";
+            context.ViewData = replacementViewData;
+            context.ViewBag.Another = "property";
+
+            // Assert
+            Assert.NotSame(originalViewData, context.ViewData);
+            Assert.Same(replacementViewData, context.ViewData);
+            Assert.Null(context.ViewBag.Hello);
+            Assert.Equal("property", context.ViewBag.Another);
+            Assert.Equal("property", context.ViewData["Another"]);
+        }
+    }
+}

test/Microsoft.AspNet.Mvc.Rendering.Test/ViewDataOfTTest.cs

@@ -10,24 +10,24 @@ namespace Microsoft.AspNet.Mvc.Rendering
         public void SettingModelThrowsIfTheModelIsNull()
         {
             // Arrange
-            var viewDataOfT = new ViewData<int>(new DataAnnotationsModelMetadataProvider());
-            ViewData viewData = viewDataOfT;
+            var viewDataOfT = new ViewDataDictionary<int>(new DataAnnotationsModelMetadataProvider());
+            ViewDataDictionary viewData = viewDataOfT;
 
             // Act and Assert
             Exception ex = Assert.Throws<InvalidOperationException>(() => viewData.Model = null);
-            Assert.Equal("The model item passed is null, but this ViewData instance requires a non-null model item of type 'System.Int32'.", ex.Message);
+            Assert.Equal("The model item passed is null, but this ViewDataDictionary instance requires a non-null model item of type 'System.Int32'.", ex.Message);
         }
 
         [Fact]
         public void SettingModelThrowsIfTheModelIsIncompatible()
         {
             // Arrange
-            var viewDataOfT = new ViewData<string>(new DataAnnotationsModelMetadataProvider());
-            ViewData viewData = viewDataOfT;
+            var viewDataOfT = new ViewDataDictionary<string>(new DataAnnotationsModelMetadataProvider());
+            ViewDataDictionary viewData = viewDataOfT;
 
             // Act and Assert
             Exception ex = Assert.Throws<InvalidOperationException>(() => viewData.Model = DateTime.UtcNow);
-            Assert.Equal("The model item passed into the ViewData is of type 'System.DateTime', but this ViewData instance requires a model item of type 'System.String'.", ex.Message);
+            Assert.Equal("The model item passed into the ViewDataDictionary is of type 'System.DateTime', but this ViewDataDictionary instance requires a model item of type 'System.String'.", ex.Message);
         }
 
         [Fact]
@@ -35,8 +35,8 @@ namespace Microsoft.AspNet.Mvc.Rendering
         {
             // Arrange
             string value = "some value";
-            var viewDataOfT = new ViewData<object>(new DataAnnotationsModelMetadataProvider());
-            ViewData viewData = viewDataOfT;
+            var viewDataOfT = new ViewDataDictionary<object>(new DataAnnotationsModelMetadataProvider());
+            ViewDataDictionary viewData = viewDataOfT;
 
             // Act
             viewData.Model = value;