Always render antiforgery tokens if !CanRenderAtEndOfForm

- #5005
- also add `FormContext` doc comments

src/Microsoft.AspNetCore.Mvc.ViewFeatures/ViewFeatures/DefaultHtmlGenerator.cs

@@ -156,15 +156,19 @@ public virtual IHtmlContent GenerateAntiforgery(ViewContext viewContext)
                 throw new ArgumentNullException(nameof(viewContext));
             }
 
-            // If we're inside a BeginForm/BeginRouteForm, the antiforgery token might have already been
-            // created and appended to the 'end form' content OR the form tag helper might have already generated
-            // an antiforgery token.
-            if (viewContext.FormContext.HasAntiforgeryToken)
+            var formContext = viewContext.FormContext;
+            if (formContext.CanRenderAtEndOfForm)
             {
-                return HtmlString.Empty;
-            }
+                // Inside a BeginForm/BeginRouteForm or a <form> tag helper. So, the antiforgery token might have
+                // already been created and appended to the 'end form' content (the AntiForgeryToken HTML helper does
+                // this) OR the <form> tag helper might have already generated an antiforgery token.
+                if (formContext.HasAntiforgeryToken)
+                {
+                    return HtmlString.Empty;
+                }
 
-            viewContext.FormContext.HasAntiforgeryToken = true;
+                formContext.HasAntiforgeryToken = true;
+            }
 
             return _antiforgery.GetHtml(viewContext.HttpContext);
         }

src/Microsoft.AspNetCore.Mvc.ViewFeatures/ViewFeatures/FormContext.cs

@@ -7,14 +7,21 @@
 
 namespace Microsoft.AspNetCore.Mvc.ViewFeatures
 {
+    /// <summary>
+    /// Collection of information about the current &lt;form&gt;.
+    /// </summary>
+    /// <remarks>
+    /// Literal &lt;form&gt; elements in a view will share the default <see cref="FormContext"/> instance unless tag
+    /// helpers are enabled.
+    /// </remarks>
     public class FormContext
     {
         private Dictionary<string, bool> _renderedFields;
         private Dictionary<string, object> _formData;
         private IList<IHtmlContent> _endOfFormContent;
 
         /// <summary>
-        /// Property bag for any information you wish to associate with a &lt;form/&gt; in an
+        /// Gets a property bag for any information you wish to associate with a &lt;form/&gt; in an
         /// <see cref="Rendering.IHtmlHelper"/> implementation or extension method.
         /// </summary>
         public IDictionary<string, object> FormData
@@ -30,12 +37,36 @@ public IDictionary<string, object> FormData
             }
         }
 
+        /// <summary>
+        /// Gets or sets an indication the current &lt;form&gt; element contains an antiforgery token. Do not use
+        /// unless <see cref="CanRenderAtEndOfForm"/> is <c>true</c>.
+        /// </summary>
+        /// <value>
+        /// <c>true</c> if the current &lt;form&gt; element contains an antiforgery token; <c>false</c> otherwise.
+        /// </value>
         public bool HasAntiforgeryToken { get; set; }
 
+        /// <summary>
+        /// Gets an indication the <see cref="FormData"/> property bag has been used and likely contains entries.
+        /// </summary>
+        /// <value>
+        /// <c>true</c> if the backing field for <see cref="FormData"/> is non-<c>null</c>; <c>false</c> otherwise.
+        /// </value>
         public bool HasFormData => _formData != null;
 
+        /// <summary>
+        /// Gets an indication the <see cref="EndOfFormContent"/> collection has been used and likely contains entries.
+        /// </summary>
+        /// <value>
+        /// <c>true</c> if the backing field for <see cref="EndOfFormContent"/> is non-<c>null</c>; <c>false</c>
+        /// otherwise.
+        /// </value>
         public bool HasEndOfFormContent => _endOfFormContent != null;
 
+        /// <summary>
+        /// Gets an <see cref="IHtmlContent"/> collection that should be rendered just prior to the next &lt;/form&gt;
+        /// end tag. Do not use unless <see cref="CanRenderAtEndOfForm"/> is <c>true</c>.
+        /// </summary>
         public IList<IHtmlContent> EndOfFormContent
         {
             get
@@ -49,8 +80,23 @@ public IList<IHtmlContent> EndOfFormContent
             }
         }
 
+        /// <summary>
+        /// Gets or sets an indication the current &lt;form&gt; is associated with a tag helper or will be generated by
+        /// an HTML helper. In particular, <see cref="EndOfFormContent"/> will be rendered just prior to the next
+        /// &lt;/form&gt; end tag.
+        /// </summary>
+        /// <value>
+        /// <c>true</c> if the framework will render <see cref="EndOfFormContent"/>; <c>false</c> otherwise.
+        /// </value>
+        /// <remarks>
+        /// <c>true</c> for all framework-created <see cref="FormContext"/> instances except the default.
+        /// </remarks>
         public bool CanRenderAtEndOfForm { get; set; }
 
+        /// <summary>
+        /// Gets a dictionary mapping full HTML field names to indications that the named field has been rendered in
+        /// this &lt;form&gt;.
+        /// </summary>
         private Dictionary<string, bool> RenderedFields
         {
             get
@@ -64,6 +110,14 @@ private Dictionary<string, bool> RenderedFields
             }
         }
 
+        /// <summary>
+        /// Returns an indication based on <see cref="RenderedFields"/> that the given <paramref name="fieldName"/> has
+        /// been rendered in this &lt;form&gt;.
+        /// </summary>
+        /// <param name="fieldName">The full HTML name of a field that may have been rendered.</param>
+        /// <returns>
+        /// <c>true</c> if the given <paramref name="fieldName"/> has been rendered; <c>false</c> otherwise.
+        /// </returns>
         public bool RenderedField(string fieldName)
         {
             if (fieldName == null)
@@ -77,6 +131,12 @@ public bool RenderedField(string fieldName)
             return result;
         }
 
+        /// <summary>
+        /// Updates <see cref="RenderedFields"/> to indicate <paramref name="fieldName"/> has been rendered in this
+        /// &lt;form&gt;.
+        /// </summary>
+        /// <param name="fieldName">The full HTML name of a field that may have been rendered.</param>
+        /// <param name="value">If <c>true</c>, the given <paramref name="fieldName"/> has been rendered.</param>
         public void RenderedField(string fieldName, bool value)
         {
             if (fieldName == null)

test/Microsoft.AspNetCore.Mvc.ViewFeatures.Test/ViewFeatures/DefaultHtmlGeneratorTest.cs

@@ -641,6 +641,7 @@ public void GenerateAntiforgery_GeneratesAntiforgeryFieldsOnlyIfRequired(
             var metadataProvider = TestModelMetadataProvider.CreateDefaultProvider();
             var htmlGenerator = GetGenerator(metadataProvider);
             var viewContext = GetViewContext<Model>(model: null, metadataProvider: metadataProvider);
+            viewContext.FormContext.CanRenderAtEndOfForm = true;
             viewContext.FormContext.HasAntiforgeryToken = hasAntiforgeryToken;
 
             // Act
@@ -651,6 +652,26 @@ public void GenerateAntiforgery_GeneratesAntiforgeryFieldsOnlyIfRequired(
             Assert.Equal(expectedAntiforgeryHtmlField, antiforgeryField);
         }
 
+        [Theory]
+        [InlineData(false)]
+        [InlineData(true)]
+        public void GenerateAntiforgery_AlwaysGeneratesAntiforgeryToken_IfCannotRenderAtEnd(bool hasAntiforgeryToken)
+        {
+            // Arrange
+            var expected = "<input name=\"formFieldName\" type=\"hidden\" value=\"requestToken\" />";
+            var metadataProvider = TestModelMetadataProvider.CreateDefaultProvider();
+            var htmlGenerator = GetGenerator(metadataProvider);
+            var viewContext = GetViewContext<Model>(model: null, metadataProvider: metadataProvider);
+            viewContext.FormContext.HasAntiforgeryToken = hasAntiforgeryToken;
+
+            // Act
+            var result = htmlGenerator.GenerateAntiforgery(viewContext);
+
+            // Assert
+            var antiforgeryField = HtmlContentUtilities.HtmlContentToString(result, HtmlEncoder.Default);
+            Assert.Equal(expected, antiforgeryField);
+        }
+
         // GetCurrentValues uses only the IModelMetadataProvider passed to the DefaultHtmlGenerator constructor.
         private static IHtmlGenerator GetGenerator(IModelMetadataProvider metadataProvider)
         {