Inject `HtmlHelper` property into `RazorView<T>`

- add `IHtmlHelper<T>` and `ICanHasViewContext`
- adjust `HtmlHelper` and `HtmlHelper<T>` to match
- throw if `ViewContext` accessed prior to `Contextualize` call
- XML comments (from old world) all around

Note
- no current need for an `HtmlHelper` copy constructor or `Clone()` method
- expect recursion code to get another injected `IHtmlHelper<T>` and then
  "contextualize" that instance with a new `ViewContext`

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

@@ -18,7 +18,7 @@ namespace Microsoft.AspNet.Mvc.Razor
 
         public ViewDataDictionary<TModel> ViewData { get; private set; }
 
-        public HtmlHelper<TModel> Html { get; set; }
+        public IHtmlHelper<TModel> Html { get; set; }
 
         public override Task RenderAsync([NotNull] ViewContext context)
         {
@@ -46,7 +46,13 @@ namespace Microsoft.AspNet.Mvc.Razor
 
         private void InitHelpers(ViewContext context)
         {
-            Html = new HtmlHelper<TModel>(context.HttpContext, ViewData);
+            Html = context.ServiceProvider.GetService<IHtmlHelper<TModel>>();
+
+            var contextable = Html as ICanHasViewContext;
+            if (contextable != null)
+            {
+                contextable.Contextualize(context);
+            }
         }
     }
 }

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

@@ -8,7 +8,10 @@ using Microsoft.AspNet.Abstractions;
 
 namespace Microsoft.AspNet.Mvc.Rendering
 {
-    public class HtmlHelper
+    /// <summary>
+    /// Default implementation of non-generic portions of <see cref="IHtmlHelper{T}">.
+    /// </summary>
+    public class HtmlHelper : ICanHasViewContext
     {
         public static readonly string ValidationInputCssClassName = "input-validation-error";
         public static readonly string ValidationInputValidCssClassName = "input-validation-valid";
@@ -17,11 +20,13 @@ 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, ViewDataDictionary viewData)
+        private ViewContext _viewContext;
-        {
-            HttpContext = httpContext;
-            ViewData = viewData;
 
+        /// <summary>
+        /// Initializes a new instance of the <see cref="HtmlHelper"/> class.
+        /// </summary>
+        public HtmlHelper()
+        {
             // Underscores are fine characters in id's.
             IdAttributeDotReplacement = "_";
         }
@@ -30,10 +35,37 @@ namespace Microsoft.AspNet.Mvc.Rendering
 
         public HttpContext HttpContext { get; private set; }
 
+        public ViewContext ViewContext
+        {
+            get
+            {
+                if (_viewContext == null)
+                {
+                    throw new InvalidOperationException(Resources.HtmlHelper_NotContextualized);
+                }
+
+                return _viewContext;
+            }
+            private set
+            {
+                _viewContext = value;
+            }
+        }
+
+        public dynamic ViewBag
+        {
+            get
+            {
+                return ViewContext.ViewBag;
+            }
+        }
+
         public ViewDataDictionary ViewData
         {
-            get;
+            get
-            private set;
+            {
+                return ViewContext.ViewData;
+            }
         }
 
         /// <summary>
@@ -71,6 +103,11 @@ namespace Microsoft.AspNet.Mvc.Rendering
             return result;
         }
 
+        public virtual void Contextualize([NotNull] ViewContext viewContext)
+        {
+            ViewContext = viewContext;
+        }
+
         [SuppressMessage("Microsoft.Performance", "CA1822:MarkMembersAsStatic", Justification = "For consistency, all helpers are instance methods.")]
         public string Encode(string value)
         {
@@ -88,6 +125,11 @@ namespace Microsoft.AspNet.Mvc.Rendering
             return TagBuilder.CreateSanitizedId(name, IdAttributeDotReplacement);
         }
 
+        /// <summary>
+        /// Returns the HTTP method that handles form input (GET or POST) as a string.
+        /// </summary>
+        /// <param name="method">The HTTP method that handles the form.</param>
+        /// <returns>The form method string, either "get" or "post".</returns>
         public static string GetFormMethodString(FormMethod method)
         {
             switch (method)
@@ -120,24 +162,12 @@ namespace Microsoft.AspNet.Mvc.Rendering
             }
         }
 
-        /// <summary>
-        /// Wraps HTML markup in an IHtmlString, which will enable HTML markup to be
-        /// rendered to the output without getting HTML encoded.
-        /// </summary>
-        /// <param name="value">HTML markup string.</param>
-        /// <returns>An IHtmlString that represents HTML markup.</returns>
         [SuppressMessage("Microsoft.Performance", "CA1822:MarkMembersAsStatic", Justification = "For consistency, all helpers are instance methods.")]
         public HtmlString Raw(string value)
         {
             return new HtmlString(value);
         }
 
-        /// <summary>
-        /// Wraps HTML markup from the string representation of an object in an IHtmlString,
-        /// which will enable HTML markup to be rendered to the output without getting HTML encoded.
-        /// </summary>
-        /// <param name="value">object with string representation as HTML markup</param>
-        /// <returns>An IHtmlString that represents HTML markup.</returns>
         [SuppressMessage("Microsoft.Performance", "CA1822:MarkMembersAsStatic", Justification = "For consistency, all helpers are instance methods.")]
         public HtmlString Raw(object value)
         {

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

@@ -1,18 +1,40 @@
-using Microsoft.AspNet.Abstractions;
+using System;
+using Microsoft.AspNet.Abstractions;
 
 namespace Microsoft.AspNet.Mvc.Rendering
 {
-    public class HtmlHelper<TModel> : HtmlHelper
+    public class HtmlHelper<TModel> : HtmlHelper, IHtmlHelper<TModel>
     {
-        public HtmlHelper([NotNull]HttpContext httpContext, ViewDataDictionary<TModel> viewData)
+        /// <summary>
-            : base(httpContext, viewData)
+        /// Initializes a new instance of the <see cref="HtmlHelper{TModel}"/> class.
+        /// </summary>
+        public HtmlHelper()
+            : base()
         {
-            ViewData = viewData;
         }
 
-        public new ViewDataDictionary<TModel> ViewData
+        /// <inheritdoc />
+        public new ViewDataDictionary<TModel> ViewData { get; private set;}
+
+        public override void Contextualize([NotNull] ViewContext viewContext)
         {
-            get; private set;
+            if (viewContext.ViewData == null)
+            {
+                throw new ArgumentException(Resources.FormatArgumentPropertyNull("ViewData"), "viewContext");
+            }
+
+            ViewData = viewContext.ViewData as ViewDataDictionary<TModel>;
+            if (ViewData == null)
+            {
+                // viewContext may contain a base ViewDataDictionary instance. So complain about that type, not TModel.
+                throw new ArgumentException(Resources.FormatArgumentPropertyUnexpectedType(
+                        "ViewData",
+                        viewContext.ViewData.GetType().FullName,
+                        typeof(ViewDataDictionary<TModel>).FullName),
+                    "viewContext");
+            }
+
+            base.Contextualize(viewContext);
         }
     }
 }

src/Microsoft.AspNet.Mvc.Rendering/ICanHasViewContext.cs

@@ -0,0 +1,7 @@
+namespace Microsoft.AspNet.Mvc.Rendering
+{
+    public interface ICanHasViewContext
+    {
+        void Contextualize([NotNull] ViewContext viewContext);
+    }
+}

src/Microsoft.AspNet.Mvc.Rendering/IHtmlHelperOfT.cs

@@ -0,0 +1,68 @@
+using System;
+
+namespace Microsoft.AspNet.Mvc.Rendering
+{
+    /// <summary>
+    /// An <see cref="IHtmlHelper"/> for Linq expressions.
+    /// </summary>
+    /// <typeparam name="TModel">The <see cref="Type"/> of the model.</typeparam>
+    public interface IHtmlHelper<TModel>
+    {
+        /// <summary>
+        /// Gets or sets the character that replaces periods in the ID attribute of an element.
+        /// </summary>
+        string IdAttributeDotReplacement { get; set; }
+
+        /// <summary>
+        /// Gets the view bag.
+        /// </summary>
+        dynamic ViewBag { get; }
+
+        /// <summary>
+        /// Gets the context information about the view.
+        /// </summary>
+        ViewContext ViewContext { get; }
+
+        /// <summary>
+        /// Gets the current view data.
+        /// </summary>
+        ViewDataDictionary<TModel> ViewData { get; }
+
+        /// <summary>
+        /// Converts the value of the specified object to an HTML-encoded string.
+        /// </summary>
+        /// <param name="value">The object to encode.</param>
+        /// <returns>The HTML-encoded string.</returns>
+        string Encode(object value);
+
+        /// <summary>
+        /// Converts the specified string to an HTML-encoded string.
+        /// </summary>
+        /// <param name="value">The string to encode.</param>
+        /// <returns>The HTML-encoded string.</returns>
+        string Encode(string value);
+
+        /// <summary>
+        /// Creates an HTML element ID using the specified element name.
+        /// </summary>
+        /// <param name="name">The name of the HTML element.</param>
+        /// <returns>The ID of the HTML element.</returns>
+        string GenerateIdFromName(string name);
+
+        /// <summary>
+        /// Wraps HTML markup in an <see cref="HtmlString"/>, which will enable HTML markup to be
+        /// rendered to the output without getting HTML encoded.
+        /// </summary>
+        /// <param name="value">HTML markup string.</param>
+        /// <returns>An <see cref="HtmlString"/> that represents HTML markup.</returns>
+        HtmlString Raw(string value);
+
+        /// <summary>
+        /// Wraps HTML markup from the string representation of an object in an <see cref="HtmlString"/>,
+        /// which will enable HTML markup to be rendered to the output without getting HTML encoded.
+        /// </summary>
+        /// <param name="value">object with string representation as HTML markup.</param>
+        /// <returns>An <see cref="HtmlString"/> that represents HTML markup.</returns>
+        HtmlString Raw(object value);
+    }
+}

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

@@ -26,6 +26,38 @@ namespace Microsoft.AspNet.Mvc.Rendering
             return string.Format(CultureInfo.CurrentCulture, GetString("ArgumentNullOrEmpty"), p0);
         }
 
+        /// <summary>
+        /// Property '{0}' must not be null.
+        /// </summary>
+        internal static string ArgumentPropertyNull
+        {
+            get { return GetString("ArgumentPropertyNull"); }
+        }
+
+        /// <summary>
+        /// Property '{0}' must not be null.
+        /// </summary>
+        internal static string FormatArgumentPropertyNull(object p0)
+        {
+            return string.Format(CultureInfo.CurrentCulture, GetString("ArgumentPropertyNull"), p0);
+        }
+
+        /// <summary>
+        /// Property '{0}' is of type '{1}', but this method requires a value of type '{2}'.
+        /// </summary>
+        internal static string ArgumentPropertyUnexpectedType
+        {
+            get { return GetString("ArgumentPropertyUnexpectedType"); }
+        }
+
+        /// <summary>
+        /// Property '{0}' is of type '{1}', but this method requires a value of type '{2}'.
+        /// </summary>
+        internal static string FormatArgumentPropertyUnexpectedType(object p0, object p1, object p2)
+        {
+            return string.Format(CultureInfo.CurrentCulture, GetString("ArgumentPropertyUnexpectedType"), p0, p1, p2);
+        }
+
         /// <summary>
         /// The partial view '{0}' was not found or no view engine supports the searched locations. The following locations were searched:{1}
         /// </summary>
@@ -59,7 +91,23 @@ namespace Microsoft.AspNet.Mvc.Rendering
         }
 
         /// <summary>
-        /// The model item passed is null, but this ViewData instance requires a non-null model item of type '{0}'.
+        /// Must call 'Contextualize' method before using this HtmlHelper instance.
+        /// </summary>
+        internal static string HtmlHelper_NotContextualized
+        {
+            get { return GetString("HtmlHelper_NotContextualized"); }
+        }
+
+        /// <summary>
+        /// Must call 'Contextualize' method before using this HtmlHelper instance.
+        /// </summary>
+        internal static string FormatHtmlHelper_NotContextualized()
+        {
+            return GetString("HtmlHelper_NotContextualized");
+        }
+
+        /// <summary>
+        /// The model item passed is null, but this ViewDataDictionary instance requires a non-null model item of type '{0}'.
         /// </summary>
         internal static string ViewData_ModelCannotBeNull
         {
@@ -67,7 +115,7 @@ namespace Microsoft.AspNet.Mvc.Rendering
         }
 
         /// <summary>
-        /// The model item passed is null, but this ViewData instance requires a non-null model item of type '{0}'.
+        /// The model item passed is null, but this ViewDataDictionary instance requires a non-null model item of type '{0}'.
         /// </summary>
         internal static string FormatViewData_ModelCannotBeNull(object p0)
         {
@@ -75,7 +123,7 @@ namespace Microsoft.AspNet.Mvc.Rendering
         }
 
         /// <summary>
-        /// The model item passed into the ViewData is of type '{0}', but this ViewData instance requires a model item of type '{1}'.
+        /// The model item passed into the ViewDataDictionary is of type '{0}', but this ViewDataDictionary instance requires a model item of type '{1}'.
         /// </summary>
         internal static string ViewData_WrongTModelType
         {
@@ -83,7 +131,7 @@ namespace Microsoft.AspNet.Mvc.Rendering
         }
 
         /// <summary>
-        /// The model item passed into the ViewData is of type '{0}', but this ViewData instance requires a model item of type '{1}'.
+        /// The model item passed into the ViewDataDictionary is of type '{0}', but this ViewDataDictionary instance requires a model item of type '{1}'.
         /// </summary>
         internal static string FormatViewData_WrongTModelType(object p0, object p1)
         {

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

@@ -120,12 +120,21 @@
   <data name="ArgumentNullOrEmpty" xml:space="preserve">
     <value>The argument '{0}' is null or empty.</value>
   </data>
+  <data name="ArgumentPropertyNull" xml:space="preserve">
+    <value>Property '{0}' must not be null.</value>
+  </data>
+  <data name="ArgumentPropertyUnexpectedType" xml:space="preserve">
+    <value>Property '{0}' is of type '{1}', but this method requires a value of type '{2}'.</value>
+  </data>
   <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="HtmlHelper_NotContextualized" xml:space="preserve">
+    <value>Must call 'Contextualize' method before using this HtmlHelper instance.</value>
+  </data>
   <data name="ViewData_ModelCannotBeNull" xml:space="preserve">
     <value>The model item passed is null, but this ViewDataDictionary instance requires a non-null model item of type '{0}'.</value>
   </data>

src/Microsoft.AspNet.Mvc/MvcServices.cs

@@ -89,6 +89,12 @@ namespace Microsoft.AspNet.Mvc
                     implementationInstance: null,
                     lifecycle: LifecycleKind.Transient);
 
+            yield return
+                describe.Describe(
+                    typeof(IHtmlHelper<>),
+                    typeof(HtmlHelper<>),
+                    implementationInstance: null,
+                    lifecycle: LifecycleKind.Transient);
         }
     }
 }