Fix for #215 - nullref trying to generate link in a partial

The fix here is to do some cleanup we've been planning to do for a while,
rather than flowing IUrlHelper and IComponentHelper as part of the
ViewContext, they now are just grabbed from the service provider.

This simplifies the code for invoking a view, and gets us closer to the
desired API surface of ViewContext

samples/MvcSample.Web/Views/Shared/HelloWorldPartial.cshtml

@@ -2,3 +2,5 @@
 @model User
 
 <strong>Hello @Model.Name from Partial</strong>
+
+<a href="@Url.Action("Edit", "Home")">Edit Something!</a>

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

@@ -51,16 +51,10 @@ namespace Microsoft.AspNet.Mvc
 
             var viewContext = new ViewContext(_serviceProvider, actionContext.HttpContext, actionContext.RouteValues)
             {
-                Url = urlHelper,
                 ViewData = ViewData,
                 Writer = writer,
             };
 
-            viewContext.Component = new DefaultViewComponentHelper(
-                _serviceProvider.GetService<IViewComponentSelector>(),
-                _serviceProvider.GetService<IViewComponentInvokerFactory>(),
-                viewContext);
-
             return viewContext;
         }
     }

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

@@ -28,16 +28,23 @@ namespace Microsoft.AspNet.Mvc.Rendering
 
         private const string HiddenListItem = @"<li style=""display:none""></li>";
 
+        private readonly IUrlHelper _urlHelper;
+        private readonly IViewEngine _viewEngine;
+
         private ViewContext _viewContext;
-        private IViewEngine _viewEngine;
+        
 
         /// <summary>
         /// Initializes a new instance of the <see cref="HtmlHelper"/> class.
         /// </summary>
-        public HtmlHelper([NotNull] IViewEngine viewEngine, [NotNull] IModelMetadataProvider metadataProvider)
+        public HtmlHelper(
+            [NotNull] IViewEngine viewEngine, 
+            [NotNull] IModelMetadataProvider metadataProvider,
+            [NotNull] IUrlHelper urlHelper)
         {
             _viewEngine = viewEngine;
             MetadataProvider = metadataProvider;
+            _urlHelper = urlHelper;
 
             // Underscores are fine characters in id's.
             IdAttributeDotReplacement = "_";
@@ -445,8 +452,7 @@ namespace Microsoft.AspNet.Mvc.Rendering
         protected virtual MvcForm GenerateForm(string actionName, string controllerName, object routeValues,
                                                FormMethod method, IDictionary<string, object> htmlAttributes)
         {
-            var urlHelper = ViewContext.Url;
+            var formAction = _urlHelper.Action(action: actionName, controller: controllerName, values: routeValues);
-            var formAction = urlHelper.Action(action: actionName, controller: controllerName, values: routeValues);
 
             var tagBuilder = new TagBuilder("form");
             tagBuilder.MergeAttributes(htmlAttributes);

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

@@ -13,8 +13,11 @@ namespace Microsoft.AspNet.Mvc.Rendering
         /// <summary>
         /// Initializes a new instance of the <see cref="HtmlHelper{TModel}"/> class.
         /// </summary>
-        public HtmlHelper([NotNull] IViewEngine viewEngine, [NotNull] IModelMetadataProvider metadataProvider)
+        public HtmlHelper(
-             : base(viewEngine, metadataProvider)
+            [NotNull] IViewEngine viewEngine, 
+            [NotNull] IModelMetadataProvider metadataProvider,
+            [NotNull] IUrlHelper urlHelper)
+             : base(viewEngine, metadataProvider, urlHelper)
         {
         }
 

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

@@ -8,19 +8,22 @@ using Microsoft.AspNet.Mvc.Rendering;
 
 namespace Microsoft.AspNet.Mvc
 {
-    public class DefaultViewComponentHelper : IViewComponentHelper
+    public class DefaultViewComponentHelper : IViewComponentHelper, ICanHasViewContext
     {
         private readonly IViewComponentInvokerFactory _invokerFactory;
         private readonly IViewComponentSelector _selector;
-        private readonly ViewContext _viewContext;
+        private ViewContext _viewContext;
 
         public DefaultViewComponentHelper(
             [NotNull] IViewComponentSelector selector,
-            [NotNull] IViewComponentInvokerFactory invokerFactory,
+            [NotNull] IViewComponentInvokerFactory invokerFactory)
-            [NotNull] ViewContext viewContext)
         {
             _selector = selector;
             _invokerFactory = invokerFactory;
+        }
+
+        public void Contextualize([NotNull] ViewContext viewContext)
+        {
             _viewContext = viewContext;
         }
 

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

@@ -36,8 +36,6 @@ namespace Microsoft.AspNet.Mvc
                 context.ViewContext.HttpContext,
                 context.ViewContext.ViewEngineContext)
             {
-                Component = context.ViewContext.Component,
-                Url = context.ViewContext.Url,
                 ViewData = _viewData ?? context.ViewContext.ViewData,
                 Writer = context.Writer,
             };

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

@@ -33,8 +33,6 @@ namespace Microsoft.AspNet.Mvc
             ClientValidationEnabled = true;
         }
 
-        public IViewComponentHelper Component { get; set; }
-
         public virtual FormContext FormContext
         {
             get
@@ -52,8 +50,6 @@ namespace Microsoft.AspNet.Mvc
 
         public IServiceProvider ServiceProvider { get; private set; }
 
-        public IUrlHelper Url { get; set; }
-
         public bool UnobtrusiveJavaScriptEnabled { get; set; }
 
         public bool ClientValidationEnabled { get; set; }

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

@@ -15,21 +15,13 @@ namespace Microsoft.AspNet.Mvc.Razor
         private readonly HashSet<string> _renderedSections = new HashSet<string>(StringComparer.OrdinalIgnoreCase);
         private bool _renderedBody;
 
-        public IViewComponentHelper Component
-        {
-            get { return Context == null ? null : Context.Component; }
-        }
-
         public ViewContext Context { get; set; }
 
         public string Layout { get; set; }
 
         protected TextWriter Output { get; set; }
 
-        public IUrlHelper Url
+        public IUrlHelper Url { get; private set; }
-        {
-            get { return Context == null ? null : Context.Url; }
-        }
 
         public dynamic ViewBag
         {
@@ -50,6 +42,8 @@ namespace Microsoft.AspNet.Mvc.Razor
             SectionWriters = new Dictionary<string, HelperResult>(StringComparer.OrdinalIgnoreCase);
             Context = context;
 
+            Url = context.ServiceProvider.GetService<IUrlHelper>();
+
             var contentBuilder = new StringBuilder(1024);
             using (var bodyWriter = new StringWriter(contentBuilder))
             {

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

@@ -18,6 +18,8 @@ namespace Microsoft.AspNet.Mvc.Razor
 
         public ViewDataDictionary<TModel> ViewData { get; private set; }
 
+        public IViewComponentHelper Component { get; private set; }
+
         public IHtmlHelper<TModel> Html { get; set; }
 
         public override Task RenderAsync([NotNull] ViewContext context)
@@ -53,6 +55,14 @@ namespace Microsoft.AspNet.Mvc.Razor
             {
                 contextable.Contextualize(context);
             }
+
+            Component = context.ServiceProvider.GetService<IViewComponentHelper>();
+
+            contextable = Component as ICanHasViewContext;
+            if (contextable != null)
+            {
+                contextable.Contextualize(context);
+            }
         }
     }
 }

src/Microsoft.AspNet.Mvc/MvcServices.cs

@@ -74,6 +74,7 @@ namespace Microsoft.AspNet.Mvc
             yield return describe.Transient<IViewComponentInvokerFactory, DefaultViewComponentInvokerFactory>();
             yield return describe.Transient<INestedProvider<ViewComponentInvokerProviderContext>, DefaultViewComponentInvokerProvider>();
             yield return describe.Transient<IViewComponentResultHelper, DefaultViewComponentResultHelper>();
+            yield return describe.Transient<IViewComponentHelper, DefaultViewComponentHelper>();
 
             yield return
                describe.Describe(