Store `GetFullHtmlFieldName()` and `CreateSanitizedId()` values

- #3918
- don't repeat allocations for identical calls; helps w/ e.g. label / input / validation clusters
- add `NameAndIdProvider`; it stores `PreviousNameAndId` in `HttpContext.Items`
 - `PreviousNameAndId` allocated only when `string`s are allocated e.g. "id"s were sanitized

src/Microsoft.AspNetCore.Mvc.ViewFeatures/Internal/NameAndIdProvider.cs

@@ -0,0 +1,214 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using Microsoft.AspNetCore.Mvc.Rendering;
+
+namespace Microsoft.AspNetCore.Mvc.ViewFeatures.Internal
+{
+    /// <summary>
+    /// Provides cached values for "name" and "id" HTML attributes.
+    /// </summary>
+    public static class NameAndIdProvider
+    {
+        private static readonly object PreviousNameAndIdKey = typeof(PreviousNameAndId);
+
+        /// <summary>
+        /// Returns a valid HTML 4.01 "id" attribute value for an element with the given <paramref name="fullName"/>.
+        /// </summary>
+        /// <param name="viewContext">A <see cref="ViewContext"/> instance for the current scope.</param>
+        /// <param name="fullName">
+        /// The fully-qualified expression name, ignoring the current model. Also the original HTML element name.
+        /// </param>
+        /// <param name="invalidCharReplacement">
+        /// The <see cref="string"/> (normally a single <see cref="char"/>) to substitute for invalid characters in
+        /// <paramref name="fullName"/>.
+        /// </param>
+        /// <returns>
+        /// Valid HTML 4.01 "id" attribute value for an element with the given <paramref name="fullName"/>.
+        /// </returns>
+        /// <remarks>
+        /// Similar to <see cref="TagBuilder.CreateSanitizedId"/> but caches value for repeated invocations.
+        /// </remarks>
+        public static string CreateSanitizedId(ViewContext viewContext, string fullName, string invalidCharReplacement)
+        {
+            if (viewContext == null)
+            {
+                throw new ArgumentNullException(nameof(viewContext));
+            }
+
+            if (invalidCharReplacement == null)
+            {
+                throw new ArgumentNullException(nameof(invalidCharReplacement));
+            }
+
+            if (string.IsNullOrEmpty(fullName))
+            {
+                return string.Empty;
+            }
+
+            // Check cache to avoid whatever TagBuilder.CreateSanitizedId() may do.
+            var items = viewContext.HttpContext.Items;
+            object previousNameAndIdObject;
+            PreviousNameAndId previousNameAndId = null;
+            if (items.TryGetValue(PreviousNameAndIdKey, out previousNameAndIdObject) &&
+                (previousNameAndId = (PreviousNameAndId)previousNameAndIdObject) != null &&
+                string.Equals(previousNameAndId.FullName, fullName, StringComparison.Ordinal))
+            {
+                return previousNameAndId.SanitizedId;
+            }
+
+            var sanitizedId = TagBuilder.CreateSanitizedId(fullName, invalidCharReplacement);
+
+            if (previousNameAndId == null)
+            {
+                // Do not create a PreviousNameAndId when TagBuilder.CreateSanitizedId() only examined fullName.
+                if (string.Equals(fullName, sanitizedId, StringComparison.Ordinal))
+                {
+                    return sanitizedId;
+                }
+
+                previousNameAndId = new PreviousNameAndId();
+                items[PreviousNameAndIdKey] = previousNameAndId;
+            }
+
+            previousNameAndId.FullName = fullName;
+            previousNameAndId.SanitizedId = sanitizedId;
+
+            return previousNameAndId.SanitizedId;
+        }
+
+        /// <summary>
+        /// Adds a valid HTML 4.01 "id" attribute for an element with the given <paramref name="fullName"/>. Does
+        /// nothing if <see cref="TagBuilder.Attributes"/> already contains an "id" attribute or the
+        /// <paramref name="fullName"/> is <c>null</c> or empty.
+        /// </summary>
+        /// <param name="viewContext">A <see cref="ViewContext"/> instance for the current scope.</param>
+        /// <param name="tagBuilder">A <see cref="TagBuilder"/> instance that will contain the "id" attribute.</param>
+        /// <param name="fullName">
+        /// The fully-qualified expression name, ignoring the current model. Also the original HTML element name.
+        /// </param>
+        /// <param name="invalidCharReplacement">
+        /// The <see cref="string"/> (normally a single <see cref="char"/>) to substitute for invalid characters in
+        /// <paramref name="fullName"/>.
+        /// </param>
+        /// <remarks>
+        /// Similar to <see cref="TagBuilder.GenerateId"/> but caches value for repeated invocations.
+        /// </remarks>
+        /// <seealso cref="CreateSanitizedId"/>
+        public static void GenerateId(
+            ViewContext viewContext,
+            TagBuilder tagBuilder,
+            string fullName,
+            string invalidCharReplacement)
+        {
+            if (viewContext == null)
+            {
+                throw new ArgumentNullException(nameof(viewContext));
+            }
+
+            if (tagBuilder == null)
+            {
+                throw new ArgumentNullException(nameof(tagBuilder));
+            }
+
+            if (invalidCharReplacement == null)
+            {
+                throw new ArgumentNullException(nameof(invalidCharReplacement));
+            }
+
+            if (string.IsNullOrEmpty(fullName))
+            {
+                return;
+            }
+
+            if (!tagBuilder.Attributes.ContainsKey("id"))
+            {
+                var sanitizedId = CreateSanitizedId(viewContext, fullName, invalidCharReplacement);
+
+                // Duplicate check for null or empty to cover the corner case where fullName contains only invalid
+                // characters and invalidCharReplacement is empty.
+                if (!string.IsNullOrEmpty(sanitizedId))
+                {
+                    tagBuilder.Attributes["id"] = sanitizedId;
+                }
+            }
+        }
+
+        /// <summary>
+        /// Returns the full HTML element name for the specified <paramref name="expression"/>.
+        /// </summary>
+        /// <param name="viewContext">A <see cref="ViewContext"/> instance for the current scope.</param>
+        /// <param name="expression">Expression name, relative to the current model.</param>
+        /// <returns>Fully-qualified expression name for <paramref name="expression"/>.</returns>
+        /// <remarks>
+        /// Similar to <see cref="TemplateInfo.GetFullHtmlFieldName"/> but caches value for repeated invocations.
+        /// </remarks>
+        public static string GetFullHtmlFieldName(ViewContext viewContext, string expression)
+        {
+            var htmlFieldPrefix = viewContext.ViewData.TemplateInfo.HtmlFieldPrefix;
+            if (string.IsNullOrEmpty(expression))
+            {
+                return htmlFieldPrefix;
+            }
+
+            if (string.IsNullOrEmpty(htmlFieldPrefix))
+            {
+                return expression;
+            }
+
+            // Need to concatenate. See if we've already done that.
+            var items = viewContext.HttpContext.Items;
+            object previousNameAndIdObject;
+            PreviousNameAndId previousNameAndId = null;
+            if (items.TryGetValue(PreviousNameAndIdKey, out previousNameAndIdObject) &&
+                (previousNameAndId = (PreviousNameAndId)previousNameAndIdObject) != null &&
+                string.Equals(previousNameAndId.HtmlFieldPrefix, htmlFieldPrefix, StringComparison.Ordinal) &&
+                string.Equals(previousNameAndId.Expression, expression, StringComparison.Ordinal))
+            {
+                return previousNameAndId.OutputFullName;
+            }
+
+            if (previousNameAndId == null)
+            {
+                previousNameAndId = new PreviousNameAndId();
+                items[PreviousNameAndIdKey] = previousNameAndId;
+            }
+
+            previousNameAndId.HtmlFieldPrefix = htmlFieldPrefix;
+            previousNameAndId.Expression = expression;
+            if (expression.StartsWith("[", StringComparison.Ordinal))
+            {
+                // The expression might represent an indexer access, in which case  with a 'dot' would be invalid.
+                previousNameAndId.OutputFullName = htmlFieldPrefix + expression;
+            }
+            else
+            {
+                previousNameAndId.OutputFullName = htmlFieldPrefix + "." + expression;
+            }
+
+            return previousNameAndId.OutputFullName;
+        }
+
+        private class PreviousNameAndId
+        {
+            // Cached ambient input for NameAndIdProvider.GetFullHtmlFieldName(). TemplateInfo.HtmlFieldPrefix may
+            // change during the lifetime of a ViewContext.
+            public string HtmlFieldPrefix { get; set; }
+
+            // Cached input for NameAndIdProvider.GetFullHtmlFieldName().
+            public string Expression { get; set; }
+
+            // Cached return value for NameAndIdProvider.GetFullHtmlFieldName().
+            public string OutputFullName { get; set; }
+
+            // Cached input for NameAndIdProvider.CreateSanitizedId(). Since IHtmlHelper.GenerateIdFromName() is
+            // available to all, there is no guarantee this is equal to OutputFullName when CreateSanitizedId() is
+            // called.
+            public string FullName { get; set; }
+
+            // Cached return value for NameAndIdProvider.CreateSanitizedId().
+            public string SanitizedId { get; set; }
+        }
+    }
+}

src/Microsoft.AspNetCore.Mvc.ViewFeatures/Rendering/TagBuilder.cs

@@ -109,7 +109,9 @@ namespace Microsoft.AspNetCore.Mvc.Rendering
         /// <summary>
         /// Returns a valid HTML 4.01 "id" attribute value for an element with the given <paramref name="name"/>.
         /// </summary>
-        /// <param name="name">The original element name.</param>
+        /// <param name="name">
+        /// The fully-qualified expression name, ignoring the current model. Also the original HTML element name.
+        /// </param>
         /// <param name="invalidCharReplacement">
         /// The <see cref="string"/> (normally a single <see cref="char"/>) to substitute for invalid characters in
         /// <paramref name="name"/>.
@@ -159,7 +161,7 @@ namespace Microsoft.AspNetCore.Mvc.Rendering
             stringBuffer.Append(firstChar);
 
             // Characters until 'firstIndexOfInvalidCharacter' have already been checked for validity.
-            // So just copying them. This avoids running them through Html401IdUtil.IsValidIdCharacter again.
+            // So just copy them. This avoids running them through Html401IdUtil.IsValidIdCharacter again.
             for (var index = 1; index < firstIndexOfInvalidCharacter; index++)
             {
                 stringBuffer.Append(name[index]);
@@ -182,13 +184,18 @@ namespace Microsoft.AspNetCore.Mvc.Rendering
         }
 
         /// <summary>
-        /// Generates a sanitized ID attribute for the tag by using the specified name.
+        /// Adds a valid HTML 4.01 "id" attribute for an element with the given <paramref name="name"/>. Does
+        /// nothing if <see cref="Attributes"/> already contains an "id" attribute or the <paramref name="name"/>
+        /// is <c>null</c> or empty.
         /// </summary>
-        /// <param name="name">The name to use to generate an ID attribute.</param>
+        /// <param name="name">
+        /// The fully-qualified expression name, ignoring the current model. Also the original HTML element name.
+        /// </param>
         /// <param name="invalidCharReplacement">
         /// The <see cref="string"/> (normally a single <see cref="char"/>) to substitute for invalid characters in
         /// <paramref name="name"/>.
         /// </param>
+        /// <seealso cref="CreateSanitizedId(string, string)"/>
         public void GenerateId(string name, string invalidCharReplacement)
         {
             if (invalidCharReplacement == null)
@@ -196,9 +203,17 @@ namespace Microsoft.AspNetCore.Mvc.Rendering
                 throw new ArgumentNullException(nameof(invalidCharReplacement));
             }
 
+            if (string.IsNullOrEmpty(name))
+            {
+                return;
+            }
+
             if (!Attributes.ContainsKey("id"))
             {
                 var sanitizedId = CreateSanitizedId(name, invalidCharReplacement);
+
+                // Duplicate check for null or empty to cover the corner case where name contains only invalid
+                // characters and invalidCharReplacement is empty.
                 if (!string.IsNullOrEmpty(sanitizedId))
                 {
                     Attributes["id"] = sanitizedId;

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

@@ -54,8 +54,9 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures
         /// <param name="metadataProvider">The <see cref="IModelMetadataProvider"/>.</param>
         /// <param name="urlHelperFactory">The <see cref="IUrlHelperFactory"/>.</param>
         /// <param name="htmlEncoder">The <see cref="HtmlEncoder"/>.</param>
-        /// <param name="clientValidatorCache">The <see cref="ClientValidatorCache"/> that provides
+        /// <param name="clientValidatorCache">
-        /// a list of <see cref="IClientModelValidator"/>s.</param>
+        /// The <see cref="ClientValidatorCache"/> that provides a list of <see cref="IClientModelValidator"/>s.
+        /// </param>
         [Obsolete("This constructor is obsolete and will be removed in a future version. The recommended " +
             "alternative is to use the other public constructor.")]
         public DefaultHtmlGenerator(
@@ -64,14 +65,15 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures
             IModelMetadataProvider metadataProvider,
             IUrlHelperFactory urlHelperFactory,
             HtmlEncoder htmlEncoder,
-            ClientValidatorCache clientValidatorCache) : this(
+            ClientValidatorCache clientValidatorCache)
-                antiforgery,
+            : this(
-                optionsAccessor,
+                  antiforgery,
-                metadataProvider,
+                  optionsAccessor,
-                urlHelperFactory,
+                  metadataProvider,
-                htmlEncoder,
+                  urlHelperFactory,
-                clientValidatorCache,
+                  htmlEncoder,
-                new DefaultValidationHtmlAttributeProvider(optionsAccessor, metadataProvider, clientValidatorCache))
+                  clientValidatorCache,
+                  new DefaultValidationHtmlAttributeProvider(optionsAccessor, metadataProvider, clientValidatorCache))
         {
         }
 
@@ -84,8 +86,9 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures
         /// <param name="metadataProvider">The <see cref="IModelMetadataProvider"/>.</param>
         /// <param name="urlHelperFactory">The <see cref="IUrlHelperFactory"/>.</param>
         /// <param name="htmlEncoder">The <see cref="HtmlEncoder"/>.</param>
-        /// <param name="clientValidatorCache">The <see cref="ClientValidatorCache"/> that provides
+        /// <param name="clientValidatorCache">
-        /// a list of <see cref="IClientModelValidator"/>s.</param>
+        /// The <see cref="ClientValidatorCache"/> that provides a list of <see cref="IClientModelValidator"/>s.
+        /// </param>
         /// <param name="validationAttributeProvider">The <see cref="ValidationHtmlAttributeProvider"/>.</param>
         public DefaultHtmlGenerator(
             IAntiforgery antiforgery,
@@ -280,7 +283,7 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures
             tagBuilder.MergeAttribute("value", "false");
             tagBuilder.TagRenderMode = TagRenderMode.SelfClosing;
 
-            var fullName = GetFullHtmlFieldName(viewContext, expression);
+            var fullName = NameAndIdProvider.GetFullHtmlFieldName(viewContext, expression);
             tagBuilder.MergeAttribute("name", fullName);
 
             return tagBuilder;
@@ -424,8 +427,8 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures
             }
 
             var tagBuilder = new TagBuilder("label");
-            var idString =
+            var fullName = NameAndIdProvider.GetFullHtmlFieldName(viewContext, expression);
-                TagBuilder.CreateSanitizedId(GetFullHtmlFieldName(viewContext, expression), IdAttributeDotReplacement);
+            var idString = NameAndIdProvider.CreateSanitizedId(viewContext, fullName, IdAttributeDotReplacement);
             tagBuilder.Attributes.Add("for", idString);
             tagBuilder.InnerHtml.SetContent(resolvedLabelText);
             tagBuilder.MergeAttributes(GetHtmlAttributeDictionaryOrNull(htmlAttributes), replaceExisting: true);
@@ -599,7 +602,7 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures
                 throw new ArgumentNullException(nameof(viewContext));
             }
 
-            var fullName = GetFullHtmlFieldName(viewContext, expression);
+            var fullName = NameAndIdProvider.GetFullHtmlFieldName(viewContext, expression);
             if (string.IsNullOrEmpty(fullName))
             {
                 throw new ArgumentException(
@@ -628,7 +631,7 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures
             tagBuilder.InnerHtml.SetHtmlContent(listItemBuilder);
             tagBuilder.MergeAttributes(GetHtmlAttributeDictionaryOrNull(htmlAttributes));
             tagBuilder.MergeAttribute("name", fullName, true /* replaceExisting */);
-            tagBuilder.GenerateId(fullName, IdAttributeDotReplacement);
+            NameAndIdProvider.GenerateId(viewContext, tagBuilder, fullName, IdAttributeDotReplacement);
             if (allowMultiple)
             {
                 tagBuilder.MergeAttribute("multiple", "multiple");
@@ -675,7 +678,7 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures
                     Resources.HtmlHelper_TextAreaParameterOutOfRange);
             }
 
-            var fullName = GetFullHtmlFieldName(viewContext, expression);
+            var fullName = NameAndIdProvider.GetFullHtmlFieldName(viewContext, expression);
             if (string.IsNullOrEmpty(fullName))
             {
                 throw new ArgumentException(
@@ -702,7 +705,7 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures
             }
 
             var tagBuilder = new TagBuilder("textarea");
-            tagBuilder.GenerateId(fullName, IdAttributeDotReplacement);
+            NameAndIdProvider.GenerateId(viewContext, tagBuilder, fullName, IdAttributeDotReplacement);
             tagBuilder.MergeAttributes(GetHtmlAttributeDictionaryOrNull(htmlAttributes), true);
             if (rows > 0)
             {
@@ -776,7 +779,7 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures
                 throw new ArgumentNullException(nameof(viewContext));
             }
 
-            var fullName = GetFullHtmlFieldName(viewContext, expression);
+            var fullName = NameAndIdProvider.GetFullHtmlFieldName(viewContext, expression);
             if (string.IsNullOrEmpty(fullName))
             {
                 throw new ArgumentException(
@@ -967,7 +970,7 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures
                 throw new ArgumentNullException(nameof(viewContext));
             }
 
-            var fullName = GetFullHtmlFieldName(viewContext, expression);
+            var fullName = NameAndIdProvider.GetFullHtmlFieldName(viewContext, expression);
             if (string.IsNullOrEmpty(fullName))
             {
                 throw new ArgumentException(
@@ -1135,12 +1138,6 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures
             return tagBuilder;
         }
 
-        internal static string GetFullHtmlFieldName(ViewContext viewContext, string expression)
-        {
-            var fullName = viewContext.ViewData.TemplateInfo.GetFullHtmlFieldName(expression);
-            return fullName;
-        }
-
         internal static object GetModelStateValue(ViewContext viewContext, string key, Type destinationType)
         {
             ModelStateEntry entry;
@@ -1216,7 +1213,7 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures
             // Not valid to use TextBoxForModel() and so on in a top-level view; would end up with an unnamed input
             // elements. But we support the *ForModel() methods in any lower-level template, once HtmlFieldPrefix is
             // non-empty.
-            var fullName = GetFullHtmlFieldName(viewContext, expression);
+            var fullName = NameAndIdProvider.GetFullHtmlFieldName(viewContext, expression);
             if (string.IsNullOrEmpty(fullName))
             {
                 throw new ArgumentException(
@@ -1319,7 +1316,7 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures
 
             if (setId)
             {
-                tagBuilder.GenerateId(fullName, IdAttributeDotReplacement);
+                NameAndIdProvider.GenerateId(viewContext, tagBuilder, fullName, IdAttributeDotReplacement);
             }
 
             // If there are any errors for a named field, we add the CSS attribute.

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

@@ -342,7 +342,7 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures
                 throw new ArgumentNullException(nameof(fullName));
             }
 
-            return TagBuilder.CreateSanitizedId(fullName, IdAttributeDotReplacement);
+            return NameAndIdProvider.CreateSanitizedId(ViewContext, fullName, IdAttributeDotReplacement);
         }
 
         /// <inheritdoc />
@@ -1001,10 +1001,9 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures
 
         protected virtual string GenerateId(string expression)
         {
-            var fullName = DefaultHtmlGenerator.GetFullHtmlFieldName(ViewContext, expression: expression);
+            var fullName = NameAndIdProvider.GetFullHtmlFieldName(ViewContext, expression);
-            var id = TagBuilder.CreateSanitizedId(fullName, IdAttributeDotReplacement);
 
-            return id;
+            return GenerateIdFromName(fullName);
         }
 
         protected virtual IHtmlContent GenerateLabel(
@@ -1056,7 +1055,7 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures
 
         protected virtual string GenerateName(string expression)
         {
-            var fullName = DefaultHtmlGenerator.GetFullHtmlFieldName(ViewContext, expression);
+            var fullName = NameAndIdProvider.GetFullHtmlFieldName(ViewContext, expression);
             return fullName;
         }
 
@@ -1190,7 +1189,7 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures
 
         protected virtual string GenerateValue(string expression, object value, string format, bool useViewData)
         {
-            var fullName = DefaultHtmlGenerator.GetFullHtmlFieldName(ViewContext, expression);
+            var fullName = NameAndIdProvider.GetFullHtmlFieldName(ViewContext, expression);
             var attemptedValue =
                 (string)DefaultHtmlGenerator.GetModelStateValue(ViewContext, fullName, typeof(string));
 

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

@@ -29,12 +29,12 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures
             UrlEncoder urlEncoder,
             ExpressionTextCache expressionTextCache)
             : base(
-                htmlGenerator,
+                  htmlGenerator,
-                viewEngine,
+                  viewEngine,
-                metadataProvider,
+                  metadataProvider,
-                bufferScope,
+                  bufferScope,
-                htmlEncoder,
+                  htmlEncoder,
-                urlEncoder)
+                  urlEncoder)
         {
             if (expressionTextCache == null)
             {

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

@@ -1,6 +1,7 @@
 // Copyright (c) .NET Foundation. All rights reserved.
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
+using System;
 using Microsoft.AspNetCore.Mvc.Rendering;
 
 namespace Microsoft.AspNetCore.Mvc.ViewFeatures
@@ -10,6 +11,8 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures
     /// </summary>
     public class HtmlHelperOptions
     {
+        private string _idAttributeDotReplacement = "_";
+
         /// <summary>
         /// Gets or sets the <see cref="Html5DateRenderingMode.Html5DateRenderingMode"/> value.
         /// </summary>
@@ -23,7 +26,22 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures
         /// <summary>
         /// Gets or sets the <see cref="string"/> that replaces periods in the ID attribute of an element.
         /// </summary>
-        public string IdAttributeDotReplacement { get; set; } = "_";
+        public string IdAttributeDotReplacement
+        {
+            get
+            {
+                return _idAttributeDotReplacement;
+            }
+            set
+            {
+                if (value == null)
+                {
+                    throw new ArgumentNullException(nameof(value));
+                }
+
+                _idAttributeDotReplacement = value;
+            }
+        }
 
         /// <summary>
         /// Gets or sets a value that indicates whether client-side validation is enabled.
@@ -31,13 +49,13 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures
         public bool ClientValidationEnabled { get; set; } = true;
 
         /// <summary>
-        /// Gets or sets the element name used to wrap a top-level message generated by 
+        /// Gets or sets the element name used to wrap a top-level message generated by
         /// <see cref="IHtmlHelper.ValidationMessage"/> and other overloads.
         /// </summary>
         public string ValidationMessageElement { get; set; } = "span";
 
         /// <summary>
-        /// Gets or sets the element name used to wrap a top-level message generated by 
+        /// Gets or sets the element name used to wrap a top-level message generated by
         /// <see cref="IHtmlHelper.ValidationSummary"/> and other overloads.
         /// </summary>
         public string ValidationSummaryMessageElement { get; set; } = "span";

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

@@ -8,11 +8,11 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures
 {
     public class TemplateInfo
     {
-        private string _htmlFieldPrefix;
-        private object _formattedModelValue;
-
         // Keep a collection of visited objects to prevent infinite recursion.
-        private HashSet<object> _visitedObjects;
+        private readonly HashSet<object> _visitedObjects;
+
+        private object _formattedModelValue;
+        private string _htmlFieldPrefix;
 
         public TemplateInfo()
         {
@@ -65,26 +65,31 @@ namespace Microsoft.AspNetCore.Mvc.ViewFeatures
             return _visitedObjects.Add(value);
         }
 
+        /// <summary>
+        /// Returns the full HTML element name for the specified <paramref name="partialFieldName"/>.
+        /// </summary>
+        /// <param name="partialFieldName">Expression name, relative to the current model.</param>
+        /// <returns>Fully-qualified expression name for <paramref name="partialFieldName"/>.</returns>
         public string GetFullHtmlFieldName(string partialFieldName)
         {
             if (string.IsNullOrEmpty(partialFieldName))
             {
                 return HtmlFieldPrefix;
             }
-            else if (string.IsNullOrEmpty(HtmlFieldPrefix))
+
+            if (string.IsNullOrEmpty(HtmlFieldPrefix))
             {
                 return partialFieldName;
             }
-            else if (partialFieldName.StartsWith("[", StringComparison.Ordinal))
+
+            if (partialFieldName.StartsWith("[", StringComparison.Ordinal))
             {
                 // The partialFieldName might represent an indexer access, in which case combining
                 // with a 'dot' would be invalid.
                 return HtmlFieldPrefix + partialFieldName;
             }
-            else
+
-            {
+            return HtmlFieldPrefix + "." + partialFieldName;
-                return HtmlFieldPrefix + "." + partialFieldName;
-            }
         }
 
         public bool Visited(ModelExplorer modelExplorer)