From 3002f4f438c2224b3be37a46246f9ff212564a7a Mon Sep 17 00:00:00 2001
From: Pranav K <prkrishn@hotmail.com>
Date: Tue, 28 Nov 2017 17:52:22 -0800
Subject: [PATCH] Add a <partial /> tag helper

Fixes #5916
---
 .../PartialTagHelper.cs                       |  66 +++++++++
 .../ViewFeatures/DefaultHtmlGenerator.cs      | 133 +++++++++++-------
 .../ViewFeatures/HtmlHelper.cs                |  48 +------
 .../ViewFeatures/IHtmlGenerator.cs            |  23 +++
 .../breakingchanges.netcore.json              |  12 ++
 .../HtmlGenerationTest.cs                     |   2 +
 ...ation_Home.ProductListUsingTagHelpers.html |  64 +++++++++
 .../PartialTagHelperTest.cs                   |  81 +++++++++++
 .../TestableHtmlGenerator.cs                  |   9 +-
 .../HtmlGeneratorUtilities.cs                 |  12 +-
 .../Rendering/DefaultTemplatesUtilities.cs    |  10 +-
 .../ViewFeatures/DefaultHtmlGeneratorTest.cs  |  58 +++++++-
 .../HtmlGeneration_HomeController.cs          |   2 +
 .../ProductListUsingTagHelpers.cshtml         |  30 ++++
 14 files changed, 447 insertions(+), 103 deletions(-)
 create mode 100644 src/Microsoft.AspNetCore.Mvc.TagHelpers/PartialTagHelper.cs
 create mode 100644 src/Microsoft.AspNetCore.Mvc.ViewFeatures/breakingchanges.netcore.json
 create mode 100644 test/Microsoft.AspNetCore.Mvc.FunctionalTests/compiler/resources/HtmlGenerationWebSite.HtmlGeneration_Home.ProductListUsingTagHelpers.html
 create mode 100644 test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/PartialTagHelperTest.cs
 create mode 100644 test/WebSites/HtmlGenerationWebSite/Views/HtmlGeneration_Home/ProductListUsingTagHelpers.cshtml

diff --git a/src/Microsoft.AspNetCore.Mvc.TagHelpers/PartialTagHelper.cs b/src/Microsoft.AspNetCore.Mvc.TagHelpers/PartialTagHelper.cs
new file mode 100644
index 0000000000..7b7ba340e3
--- /dev/null
+++ b/src/Microsoft.AspNetCore.Mvc.TagHelpers/PartialTagHelper.cs
@@ -0,0 +1,66 @@
+// 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 System.Text;
+using System.Threading.Tasks;
+using Microsoft.AspNetCore.Mvc.Rendering;
+using Microsoft.AspNetCore.Mvc.ViewFeatures;
+using Microsoft.AspNetCore.Mvc.ViewFeatures.Internal;
+using Microsoft.AspNetCore.Razor.TagHelpers;
+
+namespace Microsoft.AspNetCore.Mvc.TagHelpers
+{
+    /// <summary>
+    /// Renders a partial view.
+    /// </summary>
+    [HtmlTargetElement("partial", Attributes = "name", TagStructure = TagStructure.WithoutEndTag)]
+    public class PartialTagHelper : TagHelper
+    {
+        private readonly IHtmlGenerator _htmlGenerator;
+        private readonly IViewBufferScope _viewBufferScope;
+
+        public PartialTagHelper(
+            IHtmlGenerator htmlGenerator,
+            IViewBufferScope viewBufferScope)
+        {
+            _htmlGenerator = htmlGenerator ?? throw new ArgumentNullException(nameof(htmlGenerator));
+            _viewBufferScope = viewBufferScope ?? throw new ArgumentNullException(nameof(viewBufferScope));
+        }
+
+        /// <summary>
+        /// The name of the partial view used to create the HTML markup. Must not be <c>null</c>.
+        /// </summary>
+        public string Name { get; set; }
+
+        /// <summary>
+        /// A model to pass into the partial view.
+        /// </summary>
+        public object Model { get; set; }
+
+        /// <summary>
+        /// A <see cref="ViewDataDictionary"/> to pass into the partial view.
+        /// </summary>
+        public ViewDataDictionary ViewData { get; set; }
+
+        [ViewContext]
+        public ViewContext ViewContext { get; set; }
+
+        public override async Task ProcessAsync(TagHelperContext context, TagHelperOutput output)
+        {
+            var viewBuffer = new ViewBuffer(_viewBufferScope, Name, ViewBuffer.PartialViewPageSize);
+            using (var writer = new ViewBufferTextWriter(viewBuffer, Encoding.UTF8))
+            {
+                await _htmlGenerator.RenderPartialViewAsync(
+                    ViewContext,
+                    Name,
+                    Model,
+                    ViewData,
+                    writer);
+
+                output.SuppressOutput();
+                output.Content.SetHtmlContent(viewBuffer);
+            }
+        }
+    }
+}
diff --git a/src/Microsoft.AspNetCore.Mvc.ViewFeatures/ViewFeatures/DefaultHtmlGenerator.cs b/src/Microsoft.AspNetCore.Mvc.ViewFeatures/ViewFeatures/DefaultHtmlGenerator.cs
index 0b9a75e282..41f60d96e0 100644
--- a/src/Microsoft.AspNetCore.Mvc.ViewFeatures/ViewFeatures/DefaultHtmlGenerator.cs
+++ b/src/Microsoft.AspNetCore.Mvc.ViewFeatures/ViewFeatures/DefaultHtmlGenerator.cs
@@ -6,15 +6,18 @@
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Globalization;
+using System.IO;
 using System.Linq;
 using System.Reflection;
 using System.Text.Encodings.Web;
+using System.Threading.Tasks;
 using Microsoft.AspNetCore.Antiforgery;
 using Microsoft.AspNetCore.Html;
 using Microsoft.AspNetCore.Mvc.ModelBinding;
 using Microsoft.AspNetCore.Mvc.ModelBinding.Internal;
 using Microsoft.AspNetCore.Mvc.Rendering;
 using Microsoft.AspNetCore.Mvc.Routing;
+using Microsoft.AspNetCore.Mvc.ViewEngines;
 using Microsoft.AspNetCore.Mvc.ViewFeatures.Internal;
 using Microsoft.Extensions.Options;
 
@@ -35,6 +38,7 @@ public class DefaultHtmlGenerator : IHtmlGenerator
         private readonly IUrlHelperFactory _urlHelperFactory;
         private readonly HtmlEncoder _htmlEncoder;
         private readonly ValidationHtmlAttributeProvider _validationAttributeProvider;
+        private readonly ICompositeViewEngine _viewEngine;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="DefaultHtmlGenerator"/> class.
@@ -46,49 +50,27 @@ public class DefaultHtmlGenerator : IHtmlGenerator
         /// <param name="urlHelperFactory">The <see cref="IUrlHelperFactory"/>.</param>
         /// <param name="htmlEncoder">The <see cref="HtmlEncoder"/>.</param>
         /// <param name="validationAttributeProvider">The <see cref="ValidationHtmlAttributeProvider"/>.</param>
+        /// <param name="viewEngine">The <see cref="ICompositeViewEngine"/>.</param>
         public DefaultHtmlGenerator(
             IAntiforgery antiforgery,
             IOptions<MvcViewOptions> optionsAccessor,
             IModelMetadataProvider metadataProvider,
             IUrlHelperFactory urlHelperFactory,
             HtmlEncoder htmlEncoder,
-            ValidationHtmlAttributeProvider validationAttributeProvider)
+            ValidationHtmlAttributeProvider validationAttributeProvider,
+            ICompositeViewEngine viewEngine)
         {
-            if (antiforgery == null)
-            {
-                throw new ArgumentNullException(nameof(antiforgery));
-            }
-
             if (optionsAccessor == null)
             {
                 throw new ArgumentNullException(nameof(optionsAccessor));
             }
 
-            if (metadataProvider == null)
-            {
-                throw new ArgumentNullException(nameof(metadataProvider));
-            }
-
-            if (urlHelperFactory == null)
-            {
-                throw new ArgumentNullException(nameof(urlHelperFactory));
-            }
-
-            if (htmlEncoder == null)
-            {
-                throw new ArgumentNullException(nameof(htmlEncoder));
-            }
-
-            if (validationAttributeProvider == null)
-            {
-                throw new ArgumentNullException(nameof(validationAttributeProvider));
-            }
-
-            _antiforgery = antiforgery;
-            _metadataProvider = metadataProvider;
-            _urlHelperFactory = urlHelperFactory;
-            _htmlEncoder = htmlEncoder;
-            _validationAttributeProvider = validationAttributeProvider;
+            _antiforgery = antiforgery ?? throw new ArgumentNullException(nameof(antiforgery));
+            _metadataProvider = metadataProvider ?? throw new ArgumentNullException(nameof(metadataProvider));
+            _urlHelperFactory = urlHelperFactory ?? throw new ArgumentNullException(nameof(urlHelperFactory));
+            _htmlEncoder = htmlEncoder ?? throw new ArgumentNullException(nameof(htmlEncoder));
+            _validationAttributeProvider = validationAttributeProvider ?? throw new ArgumentNullException(nameof(validationAttributeProvider));
+            _viewEngine = viewEngine;
 
             // Underscores are fine characters in id's.
             IdAttributeDotReplacement = optionsAccessor.Value.HtmlHelperOptions.IdAttributeDotReplacement;
@@ -214,8 +196,7 @@ public virtual TagBuilder GenerateCheckBox(
 
                 if (modelExplorer.Model != null)
                 {
-                    bool modelChecked;
-                    if (bool.TryParse(modelExplorer.Model.ToString(), out modelChecked))
+                    if (bool.TryParse(modelExplorer.Model.ToString(), out var modelChecked))
                     {
                         isChecked = modelChecked;
                     }
@@ -363,8 +344,7 @@ public virtual TagBuilder GenerateHidden(
             }
 
             // Special-case opaque values and arbitrary binary data.
-            var byteArrayValue = value as byte[];
-            if (byteArrayValue != null)
+            if (value is byte[] byteArrayValue)
             {
                 value = Convert.ToBase64String(byteArrayValue);
             }
@@ -631,8 +611,7 @@ public virtual TagBuilder GenerateSelect(
             }
 
             // If there are any errors for a named field, we add the css attribute.
-            ModelStateEntry entry;
-            if (viewContext.ViewData.ModelState.TryGetValue(fullName, out entry))
+            if (viewContext.ViewData.ModelState.TryGetValue(fullName, out var entry))
             {
                 if (entry.Errors.Count > 0)
                 {
@@ -684,8 +663,7 @@ public virtual TagBuilder GenerateTextArea(
                     nameof(expression));
             }
 
-            ModelStateEntry entry;
-            viewContext.ViewData.ModelState.TryGetValue(fullName, out entry);
+            viewContext.ViewData.ModelState.TryGetValue(fullName, out var entry);
 
             var value = string.Empty;
             if (entry != null && entry.AttemptedValue != null)
@@ -791,8 +769,7 @@ public virtual TagBuilder GenerateValidationMessage(
                 return null;
             }
 
-            ModelStateEntry entry;
-            var tryGetModelStateResult = viewContext.ViewData.ModelState.TryGetValue(fullName, out entry);
+            var tryGetModelStateResult = viewContext.ViewData.ModelState.TryGetValue(fullName, out var entry);
             var modelErrors = tryGetModelStateResult ? entry.Errors : null;
 
             ModelError modelError = null;
@@ -868,9 +845,8 @@ public virtual TagBuilder GenerateValidationSummary(
                 return null;
             }
 
-            ModelStateEntry entryForModel;
             if (excludePropertyErrors &&
-                (!viewData.ModelState.TryGetValue(viewData.TemplateInfo.HtmlFieldPrefix, out entryForModel) ||
+                (!viewData.ModelState.TryGetValue(viewData.TemplateInfo.HtmlFieldPrefix, out var entryForModel) ||
                  entryForModel.Errors.Count == 0))
             {
                 // Client-side validation (if enabled) will not affect the generated element and element will be empty.
@@ -1095,6 +1071,65 @@ public virtual ICollection<string> GetCurrentValues(
             return currentValues;
         }
 
+        /// <inheritdoc />
+        public async Task RenderPartialViewAsync(
+           ViewContext viewContext,
+           string partialViewName,
+           object model,
+           ViewDataDictionary viewData,
+           TextWriter writer)
+        {
+            if (viewContext == null)
+            {
+                throw new ArgumentNullException(nameof(viewContext));
+            }
+
+            if (partialViewName == null)
+            {
+                throw new ArgumentNullException(nameof(partialViewName));
+            }
+
+            if (writer == null)
+            {
+                throw new ArgumentNullException(nameof(writer));
+            }
+
+            var viewEngineResult = _viewEngine.GetView(
+                viewContext.ExecutingFilePath,
+                partialViewName,
+                isMainPage: false);
+            var getViewLocations = viewEngineResult.SearchedLocations;
+            if (!viewEngineResult.Success)
+            {
+                viewEngineResult = _viewEngine.FindView(viewContext, partialViewName, isMainPage: false);
+            }
+
+            if (!viewEngineResult.Success)
+            {
+                var searchedLocations = Enumerable.Concat(getViewLocations, viewEngineResult.SearchedLocations);
+                var locations = string.Empty;
+                if (searchedLocations.Any())
+                {
+                    locations += Environment.NewLine + string.Join(Environment.NewLine, searchedLocations);
+                }
+
+                throw new InvalidOperationException(
+                    Resources.FormatViewEngine_PartialViewNotFound(partialViewName, locations));
+            }
+
+            var view = viewEngineResult.View;
+            using (view as IDisposable)
+            {
+                // Determine which ViewData we should use to construct a new ViewData
+                var baseViewData = viewData ?? viewContext.ViewData;
+
+                var newViewData = new ViewDataDictionary<object>(baseViewData, model);
+                var partialViewContext = new ViewContext(viewContext, view, newViewData, writer);
+
+                await viewEngineResult.View.RenderAsync(partialViewContext);
+            }
+        }
+
         internal static string EvalString(ViewContext viewContext, string key, string format)
         {
             return Convert.ToString(viewContext.ViewData.Eval(key, format), CultureInfo.CurrentCulture);
@@ -1133,8 +1168,7 @@ internal static TagBuilder GenerateOption(SelectListItem item, string text, bool
 
         internal static object GetModelStateValue(ViewContext viewContext, string key, Type destinationType)
         {
-            ModelStateEntry entry;
-            if (viewContext.ViewData.ModelState.TryGetValue(key, out entry) && entry.RawValue != null)
+            if (viewContext.ViewData.ModelState.TryGetValue(key, out var entry) && entry.RawValue != null)
             {
                 return ModelBindingHelper.ConvertTo(entry.RawValue, destinationType, culture: null);
             }
@@ -1249,8 +1283,7 @@ protected virtual TagBuilder GenerateInput(
                 case InputType.Radio:
                     if (!usedModelState)
                     {
-                        var modelStateValue = GetModelStateValue(viewContext, fullName, typeof(string)) as string;
-                        if (modelStateValue != null)
+                        if (GetModelStateValue(viewContext, fullName, typeof(string)) is string modelStateValue)
                         {
                             isChecked = string.Equals(modelStateValue, valueParameter, StringComparison.Ordinal);
                             usedModelState = true;
@@ -1313,8 +1346,7 @@ protected virtual TagBuilder GenerateInput(
             }
 
             // If there are any errors for a named field, we add the CSS attribute.
-            ModelStateEntry entry;
-            if (viewContext.ViewData.ModelState.TryGetValue(fullName, out entry) && entry.Errors.Count > 0)
+            if (viewContext.ViewData.ModelState.TryGetValue(fullName, out var entry) && entry.Errors.Count > 0)
             {
                 tagBuilder.AddCssClass(HtmlHelper.ValidationInputCssClassName);
             }
@@ -1410,8 +1442,7 @@ private static Enum ConvertEnumFromInteger(object value, Type targetType)
 
         private static object ConvertEnumFromString<TEnum>(string value) where TEnum : struct
         {
-            TEnum enumValue;
-            if (Enum.TryParse(value, out enumValue))
+            if (Enum.TryParse(value, out TEnum enumValue))
             {
                 return enumValue;
             }
diff --git a/src/Microsoft.AspNetCore.Mvc.ViewFeatures/ViewFeatures/HtmlHelper.cs b/src/Microsoft.AspNetCore.Mvc.ViewFeatures/ViewFeatures/HtmlHelper.cs
index 6233cb4d94..f184f6942e 100644
--- a/src/Microsoft.AspNetCore.Mvc.ViewFeatures/ViewFeatures/HtmlHelper.cs
+++ b/src/Microsoft.AspNetCore.Mvc.ViewFeatures/ViewFeatures/HtmlHelper.cs
@@ -160,8 +160,7 @@ public static IDictionary<string, object> ObjectToDictionary(object value)
         /// </remarks>
         public static IDictionary<string, object> AnonymousObjectToHtmlAttributes(object htmlAttributes)
         {
-            var dictionary = htmlAttributes as IDictionary<string, object>;
-            if (dictionary != null)
+            if (htmlAttributes is IDictionary<string, object> dictionary)
             {
                 return new Dictionary<string, object>(dictionary, StringComparer.OrdinalIgnoreCase);
             }
@@ -504,7 +503,7 @@ protected virtual IHtmlContent GenerateDisplay(
             return templateBuilder.Build();
         }
 
-        protected virtual async Task RenderPartialCoreAsync(
+        protected virtual Task RenderPartialCoreAsync(
             string partialViewName,
             object model,
             ViewDataDictionary viewData,
@@ -515,45 +514,12 @@ protected virtual async Task RenderPartialCoreAsync(
                 throw new ArgumentNullException(nameof(partialViewName));
             }
 
-            var viewEngineResult = _viewEngine.GetView(
-                ViewContext.ExecutingFilePath,
+            return _htmlGenerator.RenderPartialViewAsync(
+                ViewContext,
                 partialViewName,
-                isMainPage: false);
-            var originalLocations = viewEngineResult.SearchedLocations;
-            if (!viewEngineResult.Success)
-            {
-                viewEngineResult = _viewEngine.FindView(ViewContext, partialViewName, isMainPage: false);
-            }
-
-            if (!viewEngineResult.Success)
-            {
-                var locations = string.Empty;
-                if (originalLocations.Any())
-                {
-                    locations = Environment.NewLine + string.Join(Environment.NewLine, originalLocations);
-                }
-
-                if (viewEngineResult.SearchedLocations.Any())
-                {
-                    locations +=
-                        Environment.NewLine + string.Join(Environment.NewLine, viewEngineResult.SearchedLocations);
-                }
-
-                throw new InvalidOperationException(
-                    Resources.FormatViewEngine_PartialViewNotFound(partialViewName, locations));
-            }
-
-            var view = viewEngineResult.View;
-            using (view as IDisposable)
-            {
-                // Determine which ViewData we should use to construct a new ViewData
-                var baseViewData = viewData ?? ViewData;
-
-                var newViewData = new ViewDataDictionary<object>(baseViewData, model);
-                var viewContext = new ViewContext(ViewContext, view, newViewData, writer);
-
-                await viewEngineResult.View.RenderAsync(viewContext);
-            }
+                model,
+                viewData,
+                writer);
         }
 
         /// <inheritdoc />
diff --git a/src/Microsoft.AspNetCore.Mvc.ViewFeatures/ViewFeatures/IHtmlGenerator.cs b/src/Microsoft.AspNetCore.Mvc.ViewFeatures/ViewFeatures/IHtmlGenerator.cs
index f0f59d4ba2..201c55058e 100644
--- a/src/Microsoft.AspNetCore.Mvc.ViewFeatures/ViewFeatures/IHtmlGenerator.cs
+++ b/src/Microsoft.AspNetCore.Mvc.ViewFeatures/ViewFeatures/IHtmlGenerator.cs
@@ -2,6 +2,8 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System.Collections.Generic;
+using System.IO;
+using System.Threading.Tasks;
 using Microsoft.AspNetCore.Html;
 using Microsoft.AspNetCore.Mvc.Rendering;
 
@@ -481,5 +483,26 @@ ICollection<string> GetCurrentValues(
             ModelExplorer modelExplorer,
             string expression,
             bool allowMultiple);
+
+        /// <summary>
+        /// Renders HTML markup for the specified partial view.
+        /// </summary>
+        /// <param name="partialViewName">
+        /// The name of the partial view used to create the HTML markup. Must not be <c>null</c>.
+        /// </param>
+        /// <param name="viewContext">A <see cref="ViewContext"/> instance for the current scope.</param>
+        /// <param name="model">A model to pass into the partial view.</param>
+        /// <param name="viewData">A <see cref="ViewDataDictionary"/> to pass into the partial view.</param>
+        /// <param name="writer">The <see cref="TextWriter"/> to render to.</param>
+        /// <returns>A <see cref="Task"/> that renders the created HTML when it executes.</returns>
+        /// <remarks>
+        /// In this context, "renders" means the method writes its output using <see cref="ViewContext.Writer"/>.
+        /// </remarks>
+        Task RenderPartialViewAsync(
+            ViewContext viewContext,
+            string partialViewName,
+            object model,
+            ViewDataDictionary viewData,
+            TextWriter writer);
     }
 }
diff --git a/src/Microsoft.AspNetCore.Mvc.ViewFeatures/breakingchanges.netcore.json b/src/Microsoft.AspNetCore.Mvc.ViewFeatures/breakingchanges.netcore.json
new file mode 100644
index 0000000000..4801f77ca8
--- /dev/null
+++ b/src/Microsoft.AspNetCore.Mvc.ViewFeatures/breakingchanges.netcore.json
@@ -0,0 +1,12 @@
+[
+  {
+    "TypeId": "public class Microsoft.AspNetCore.Mvc.ViewFeatures.DefaultHtmlGenerator : Microsoft.AspNetCore.Mvc.ViewFeatures.IHtmlGenerator",
+    "MemberId": "public .ctor(Microsoft.AspNetCore.Antiforgery.IAntiforgery antiforgery, Microsoft.Extensions.Options.IOptions<Microsoft.AspNetCore.Mvc.MvcViewOptions> optionsAccessor, Microsoft.AspNetCore.Mvc.ModelBinding.IModelMetadataProvider metadataProvider, Microsoft.AspNetCore.Mvc.Routing.IUrlHelperFactory urlHelperFactory, System.Text.Encodings.Web.HtmlEncoder htmlEncoder, Microsoft.AspNetCore.Mvc.ViewFeatures.ValidationHtmlAttributeProvider validationAttributeProvider)",
+    "Kind": "Removal"
+  },
+  {
+    "TypeId": "public interface Microsoft.AspNetCore.Mvc.ViewFeatures.IHtmlGenerator",
+    "MemberId": "System.Threading.Tasks.Task RenderPartialViewAsync(Microsoft.AspNetCore.Mvc.Rendering.ViewContext viewContext, System.String partialViewName, System.Object model, Microsoft.AspNetCore.Mvc.ViewFeatures.ViewDataDictionary viewData, System.IO.TextWriter writer)",
+    "Kind": "Addition"
+  }
+]
diff --git a/test/Microsoft.AspNetCore.Mvc.FunctionalTests/HtmlGenerationTest.cs b/test/Microsoft.AspNetCore.Mvc.FunctionalTests/HtmlGenerationTest.cs
index 1579544e7c..49b9455779 100644
--- a/test/Microsoft.AspNetCore.Mvc.FunctionalTests/HtmlGenerationTest.cs
+++ b/test/Microsoft.AspNetCore.Mvc.FunctionalTests/HtmlGenerationTest.cs
@@ -65,6 +65,8 @@ public static TheoryData<string, string> WebPagesData
                     { "ProductList", "/HtmlGeneration_Product" },
                     // Testing the ScriptTagHelper
                     { "Script", null },
+                    // Testing PartialTagHelper + InputTagHelpers
+                    { "ProductListUsingTagHelpers", "/HtmlGeneration_Product" },
                 };
 
                 return data;
diff --git a/test/Microsoft.AspNetCore.Mvc.FunctionalTests/compiler/resources/HtmlGenerationWebSite.HtmlGeneration_Home.ProductListUsingTagHelpers.html b/test/Microsoft.AspNetCore.Mvc.FunctionalTests/compiler/resources/HtmlGenerationWebSite.HtmlGeneration_Home.ProductListUsingTagHelpers.html
new file mode 100644
index 0000000000..afa2cc3bc6
--- /dev/null
+++ b/test/Microsoft.AspNetCore.Mvc.FunctionalTests/compiler/resources/HtmlGenerationWebSite.HtmlGeneration_Home.ProductListUsingTagHelpers.html
@@ -0,0 +1,64 @@
+<html>
+<head>
+    <meta charset="utf-8" />
+</head>
+<body>
+    <form action="/HtmlGeneration_Product" method="post">
+            <div>
+                <label class="product" for="z0__product_HomePage">HomePage</label>
+                <input type="url" size="50" disabled="disabled" readonly="readonly" id="z0__product_HomePage" name="[0].product.HomePage" value="http://www.contoso.com/" />
+            </div>
+            
+<div>
+    <label class="product" for="z0__Number">Number</label>
+    <input type="number" data-val="true" data-val-required="The Number field is required." id="z0__Number" name="[0].Number" value="0" />
+</div>
+<div>
+    <label class="product" for="z0__ProductName">ProductName</label>
+    <input type="text" data-val="true" data-val-required="The ProductName field is required." id="z0__ProductName" name="[0].ProductName" value="Product_0" />
+</div>
+<div>
+    <label class="product" for="z0__Description">Description</label>
+    <textarea rows="4" cols="50" class="product" id="z0__Description" name="[0].Description">
+</textarea>
+</div>
+            <div>
+                <label class="product" for="z1__product_HomePage">HomePage</label>
+                <input type="url" size="50" disabled="disabled" readonly="readonly" id="z1__product_HomePage" name="[1].product.HomePage" value="" />
+            </div>
+            
+<div>
+    <label class="product" for="z1__Number">Number</label>
+    <input type="number" data-val="true" data-val-required="The Number field is required." id="z1__Number" name="[1].Number" value="1" />
+</div>
+<div>
+    <label class="product" for="z1__ProductName">ProductName</label>
+    <input type="text" data-val="true" data-val-required="The ProductName field is required." id="z1__ProductName" name="[1].ProductName" value="Product_1" />
+</div>
+<div>
+    <label class="product" for="z1__Description">Description</label>
+    <textarea rows="4" cols="50" class="product" id="z1__Description" name="[1].Description">
+</textarea>
+</div>
+            <div>
+                <label class="product" for="z2__product_HomePage">HomePage</label>
+                <input type="url" size="50" disabled="disabled" readonly="readonly" id="z2__product_HomePage" name="[2].product.HomePage" value="" />
+            </div>
+            
+<div>
+    <label class="product" for="z2__Number">Number</label>
+    <input type="number" data-val="true" data-val-required="The Number field is required." id="z2__Number" name="[2].Number" value="2" />
+</div>
+<div>
+    <label class="product" for="z2__ProductName">ProductName</label>
+    <input type="text" data-val="true" data-val-required="The ProductName field is required." id="z2__ProductName" name="[2].ProductName" value="Product_2" />
+</div>
+<div>
+    <label class="product" for="z2__Description">Description</label>
+    <textarea rows="4" cols="50" class="product" id="z2__Description" name="[2].Description">
+Product_2 description</textarea>
+</div>
+        <input type="submit" />
+    <input name="__RequestVerificationToken" type="hidden" value="{0}" /></form>
+</body>
+</html>
diff --git a/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/PartialTagHelperTest.cs b/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/PartialTagHelperTest.cs
new file mode 100644
index 0000000000..ac7723b062
--- /dev/null
+++ b/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/PartialTagHelperTest.cs
@@ -0,0 +1,81 @@
+// 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 System.Collections.Generic;
+using System.IO;
+using System.Threading.Tasks;
+using Microsoft.AspNetCore.Mvc.ModelBinding;
+using Microsoft.AspNetCore.Mvc.Rendering;
+using Microsoft.AspNetCore.Mvc.TestCommon;
+using Microsoft.AspNetCore.Mvc.ViewFeatures;
+using Microsoft.AspNetCore.Mvc.ViewFeatures.Internal;
+using Microsoft.AspNetCore.Razor.TagHelpers;
+using Microsoft.Extensions.WebEncoders.Testing;
+using Moq;
+using Xunit;
+
+namespace Microsoft.AspNetCore.Mvc.TagHelpers
+{
+    public class PartialTagHelperTest
+    {
+        [Fact]
+        public async Task ProcessAsync_RendersPartialView()
+        {
+            // Arrange
+            var expected = "Hello world!";
+            var bufferScope = new TestViewBufferScope();
+            var generator = new Mock<IHtmlGenerator>();
+            var partialName = "_Partial";
+            var model = new object();
+            var viewData = new ViewDataDictionary(new TestModelMetadataProvider(), new ModelStateDictionary());
+            var viewContext = new ViewContext
+            {
+                ViewData = new ViewDataDictionary(new TestModelMetadataProvider(), new ModelStateDictionary()),
+            };
+
+            generator.Setup(m => m.RenderPartialViewAsync(viewContext, partialName, model, viewData, It.IsAny<TextWriter>()))
+                .Callback((ViewContext _, string __, object ___, ViewDataDictionary ____, TextWriter writer) =>
+                {
+                    writer.Write(expected);
+                })
+                .Returns(Task.CompletedTask)
+                .Verifiable();
+
+            var tagHelper = new PartialTagHelper(generator.Object, bufferScope)
+            {
+                Name = partialName,
+                Model = model,
+                ViewContext = viewContext,
+                ViewData = viewData,
+            };
+            var tagHelperContext = GetTagHelperContext();
+            var output = GetTagHelperOutput();
+
+            // Act
+            await tagHelper.ProcessAsync(tagHelperContext, output);
+
+            // Assert
+            generator.Verify();
+            var content = HtmlContentUtilities.HtmlContentToString(output.Content, new HtmlTestEncoder());
+            Assert.Equal(expected, content);
+        }
+
+        private static TagHelperContext GetTagHelperContext()
+        {
+            return new TagHelperContext(
+                "partial",
+                new TagHelperAttributeList(),
+                new Dictionary<object, object>(),
+                Guid.NewGuid().ToString("N"));
+        }
+
+        private static TagHelperOutput GetTagHelperOutput()
+        {
+            return new TagHelperOutput(
+                "partial",
+                new TagHelperAttributeList(),
+                (_, __) => Task.FromResult<TagHelperContent>(new DefaultTagHelperContent()));
+        }
+    }
+}
diff --git a/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/TestableHtmlGenerator.cs b/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/TestableHtmlGenerator.cs
index daf8314ff5..1ce8da099d 100644
--- a/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/TestableHtmlGenerator.cs
+++ b/test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/TestableHtmlGenerator.cs
@@ -35,7 +35,8 @@ public TestableHtmlGenerator(IModelMetadataProvider metadataProvider, IUrlHelper
                   metadataProvider,
                   GetOptions(),
                   urlHelper,
-                  validationAttributes: new Dictionary<string, object>(StringComparer.OrdinalIgnoreCase))
+                  validationAttributes: new Dictionary<string, object>(StringComparer.OrdinalIgnoreCase),
+                  viewEngine: Mock.Of<ICompositeViewEngine>())
         {
         }
 
@@ -43,14 +44,16 @@ public TestableHtmlGenerator(
             IModelMetadataProvider metadataProvider,
             IOptions<MvcViewOptions> options,
             IUrlHelper urlHelper,
-            IDictionary<string, object> validationAttributes)
+            IDictionary<string, object> validationAttributes,
+            ICompositeViewEngine viewEngine)
             : base(
                   Mock.Of<IAntiforgery>(),
                   options,
                   metadataProvider,
                   CreateUrlHelperFactory(urlHelper),
                   new HtmlTestEncoder(),
-                  new DefaultValidationHtmlAttributeProvider(options, metadataProvider, new ClientValidatorCache()))
+                  new DefaultValidationHtmlAttributeProvider(options, metadataProvider, new ClientValidatorCache()),
+                  viewEngine)
         {
             _validationAttributes = validationAttributes;
         }
diff --git a/test/Microsoft.AspNetCore.Mvc.TestCommon/HtmlGeneratorUtilities.cs b/test/Microsoft.AspNetCore.Mvc.TestCommon/HtmlGeneratorUtilities.cs
index a40024c7e4..b57f114311 100644
--- a/test/Microsoft.AspNetCore.Mvc.TestCommon/HtmlGeneratorUtilities.cs
+++ b/test/Microsoft.AspNetCore.Mvc.TestCommon/HtmlGeneratorUtilities.cs
@@ -5,6 +5,7 @@
 using Microsoft.AspNetCore.Mvc.Internal;
 using Microsoft.AspNetCore.Mvc.ModelBinding;
 using Microsoft.AspNetCore.Mvc.Routing;
+using Microsoft.AspNetCore.Mvc.ViewEngines;
 using Microsoft.Extensions.Options;
 using Microsoft.Extensions.WebEncoders.Testing;
 using Moq;
@@ -21,10 +22,14 @@ public static IHtmlGenerator GetHtmlGenerator(IModelMetadataProvider provider)
                 .Setup(f => f.GetUrlHelper(It.IsAny<ActionContext>()))
                 .Returns(Mock.Of<IUrlHelper>());
 
-            return GetHtmlGenerator(provider, urlHelperFactory.Object, options);
+            return GetHtmlGenerator(provider, urlHelperFactory.Object, options, Mock.Of<ICompositeViewEngine>());
         }
 
-        public static IHtmlGenerator GetHtmlGenerator(IModelMetadataProvider provider, IUrlHelperFactory urlHelperFactory, MvcViewOptions options)
+        public static IHtmlGenerator GetHtmlGenerator(
+            IModelMetadataProvider provider,
+            IUrlHelperFactory urlHelperFactory,
+            MvcViewOptions options,
+            ICompositeViewEngine viewEngine)
         {
             var optionsAccessor = new Mock<IOptions<MvcViewOptions>>();
             optionsAccessor
@@ -42,7 +47,8 @@ public static IHtmlGenerator GetHtmlGenerator(IModelMetadataProvider provider, I
                     provider,
                     urlHelperFactory,
                     new HtmlTestEncoder(),
-                    attributeProvider);
+                    attributeProvider,
+                    viewEngine);
             return htmlGenerator;
         }
     }
diff --git a/test/Microsoft.AspNetCore.Mvc.ViewFeatures.Test/Rendering/DefaultTemplatesUtilities.cs b/test/Microsoft.AspNetCore.Mvc.ViewFeatures.Test/Rendering/DefaultTemplatesUtilities.cs
index 75649f3223..390192b5c6 100644
--- a/test/Microsoft.AspNetCore.Mvc.ViewFeatures.Test/Rendering/DefaultTemplatesUtilities.cs
+++ b/test/Microsoft.AspNetCore.Mvc.ViewFeatures.Test/Rendering/DefaultTemplatesUtilities.cs
@@ -247,7 +247,11 @@ private static HtmlHelper<TModel> GetHtmlHelper<TModel>(
 
             if (htmlGenerator == null)
             {
-                htmlGenerator = HtmlGeneratorUtilities.GetHtmlGenerator(provider, urlHelperFactory.Object, options);
+                htmlGenerator = HtmlGeneratorUtilities.GetHtmlGenerator(
+                    provider,
+                    urlHelperFactory.Object,
+                    options,
+                    viewEngine);
             }
 
             // TemplateRenderer will Contextualize this transient service.
@@ -298,7 +302,7 @@ private static HtmlHelper<TModel> GetHtmlHelper<TModel>(
             return htmlHelper;
         }
 
-        private static ICompositeViewEngine CreateViewEngine()
+        public static ICompositeViewEngine CreateViewEngine()
         {
             var view = new Mock<IView>();
             view
@@ -329,7 +333,7 @@ public static string FormatOutput(IHtmlHelper helper, object model)
             return FormatOutput(modelExplorer);
         }
 
-        private static string FormatOutput(ModelExplorer modelExplorer)
+        public static string FormatOutput(ModelExplorer modelExplorer)
         {
             var metadata = modelExplorer.Metadata;
             return string.Format(
diff --git a/test/Microsoft.AspNetCore.Mvc.ViewFeatures.Test/ViewFeatures/DefaultHtmlGeneratorTest.cs b/test/Microsoft.AspNetCore.Mvc.ViewFeatures.Test/ViewFeatures/DefaultHtmlGeneratorTest.cs
index 3df4cf0438..7514fa68aa 100644
--- a/test/Microsoft.AspNetCore.Mvc.ViewFeatures.Test/ViewFeatures/DefaultHtmlGeneratorTest.cs
+++ b/test/Microsoft.AspNetCore.Mvc.ViewFeatures.Test/ViewFeatures/DefaultHtmlGeneratorTest.cs
@@ -6,6 +6,7 @@
 using System.IO;
 using System.Linq;
 using System.Text.Encodings.Web;
+using System.Threading.Tasks;
 using Microsoft.AspNetCore.Antiforgery;
 using Microsoft.AspNetCore.Http;
 using Microsoft.AspNetCore.Mvc.Abstractions;
@@ -676,8 +677,58 @@ public void GenerateAntiforgery_AlwaysGeneratesAntiforgeryToken_IfCannotRenderAt
             Assert.Equal(expected, antiforgeryField);
         }
 
+        [Fact]
+        public async Task RenderPartialViewAsync_Throws_IfViewCannotBeFound()
+        {
+            // Arrange
+            var expected = "The partial view 'test-view' was not found. The following locations were searched:" +
+                Environment.NewLine +
+                "location1" + Environment.NewLine +
+                "location2" + Environment.NewLine +
+                "location3" + Environment.NewLine +
+                "location4";
+
+            var viewEngine = new Mock<ICompositeViewEngine>(MockBehavior.Strict);
+            viewEngine
+                .Setup(v => v.GetView(/*executingFilePath*/ null, It.IsAny<string>(), /*isMainPage*/ false))
+                .Returns(ViewEngineResult.NotFound("test-view", new[] { "location1", "location2" }))
+                .Verifiable();
+            viewEngine
+                .Setup(v => v.FindView(It.IsAny<ActionContext>(), It.IsAny<string>(), /*isMainPage*/ false))
+                .Returns(ViewEngineResult.NotFound("test-view", new[] { "location3", "location4" }))
+                .Verifiable();
+            var metadataProvider = TestModelMetadataProvider.CreateDefaultProvider();
+            var htmlGenerator = GetGenerator(metadataProvider, viewEngine.Object);
+            var writer = new StringWriter();
+
+            // Act & Assert
+            var exception = await Assert.ThrowsAsync<InvalidOperationException>(
+                () => htmlGenerator.RenderPartialViewAsync(new ViewContext(), "test-view", null, null, writer));
+            Assert.Equal(expected, exception.Message);
+        }
+
+        [Fact]
+        public async Task RenderPartialAsync_RendersViewToWriter()
+        {
+            // Arrange
+            var model = new Model();
+            var metadataProvider = TestModelMetadataProvider.CreateDefaultProvider();
+            var expected = DefaultTemplatesUtilities.FormatOutput(metadataProvider.GetModelExplorerForType(model.GetType(), model));
+            var htmlGenerator = GetGenerator(metadataProvider);
+            var writer = new StringWriter();
+            var viewContext = GetViewContext<Model>(model: null, metadataProvider: metadataProvider);
+
+            // Act
+            await htmlGenerator.RenderPartialViewAsync(viewContext, "test-view", model, null, writer);
+
+            // Assert
+            Assert.Equal(expected, writer.ToString());
+        }
+
         // GetCurrentValues uses only the IModelMetadataProvider passed to the DefaultHtmlGenerator constructor.
-        private static IHtmlGenerator GetGenerator(IModelMetadataProvider metadataProvider)
+        private static IHtmlGenerator GetGenerator(
+            IModelMetadataProvider metadataProvider,
+            ICompositeViewEngine viewEngine = null)
         {
             var mvcViewOptionsAccessor = new Mock<IOptions<MvcViewOptions>>();
             mvcViewOptionsAccessor.SetupGet(accessor => accessor.Value).Returns(new MvcViewOptions());
@@ -696,13 +747,16 @@ private static IHtmlGenerator GetGenerator(IModelMetadataProvider metadataProvid
                 metadataProvider,
                 new ClientValidatorCache());
 
+            viewEngine = viewEngine ?? DefaultTemplatesUtilities.CreateViewEngine();
+
             return new DefaultHtmlGenerator(
                 antiforgery.Object,
                 mvcViewOptionsAccessor.Object,
                 metadataProvider,
                 new UrlHelperFactory(),
                 htmlEncoder,
-                attributeProvider);
+                attributeProvider,
+                viewEngine);
         }
 
         // GetCurrentValues uses only the ModelStateDictionary and ViewDataDictionary from the passed ViewContext.
diff --git a/test/WebSites/HtmlGenerationWebSite/Controllers/HtmlGeneration_HomeController.cs b/test/WebSites/HtmlGenerationWebSite/Controllers/HtmlGeneration_HomeController.cs
index 3736f9554c..3402b1fbdf 100644
--- a/test/WebSites/HtmlGenerationWebSite/Controllers/HtmlGeneration_HomeController.cs
+++ b/test/WebSites/HtmlGenerationWebSite/Controllers/HtmlGeneration_HomeController.cs
@@ -108,6 +108,8 @@ public IActionResult ProductList()
             return View(_products);
         }
 
+        public IActionResult ProductListUsingTagHelpers() => View(_products);
+
         public IActionResult EmployeeList()
         {
             var employees = new List<Employee>
diff --git a/test/WebSites/HtmlGenerationWebSite/Views/HtmlGeneration_Home/ProductListUsingTagHelpers.cshtml b/test/WebSites/HtmlGenerationWebSite/Views/HtmlGeneration_Home/ProductListUsingTagHelpers.cshtml
new file mode 100644
index 0000000000..e5ad50be52
--- /dev/null
+++ b/test/WebSites/HtmlGenerationWebSite/Views/HtmlGeneration_Home/ProductListUsingTagHelpers.cshtml
@@ -0,0 +1,30 @@
+@using HtmlGenerationWebSite.Models
+@model IEnumerable<Product>
+
+@addTagHelper *, Microsoft.AspNetCore.Mvc.TagHelpers
+
+<html>
+<head>
+    <meta charset="utf-8" />
+</head>
+<body>
+    <form asp-action="Index" asp-controller="HtmlGeneration_Product" asp-antiforgery="true">
+    @{
+        var index = 0;
+        var fieldPrefix = ViewData.TemplateInfo.HtmlFieldPrefix;
+        foreach (var product in Model)
+        {
+            @* Update HtmlFieldPrefix so generated for, id and name attribute values are correct. *@
+            ViewData.TemplateInfo.HtmlFieldPrefix = fieldPrefix + string.Format("[{0}]", index++);
+            <div>
+                <label asp-for="@(product.HomePage)" class="product"></label>
+                <input asp-for="@(product.HomePage)" type="url" size="50" disabled="disabled" readonly="readonly" />
+            </div>
+            <partial name="_ProductPartial" model="@product" />
+            ViewData.TemplateInfo.HtmlFieldPrefix = fieldPrefix;
+        }
+        <input type="submit" />
+    }
+    </form>
+</body>
+</html>