Add `ModelMetadata.HtmlEncode` property

- use new `ModelMetadata.HtmlEncode` property in HTML helpers
 - specifically in default HTML display and editor object templates (e.g.
   `@Html.DisplayFor()`) when value is non-`null` and the template is invoked
   with template depth greater than 1
- similar to MVC 5.2 commit [2b12791aee4f](https://aspnetwebstack.codeplex.com/SourceControl/changeset/2b12791aee4ffc56c7928b623bb45ee425813021)

nits:
- remove dupe `null` check in `DefaultDisplayTemplates.ObjectTemplate()`
- move backing fields initialized with constants together in `ModelMetadata`

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

@@ -213,7 +213,13 @@ namespace Microsoft.AspNet.Mvc.Rendering
 
             if (templateInfo.TemplateDepth > 1)
             {
-                return modelMetadata.Model == null ? modelMetadata.NullDisplayText : modelMetadata.SimpleDisplayText;
+                var text = modelMetadata.SimpleDisplayText;
+                if (modelMetadata.HtmlEncode)
+                {
+                    text = html.Encode(text);
+                }
+
+                return text;
             }
 
             var serviceProvider = html.ViewContext.HttpContext.RequestServices;

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

@@ -232,7 +232,18 @@ namespace Microsoft.AspNet.Mvc.Rendering
 
             if (templateInfo.TemplateDepth > 1)
             {
-                return modelMetadata.Model == null ? modelMetadata.NullDisplayText : modelMetadata.SimpleDisplayText;
+                if (modelMetadata.Model == null)
+                {
+                    return modelMetadata.NullDisplayText;
+                }
+
+                var text = modelMetadata.SimpleDisplayText;
+                if (modelMetadata.HtmlEncode)
+                {
+                    text = html.Encode(text);
+                }
+
+                return text;
             }
 
             var serviceProvider = html.ViewContext.HttpContext.RequestServices;

src/Microsoft.AspNet.Mvc.ModelBinding/Metadata/CachedDataAnnotationsModelMetadata.cs

@@ -205,6 +205,25 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
             return base.ComputeHideSurroundingHtml();
         }
 
+        /// <summary>
+        /// Calculate <see cref="ModelMetadata.HtmlEncode"/> based on presence of a
+        /// <see cref="DisplayFormatAttribute"/> and its <see cref="DisplayFormatAttribute.HtmlEncode"/> value.
+        /// </summary>
+        /// <returns>
+        /// Calculated <see cref="ModelMetadata.HtmlEncode"/> value. <c>false</c> if a
+        /// <see cref="DisplayFormatAttribute"/> exists and its <see cref="DisplayFormatAttribute.HtmlEncode"/> value
+        /// is <c>false</c>. <c>true</c> otherwise.
+        /// </returns>
+        protected override bool ComputeHtmlEncode()
+        {
+            if (PrototypeCache.DisplayFormat != null)
+            {
+                return PrototypeCache.DisplayFormat.HtmlEncode;
+            }
+
+            return base.ComputeHtmlEncode();
+        }
+
         protected override bool ComputeIsReadOnly()
         {
             if (PrototypeCache.Editable != null)

src/Microsoft.AspNet.Mvc.ModelBinding/Metadata/CachedModelMetadata.cs

@@ -24,6 +24,7 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
         private string _editFormatString;
         private bool _hasNonDefaultEditFormat;
         private bool _hideSurroundingHtml;
+        private bool _htmlEncode;
         private bool _isReadOnly;
         private bool _isComplexType;
         private bool _isRequired;
@@ -39,6 +40,7 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
         private bool _editFormatStringComputed;
         private bool _hasNonDefaultEditFormatComputed;
         private bool _hideSurroundingHtmlComputed;
+        private bool _htmlEncodeComputed;
         private bool _isReadOnlyComputed;
         private bool _isComplexTypeComputed;
         private bool _isRequiredComputed;
@@ -252,6 +254,27 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
             }
         }
 
+        /// <inheritdoc />
+        public sealed override bool HtmlEncode
+        {
+            get
+            {
+                if (!_htmlEncodeComputed)
+                {
+                    _htmlEncode = ComputeHtmlEncode();
+                    _htmlEncodeComputed = true;
+                }
+
+                return _htmlEncode;
+            }
+
+            set
+            {
+                _htmlEncode = value;
+                _htmlEncodeComputed = true;
+            }
+        }
+
         public sealed override bool IsReadOnly
         {
             get
@@ -419,6 +442,15 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
             return base.HideSurroundingHtml;
         }
 
+        /// <summary>
+        /// Calculate the <see cref="HtmlEncode"/> value.
+        /// </summary>
+        /// <returns>Calculated <see cref="HtmlEncode"/> value.</returns>
+        protected virtual bool ComputeHtmlEncode()
+        {
+            return base.HtmlEncode;
+        }
+
         protected virtual bool ComputeIsReadOnly()
         {
             return base.IsReadOnly;

src/Microsoft.AspNet.Mvc.ModelBinding/Metadata/ModelMetadata.cs

@@ -19,17 +19,18 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
         private EfficientTypePropertyKey<Type, string> _cacheKey;
 
         // Backing fields for virtual properties with default values.
-        private bool _convertEmptyStringToNull;
+        private bool _convertEmptyStringToNull = true;
-        private bool _isRequired;
+        private bool _htmlEncode = true;
+        private bool _showForDisplay = true;
+        private bool _showForEdit = true;
 
         private object _model;
         private Func<object> _modelAccessor;
         private int _order = DefaultOrder;
+        private bool _isRequired;
         private IEnumerable<ModelMetadata> _properties;
         private Type _realModelType;
         private string _simpleDisplayText;
-        private bool _showForDisplay = true;
-        private bool _showForEdit = true;
 
         public ModelMetadata([NotNull] IModelMetadataProvider provider,
                              Type containerType,
@@ -43,7 +44,6 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
             _modelAccessor = modelAccessor;
             _modelType = modelType;
             _propertyName = propertyName;
-            _convertEmptyStringToNull = true;
             _isRequired = !modelType.AllowsNullValue();
         }
 
@@ -111,6 +111,16 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
         /// </summary>
         public virtual bool HasNonDefaultEditFormat { get; set; }
 
+        /// <summary>
+        /// Gets or sets a value indicating whether the value should be HTML-encoded.
+        /// </summary>
+        /// <value>If <c>true</c>, value should be HTML-encoded. Default is <c>true</c>.</value>
+        public virtual bool HtmlEncode
+        {
+            get { return _htmlEncode; }
+            set { _htmlEncode = value; }
+        }
+
         /// <summary>
         /// Gets or sets a value indicating whether the "HiddenInput" display template should return
         /// <c>string.Empty</c> (not the expression value) and whether the "HiddenInput" editor template should not

test/Microsoft.AspNet.Mvc.Core.Test/Rendering/DefaultDisplayTemplatesTests.cs

@@ -2,7 +2,6 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
-using System.Collections.Generic;
 using System.Linq;
 using System.Threading.Tasks;
 using Microsoft.AspNet.Mvc.ModelBinding;
@@ -14,6 +13,33 @@ namespace Microsoft.AspNet.Mvc.Core
 {
     public class DefaultDisplayTemplateTests
     {
+        // Input value; HTML encode; expected value.
+        public static TheoryData<string, bool, string> HtmlEncodeData
+        {
+            get
+            {
+                return new TheoryData<string, bool, string>
+                {
+                    { "Simple Display Text", false, "Simple Display Text" },
+                    { "Simple Display Text", true, "Simple Display Text" },
+                    { "<blink>text</blink>", false, "<blink>text</blink>" },
+                    { "<blink>text</blink>", true, "&lt;blink&gt;text&lt;/blink&gt;" },
+                    { "&'\"", false, "&'\"" },
+                    { "&'\"", true, "&amp;&#39;&quot;" },
+                    { " ¡ÿĀ", false, " ¡ÿĀ" },                                           // high ASCII
+                    { " ¡ÿĀ", true, "&#160;&#161;&#255;Ā" },
+                    { "Chinese西雅图Chars", false, "Chinese西雅图Chars" },
+                    { "Chinese西雅图Chars", true, "Chinese西雅图Chars" },
+                    { "Unicode؃Format؃Char", false, "Unicode؃Format؃Char" },            // class Cf
+                    { "Unicode؃Format؃Char", true, "Unicode؃Format؃Char" },
+                    { "UnicodeῼTitlecaseῼChar", false, "UnicodeῼTitlecaseῼChar" },       // class Lt
+                    { "UnicodeῼTitlecaseῼChar", true, "UnicodeῼTitlecaseῼChar" },
+                    { "UnicodeःCombiningःChar", false, "UnicodeःCombiningःChar" },    // class Mc
+                    { "UnicodeःCombiningःChar", true, "UnicodeःCombiningःChar" },
+                };
+            }
+        }
+
         [Fact]
         public void ObjectTemplateDisplaysSimplePropertiesOnObjectByDefault()
         {
@@ -54,8 +80,12 @@ namespace Microsoft.AspNet.Mvc.Core
             Assert.Equal(metadata.NullDisplayText, result);
         }
 
-        [Fact]
+        [Theory]
-        public void ObjectTemplateDisplaysSimpleDisplayTextWhenTemplateDepthGreaterThanOne()
+        [MemberData(nameof(HtmlEncodeData))]
+        public void ObjectTemplateDisplaysSimpleDisplayTextWhenTemplateDepthGreaterThanOne(
+            string simpleDisplayText,
+            bool htmlEncode,
+            string expectedResult)
         {
             // Arrange
             var model = new DefaultTemplatesUtilities.ObjectTemplateModel();
@@ -63,7 +93,8 @@ namespace Microsoft.AspNet.Mvc.Core
             var metadata =
                 new EmptyModelMetadataProvider()
                     .GetMetadataForType(() => model, typeof(DefaultTemplatesUtilities.ObjectTemplateModel));
-            metadata.SimpleDisplayText = "Simple Display Text";
+            metadata.HtmlEncode = htmlEncode;
+            metadata.SimpleDisplayText = simpleDisplayText;
             html.ViewData.ModelMetadata = metadata;
             html.ViewData.TemplateInfo.AddVisited("foo");
             html.ViewData.TemplateInfo.AddVisited("bar");
@@ -72,7 +103,7 @@ namespace Microsoft.AspNet.Mvc.Core
             var result = DefaultDisplayTemplates.ObjectTemplate(html);
 
             // Assert
-            Assert.Equal(metadata.SimpleDisplayText, result);
+            Assert.Equal(expectedResult, result);
         }
 
         [Fact]

test/Microsoft.AspNet.Mvc.Core.Test/Rendering/DefaultEditorTemplatesTests.cs

@@ -115,8 +115,12 @@ namespace Microsoft.AspNet.Mvc.Core
             Assert.Equal(metadata.NullDisplayText, result);
         }
 
-        [Fact]
+        [Theory]
-        public void ObjectTemplateDisplaysSimpleDisplayTextWithNonNullModelTemplateDepthGreaterThanOne()
+        [MemberData(nameof(DefaultDisplayTemplateTests.HtmlEncodeData), MemberType = typeof(DefaultDisplayTemplateTests))]
+        public void ObjectTemplateDisplaysSimpleDisplayTextWithNonNullModelTemplateDepthGreaterThanOne(
+            string simpleDisplayText,
+            bool htmlEncode,
+            string expectedResult)
         {
             // Arrange
             var model = new DefaultTemplatesUtilities.ObjectTemplateModel();
@@ -124,9 +128,10 @@ namespace Microsoft.AspNet.Mvc.Core
             var metadata =
                 new EmptyModelMetadataProvider()
                     .GetMetadataForType(() => model, typeof(DefaultTemplatesUtilities.ObjectTemplateModel));
-            html.ViewData.ModelMetadata = metadata;
+            metadata.HtmlEncode = htmlEncode;
             metadata.NullDisplayText = "Null Display Text";
-            metadata.SimpleDisplayText = "Simple Display Text";
+            metadata.SimpleDisplayText = simpleDisplayText;
+            html.ViewData.ModelMetadata = metadata;
             html.ViewData.TemplateInfo.AddVisited("foo");
             html.ViewData.TemplateInfo.AddVisited("bar");
 
@@ -134,7 +139,7 @@ namespace Microsoft.AspNet.Mvc.Core
             var result = DefaultEditorTemplates.ObjectTemplate(html);
 
             // Assert
-            Assert.Equal(metadata.SimpleDisplayText, result);
+            Assert.Equal(expectedResult, result);
         }
 
         [Fact]

test/Microsoft.AspNet.Mvc.ModelBinding.Test/Metadata/CachedDataAnnotationsModelMetadataTest.cs

@@ -32,6 +32,7 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
             Assert.True(metadata.ConvertEmptyStringToNull);
             Assert.False(metadata.HasNonDefaultEditFormat);
             Assert.False(metadata.HideSurroundingHtml);
+            Assert.True(metadata.HtmlEncode);
             Assert.True(metadata.IsComplexType);
             Assert.False(metadata.IsReadOnly);
             Assert.False(metadata.IsRequired);
@@ -157,6 +158,16 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
                         metadata => metadata.HasNonDefaultEditFormat,
                         true
                     },
+                    {
+                        new DisplayFormatAttribute { HtmlEncode = false },
+                        metadata => metadata.HtmlEncode,
+                        false
+                    },
+                    {
+                        new DisplayFormatAttribute { HtmlEncode = true },
+                        metadata => metadata.HtmlEncode,
+                        true
+                    },
                     {
                         new EditableAttribute(allowEdit: false),
                         metadata => metadata.IsReadOnly,

test/Microsoft.AspNet.Mvc.ModelBinding.Test/Metadata/ModelMetadataTest.cs

@@ -22,6 +22,7 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
                     { m => m.ConvertEmptyStringToNull = false, m => m.ConvertEmptyStringToNull, false },
                     { m => m.HasNonDefaultEditFormat = true, m => m.HasNonDefaultEditFormat, true },
                     { m => m.HideSurroundingHtml = true, m => m.HideSurroundingHtml, true },
+                    { m => m.HtmlEncode = false, m => m.HtmlEncode, false },
                     { m => m.IsReadOnly = true, m => m.IsReadOnly, true },
                     { m => m.IsRequired = true, m => m.IsRequired, true },
                     { m => m.ShowForDisplay = false, m => m.ShowForDisplay, false },
@@ -59,6 +60,7 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
             Assert.True(metadata.ConvertEmptyStringToNull);
             Assert.False(metadata.HasNonDefaultEditFormat);
             Assert.False(metadata.HideSurroundingHtml);
+            Assert.True(metadata.HtmlEncode);
             Assert.False(metadata.IsComplexType);
             Assert.False(metadata.IsNullableValueType);
             Assert.False(metadata.IsReadOnly);