Add model attribute for PartialTagHelper.

- The model attribute is used to define any object based model to be passed to a `TagHelper`. It enables scenarios when users want to pass in `new` poco types.
- Added unit tests for the new `ResolveModel` method in `PartialTagHelper`.
- Added a single functional test to verify the end-to-end.

#7374

src/Microsoft.AspNetCore.Mvc.TagHelpers/PartialTagHelper.cs

@@ -20,6 +20,9 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
     [HtmlTargetElement("partial", Attributes = "name", TagStructure = TagStructure.WithoutEndTag)]
     public class PartialTagHelper : TagHelper
     {
+        private const string ForAttributeName = "for";
+        private const string ModelAttributeName = "model";
+
         private readonly ICompositeViewEngine _viewEngine;
         private readonly IViewBufferScope _viewBufferScope;
 
@@ -37,10 +40,17 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
         public string Name { get; set; }
 
         /// <summary>
-        /// An expression to be evaluated against the current model.
+        /// An expression to be evaluated against the current model. Cannot be used together with <see cref="Model"/>.
         /// </summary>
+        [HtmlAttributeName(ForAttributeName)]
         public ModelExpression For { get; set; }
 
+        /// <summary>
+        /// The model to pass into the partial view. Cannot be used together with <see cref="For"/>.
+        /// </summary>
+        [HtmlAttributeName(ModelAttributeName)]
+        public object Model { get; set; }
+
         /// <summary>
         /// A <see cref="ViewDataDictionary"/> to pass into the partial view.
         /// </summary>
@@ -63,10 +73,11 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
                 throw new ArgumentNullException(nameof(context));
             }
 
+            var model = ResolveModel();
             var viewBuffer = new ViewBuffer(_viewBufferScope, Name, ViewBuffer.PartialViewPageSize);
             using (var writer = new ViewBufferTextWriter(viewBuffer, Encoding.UTF8))
             {
-                await RenderPartialViewAsync(writer);
+                await RenderPartialViewAsync(writer, model);
 
                 // Reset the TagName. We don't want `partial` to render.
                 output.TagName = null;
@@ -74,7 +85,33 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
             }
         }
 
-        private async Task RenderPartialViewAsync(TextWriter writer)
+        // Internal for testing
+        internal object ResolveModel()
+        {
+            if (Model != null & For != null)
+            {
+                throw new InvalidOperationException(
+                    Resources.FormatPartialTagHelper_InvalidModelAttributes(
+                        typeof(PartialTagHelper).FullName,
+                        ForAttributeName,
+                        ModelAttributeName));
+            }
+
+            if (Model != null)
+            {
+                return Model;
+            }
+
+            if (For != null)
+            {
+                return For.Model;
+            }
+
+            // Model and For are null, fallback to the ViewContext's ViewData model.
+            return ViewContext.ViewData.Model;
+        }
+
+        private async Task RenderPartialViewAsync(TextWriter writer, object model)
         {
             var viewEngineResult = _viewEngine.GetView(ViewContext.ExecutingFilePath, Name, isMainPage: false);
             var getViewLocations = viewEngineResult.SearchedLocations;
@@ -99,9 +136,6 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
             var view = viewEngineResult.View;
             // Determine which ViewData we should use to construct a new ViewData
             var baseViewData = ViewData ?? ViewContext.ViewData;
-
-            // Use the rendering View's model only if an for expression does not exist
-            var model = For != null ? For.Model : ViewContext.ViewData.Model;
             var newViewData = new ViewDataDictionary<object>(baseViewData, model);
             var partialViewContext = new ViewContext(ViewContext, view, newViewData, writer);
 

src/Microsoft.AspNetCore.Mvc.TagHelpers/Properties/Resources.Designer.cs

@@ -192,6 +192,20 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
         internal static string FormatViewEngine_PartialViewNotFound(object p0, object p1)
             => string.Format(CultureInfo.CurrentCulture, GetString("ViewEngine_PartialViewNotFound"), p0, p1);
 
+        /// <summary>
+        /// Cannot use '{0}' with both '{1}' and '{2}' attributes.
+        /// </summary>
+        internal static string PartialTagHelper_InvalidModelAttributes
+        {
+            get => GetString("PartialTagHelper_InvalidModelAttributes");
+        }
+
+        /// <summary>
+        /// Cannot use '{0}' with both '{1}' and '{2}' attributes.
+        /// </summary>
+        internal static string FormatPartialTagHelper_InvalidModelAttributes(object p0, object p1, object p2)
+            => string.Format(CultureInfo.CurrentCulture, GetString("PartialTagHelper_InvalidModelAttributes"), p0, p1, p2);
+
         private static string GetString(string name, params string[] formatterNames)
         {
             var value = _resourceManager.GetString(name);

src/Microsoft.AspNetCore.Mvc.TagHelpers/Resources.resx

@@ -156,4 +156,7 @@
   <data name="ViewEngine_PartialViewNotFound" xml:space="preserve">
     <value>The partial view '{0}' was not found. The following locations were searched:{1}</value>
   </data>
+  <data name="PartialTagHelper_InvalidModelAttributes" xml:space="preserve">
+    <value>Cannot use '{0}' with both '{1}' and '{2}' attributes.</value>
+  </data>
 </root>

test/Microsoft.AspNetCore.Mvc.FunctionalTests/compiler/resources/HtmlGenerationWebSite.HtmlGeneration_Home.ProductListUsingTagHelpers.html

@@ -62,5 +62,13 @@ Product_2 description</textarea>
         <div>HtmlFieldPrefix = </div>
         <input type="submit" />
     <input name="__RequestVerificationToken" type="hidden" value="{0}" /></form>
+
+    
+
+<hr />
+<h2>You might also like these products!</h2>
+<ul>
+        <li><a href="http://www.contoso.com/">THE Best Product</a></li>
+</ul>
 </body>
 </html>

test/Microsoft.AspNetCore.Mvc.FunctionalTests/compiler/resources/HtmlGenerationWebSite.HtmlGeneration_Home.ProductListUsingTagHelpersWithNullModel.html

@@ -26,5 +26,13 @@
         <div>HtmlFieldPrefix = </div>
         <input type="submit" />
     <input name="__RequestVerificationToken" type="hidden" value="{0}" /></form>
+
+    
+
+<hr />
+<h2>You might also like these products!</h2>
+<ul>
+        <li><a href="http://www.contoso.com/">THE Best Product</a></li>
+</ul>
 </body>
 </html>

test/Microsoft.AspNetCore.Mvc.TagHelpers.Test/PartialTagHelperTest.cs

@@ -23,6 +23,90 @@ namespace Microsoft.AspNetCore.Mvc.TagHelpers
 {
     public class PartialTagHelperTest
     {
+        [Fact]
+        public void ResolveModel_ReturnsModelWhenProvided()
+        {
+            // Arrange
+            var expectedModel = new object();
+            var tagHelper = new PartialTagHelper(Mock.Of<ICompositeViewEngine>(), Mock.Of<IViewBufferScope>())
+            {
+                Model = expectedModel,
+            };
+
+            // Act
+            var model = tagHelper.ResolveModel();
+
+            // Assert
+            Assert.Same(expectedModel, model);
+        }
+
+        [Fact]
+        public void ResolveModel_ReturnsForModelWhenProvided()
+        {
+            // Arrange
+            var expectedModel = new PropertyModel();
+            var modelMetadataProvider = new TestModelMetadataProvider();
+            var containerModel = new TestModel()
+            {
+                Property = expectedModel
+            };
+            var containerModelExplorer = modelMetadataProvider.GetModelExplorerForType(
+                typeof(TestModel),
+                containerModel);
+            var propertyModelExplorer = containerModelExplorer.GetExplorerForProperty(nameof(TestModel.Property));
+            var tagHelper = new PartialTagHelper(Mock.Of<ICompositeViewEngine>(), Mock.Of<IViewBufferScope>())
+            {
+                For = new ModelExpression("Property", propertyModelExplorer),
+            };
+
+            // Act
+            var model = tagHelper.ResolveModel();
+
+            // Assert
+            Assert.Same(expectedModel, model);
+        }
+
+        [Fact]
+        public void ResolveModel_ReturnsViewContextsViewDataModelWhenModelAndForAreNull()
+        {
+            // Arrange
+            var expectedModel = new object();
+            var viewContext = GetViewContext();
+            viewContext.ViewData.Model = expectedModel;
+            var tagHelper = new PartialTagHelper(Mock.Of<ICompositeViewEngine>(), Mock.Of<IViewBufferScope>())
+            {
+                ViewContext = viewContext
+            };
+
+            // Act
+            var model = tagHelper.ResolveModel();
+
+            // Assert
+            Assert.Same(expectedModel, model);
+        }
+
+        [Fact]
+        public void ResolveModel_ThrowsWhenModelAndForProvided()
+        {
+            // Arrange
+            var modelMetadataProvider = new TestModelMetadataProvider();
+            var containerModel = new TestModel();
+            var containerModelExplorer = modelMetadataProvider.GetModelExplorerForType(
+                typeof(TestModel),
+                containerModel);
+            var propertyModelExplorer = containerModelExplorer.GetExplorerForProperty(nameof(TestModel.Property));
+            var tagHelper = new PartialTagHelper(Mock.Of<ICompositeViewEngine>(), Mock.Of<IViewBufferScope>())
+            {
+                Model = new object(),
+                For = new ModelExpression("Property", propertyModelExplorer),
+            };
+            var expectedMessage = Resources.FormatPartialTagHelper_InvalidModelAttributes(typeof(PartialTagHelper).FullName, "for", "model");
+
+            // Act & Assert
+            var exception = Assert.Throws<InvalidOperationException>(() => tagHelper.ResolveModel());
+            Assert.Equal(expectedMessage, exception.Message);
+        }
+
         [Fact]
         public async Task ProcessAsync_RendersPartialView_IfGetViewReturnsView()
         {

test/WebSites/HtmlGenerationWebSite/Models/ProductRecommendations.cs

@@ -0,0 +1,23 @@
+// 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;
+
+namespace HtmlGenerationWebSite.Models
+{
+    public class ProductRecommendations
+    {
+        public ProductRecommendations(params Product[] products)
+        {
+            if (products == null)
+            {
+                throw new ArgumentNullException(nameof(products));
+            }
+
+            Products = products;
+        }
+
+        public IEnumerable<Product> Products { get; }
+    }
+}

test/WebSites/HtmlGenerationWebSite/Views/HtmlGeneration_Home/ProductListUsingTagHelpers.cshtml

@@ -21,5 +21,7 @@
         <div>HtmlFieldPrefix = @ViewData.TemplateInfo.HtmlFieldPrefix</div>
         <input type="submit" />
     </form>
+
+    <partial name="_ProductRecommendations" model='new ProductRecommendations(new Product() { ProductName = "THE Best Product", HomePage = new Uri("http://www.contoso.com")})' />
 </body>
 </html>

test/WebSites/HtmlGenerationWebSite/Views/HtmlGeneration_Home/_ProductRecommendations.cshtml

@@ -0,0 +1,13 @@
+@using HtmlGenerationWebSite.Models
+@model ProductRecommendations
+
+@addTagHelper *, Microsoft.AspNetCore.Mvc.TagHelpers
+
+<hr />
+<h2>You might also like these products!</h2>
+<ul>
+    @foreach (var product in Model.Products)
+    {
+        <li><a href="@product.HomePage">@product.ProductName</a></li>
+    }
+</ul>