PR comments and some smallish cleanup

- `IRazorViewEngine.MakePathAbsolute()` -> `GetAbsolutePath()`
- set `IsPartial` on all `IRazorPage` instances
- improve consistency of methods in `HtmlHelperPartialExtensions`
  - a couple unnecessarily passed `htmlHelper.ViewData`
  - add missing tests of these extension methods
- restore parameter checks in `CompositeViewEngine`
- reduce `List<string>` and remove enumerator allocations in `CompositeViewEngine`

nits:
- correct a few comments
- use `<seealso/>`

src/Microsoft.AspNet.Mvc.Razor/IRazorViewEngine.cs

@@ -18,7 +18,7 @@ namespace Microsoft.AspNet.Mvc.Razor
         /// <param name="pageName">The name of the page.</param>
         /// <param name="isPartial">Determines if the page being found is a partial.</param>
         /// <returns>The <see cref="RazorPageResult"/> of locating the page.</returns>
-        /// <remarks>Page search semantics match <see cref="IViewEngine.FindView"/>.</remarks>
+        /// <remarks><seealso cref="IViewEngine.FindView"/>.</remarks>
         RazorPageResult FindPage(ActionContext context, string pageName, bool isPartial);
 
         /// <summary>
@@ -29,7 +29,7 @@ namespace Microsoft.AspNet.Mvc.Razor
         /// <param name="pagePath">The path to the page.</param>
         /// <param name="isPartial">Determines if the page being found is a partial.</param>
         /// <returns>The <see cref="RazorPageResult"/> of locating the page.</returns>
-        /// <remarks>See also <see cref="IViewEngine.GetView"/>.</remarks>
+        /// <remarks><seealso cref="IViewEngine.GetView"/>.</remarks>
         RazorPageResult GetPage(string executingFilePath, string pagePath, bool isPartial);
 
         /// <summary>
@@ -43,6 +43,6 @@ namespace Microsoft.AspNet.Mvc.Razor
         /// <paramref name="pagePath"/> is a relative path. The <paramref name="pagePath"/> value (unchanged)
         /// otherwise.
         /// </returns>
-        string MakePathAbsolute(string executingFilePath, string pagePath);
+        string GetAbsolutePath(string executingFilePath, string pagePath);
     }
 }

src/Microsoft.AspNet.Mvc.Razor/RazorView.cs

@@ -174,7 +174,7 @@ namespace Microsoft.AspNet.Mvc.Razor
 
                     // Pass correct absolute path to next layout or the entry page if this view start set Layout to a
                     // relative path.
-                    layout = _viewEngine.MakePathAbsolute(viewStart.Path, viewStart.Layout);
+                    layout = _viewEngine.GetAbsolutePath(viewStart.Path, viewStart.Layout);
                 }
             }
             finally

src/Microsoft.AspNet.Mvc.Razor/RazorViewEngine.cs

@@ -276,7 +276,7 @@ namespace Microsoft.AspNet.Mvc.Razor
 
         private ViewLocationCacheResult LocatePageFromPath(string executingFilePath, string pagePath, bool isPartial)
         {
-            var applicationRelativePath = MakePathAbsolute(executingFilePath, pagePath);
+            var applicationRelativePath = GetAbsolutePath(executingFilePath, pagePath);
             var cacheKey = new ViewLocationCacheKey(applicationRelativePath, isPartial);
             ViewLocationCacheResult cacheResult;
             if (!ViewLookupCache.TryGetValue(cacheKey, out cacheResult))
@@ -350,7 +350,7 @@ namespace Microsoft.AspNet.Mvc.Razor
         }
 
         /// <inheritdoc />
-        public string MakePathAbsolute(string executingFilePath, string pagePath)
+        public string GetAbsolutePath(string executingFilePath, string pagePath)
         {
             if (string.IsNullOrEmpty(pagePath))
             {
@@ -505,11 +505,14 @@ namespace Microsoft.AspNet.Mvc.Razor
             }
 
             var page = result.ViewEntry.PageFactory();
+            page.IsPartial = isPartial;
+
             var viewStarts = new IRazorPage[result.ViewStartEntries.Count];
             for (var i = 0; i < viewStarts.Length; i++)
             {
                 var viewStartItem = result.ViewStartEntries[i];
                 viewStarts[i] = viewStartItem.PageFactory();
+                viewStarts[i].IsPartial = true;
             }
 
             var view = new RazorView(

src/Microsoft.AspNet.Mvc.ViewFeatures/Rendering/HtmlHelperPartialExtensions.cs

@@ -68,7 +68,7 @@ namespace Microsoft.AspNet.Mvc.Rendering
                 throw new ArgumentNullException(nameof(partialViewName));
             }
 
-            return htmlHelper.PartialAsync(partialViewName, htmlHelper.ViewData.Model, viewData: viewData);
+            return htmlHelper.PartialAsync(partialViewName, htmlHelper.ViewData.Model, viewData);
         }
 
         /// <summary>
@@ -259,8 +259,7 @@ namespace Microsoft.AspNet.Mvc.Rendering
                 throw new ArgumentNullException(nameof(partialViewName));
             }
 
-            return htmlHelper.RenderPartialAsync(partialViewName, htmlHelper.ViewData.Model,
+            return htmlHelper.RenderPartialAsync(partialViewName, htmlHelper.ViewData.Model, viewData: null);
-                                                 viewData: htmlHelper.ViewData);
         }
 
         /// <summary>
@@ -290,7 +289,7 @@ namespace Microsoft.AspNet.Mvc.Rendering
                 throw new ArgumentNullException(nameof(partialViewName));
             }
 
-            return htmlHelper.RenderPartialAsync(partialViewName, htmlHelper.ViewData.Model, viewData: viewData);
+            return htmlHelper.RenderPartialAsync(partialViewName, htmlHelper.ViewData.Model, viewData);
         }
 
         /// <summary>
@@ -320,7 +319,7 @@ namespace Microsoft.AspNet.Mvc.Rendering
                 throw new ArgumentNullException(nameof(partialViewName));
             }
 
-            return htmlHelper.RenderPartialAsync(partialViewName, model, htmlHelper.ViewData);
+            return htmlHelper.RenderPartialAsync(partialViewName, model, viewData: null);
         }
     }
 }

src/Microsoft.AspNet.Mvc.ViewFeatures/Rendering/ViewContext.cs

@@ -14,7 +14,7 @@ namespace Microsoft.AspNet.Mvc.Rendering
     /// </summary>
     public class ViewContext : ActionContext
     {
-        // We need a default FormContext if the user uses html <form> instead of an MvcForm
+        // We need a default FormContext if the user uses HTML <form> instead of an MvcForm
         private readonly FormContext _defaultFormContext = new FormContext();
 
         private FormContext _formContext;

src/Microsoft.AspNet.Mvc.ViewFeatures/ViewEngines/CompositeViewEngine.cs

@@ -1,8 +1,10 @@
 // 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.Linq;
+using Microsoft.AspNet.Mvc.ViewFeatures;
 using Microsoft.Extensions.OptionsModel;
 
 namespace Microsoft.AspNet.Mvc.ViewEngines
@@ -10,8 +12,6 @@ namespace Microsoft.AspNet.Mvc.ViewEngines
     /// <inheritdoc />
     public class CompositeViewEngine : ICompositeViewEngine
     {
-        private const string ViewExtension = ".cshtml";
-
         /// <summary>
         /// Initializes a new instance of <see cref="CompositeViewEngine"/>.
         /// </summary>
@@ -27,10 +27,22 @@ namespace Microsoft.AspNet.Mvc.ViewEngines
         /// <inheritdoc />
         public ViewEngineResult FindView(ActionContext context, string viewName, bool isPartial)
         {
-            List<string> searchedLocations = null;
+            if (context == null)
-            foreach (var engine in ViewEngines)
             {
-                var result = engine.FindView(context, viewName, isPartial);
+                throw new ArgumentNullException(nameof(context));
+            }
+
+            if (string.IsNullOrEmpty(viewName))
+            {
+                throw new ArgumentException(Resources.ArgumentCannotBeNullOrEmpty, nameof(viewName));
+            }
+
+            // Do not allocate in the common cases: ViewEngines contains one entry or initial attempt is successful.
+            IEnumerable<string> searchedLocations = null;
+            List<string> searchedList = null;
+            for (var index = 0; index < ViewEngines.Count; index++)
+            {
+                var result = ViewEngines[index].FindView(context, viewName, isPartial);
                 if (result.Success)
                 {
                     return result;
@@ -38,11 +50,19 @@ namespace Microsoft.AspNet.Mvc.ViewEngines
 
                 if (searchedLocations == null)
                 {
-                    searchedLocations = new List<string>(result.SearchedLocations);
+                    // First failure.
+                    searchedLocations = result.SearchedLocations;
                 }
                 else
                 {
-                    searchedLocations.AddRange(result.SearchedLocations);
+                    if (searchedList == null)
+                    {
+                        // Second failure.
+                        searchedList = new List<string>(searchedLocations);
+                        searchedLocations = searchedList;
+                    }
+
+                    searchedList.AddRange(result.SearchedLocations);
                 }
             }
 
@@ -52,10 +72,17 @@ namespace Microsoft.AspNet.Mvc.ViewEngines
         /// <inheritdoc />
         public ViewEngineResult GetView(string executingFilePath, string viewPath, bool isPartial)
         {
-            List<string> searchedLocations = null;
+            if (string.IsNullOrEmpty(viewPath))
-            foreach (var engine in ViewEngines)
             {
-                var result = engine.GetView(executingFilePath, viewPath, isPartial);
+                throw new ArgumentException(Resources.ArgumentCannotBeNullOrEmpty, nameof(viewPath));
+            }
+
+            // Do not allocate in the common cases: ViewEngines contains one entry or initial attempt is successful.
+            IEnumerable<string> searchedLocations = null;
+            List<string> searchedList = null;
+            for (var index = 0; index < ViewEngines.Count; index++)
+            {
+                var result = ViewEngines[index].GetView(executingFilePath, viewPath, isPartial);
                 if (result.Success)
                 {
                     return result;
@@ -63,11 +90,19 @@ namespace Microsoft.AspNet.Mvc.ViewEngines
 
                 if (searchedLocations == null)
                 {
-                    searchedLocations = new List<string>(result.SearchedLocations);
+                    // First failure.
+                    searchedLocations = result.SearchedLocations;
                 }
                 else
                 {
-                    searchedLocations.AddRange(result.SearchedLocations);
+                    if (searchedList == null)
+                    {
+                        // Second failure.
+                        searchedList = new List<string>(searchedLocations);
+                        searchedLocations = searchedList;
+                    }
+
+                    searchedList.AddRange(result.SearchedLocations);
                 }
             }
 

test/Microsoft.AspNet.Mvc.FunctionalTests/InlineConstraintTests.cs

@@ -46,7 +46,7 @@ namespace Microsoft.AspNet.Mvc.FunctionalTests
                 " The following locations were searched:" + PlatformNormalizer.GetNewLinesAsUnderscores(1) +
                 "/Areas/Users/Views/Home/Index.cshtml" + PlatformNormalizer.GetNewLinesAsUnderscores(1) +
                 "/Areas/Users/Views/Shared/Index.cshtml" + PlatformNormalizer.GetNewLinesAsUnderscores(1) +
-                "/Views/Shared/Index.cshtml.",
+                "/Views/Shared/Index.cshtml",
                 exception.ExceptionMessage);
         }
 

test/Microsoft.AspNet.Mvc.Razor.Test/RazorViewEngineTest.cs

@@ -1306,13 +1306,13 @@ namespace Microsoft.AspNet.Mvc.Razor.Test
         [InlineData("/Home/Index.cshtml", "Page")]
         [InlineData("/Home/Index.cshtml", "Folder/Page")]
         [InlineData("/Home/Index.cshtml", "Folder1/Folder2/Page")]
-        public void MakePathAbsolute_ReturnsPagePathUnchanged_IfNotAPath(string executingFilePath, string pagePath)
+        public void GetAbsolutePath_ReturnsPagePathUnchanged_IfNotAPath(string executingFilePath, string pagePath)
         {
             // Arrange
             var viewEngine = CreateViewEngine();
 
             // Act
-            var result = viewEngine.MakePathAbsolute(executingFilePath, pagePath);
+            var result = viewEngine.GetAbsolutePath(executingFilePath, pagePath);
 
             // Assert
             Assert.Same(pagePath, result);
@@ -1325,13 +1325,13 @@ namespace Microsoft.AspNet.Mvc.Razor.Test
         [InlineData("/Home/Index.cshtml", "~/Page")]
         [InlineData("/Home/Index.cshtml", "/Folder/Page.cshtml")]
         [InlineData("/Home/Index.cshtml", "~/Folder1/Folder2/Page.rzr")]
-        public void MakePathAbsolute_ReturnsPageUnchanged_IfAppRelative(string executingFilePath, string pagePath)
+        public void GetAbsolutePath_ReturnsPageUnchanged_IfAppRelative(string executingFilePath, string pagePath)
         {
             // Arrange
             var viewEngine = CreateViewEngine();
 
             // Act
-            var result = viewEngine.MakePathAbsolute(executingFilePath, pagePath);
+            var result = viewEngine.GetAbsolutePath(executingFilePath, pagePath);
 
             // Assert
             Assert.Same(pagePath, result);
@@ -1341,14 +1341,14 @@ namespace Microsoft.AspNet.Mvc.Razor.Test
         [InlineData("Page.cshtml")]
         [InlineData("Folder/Page.cshtml")]
         [InlineData("../../Folder1/Folder2/Page.cshtml")]
-        public void MakePathAbsolute_ResolvesRelativeToExecutingPage(string pagePath)
+        public void GetAbsolutePath_ResolvesRelativeToExecutingPage(string pagePath)
         {
             // Arrange
             var expectedPagePath = "/Home/" + pagePath;
             var viewEngine = CreateViewEngine();
 
             // Act
-            var result = viewEngine.MakePathAbsolute("/Home/Page.cshtml", pagePath);
+            var result = viewEngine.GetAbsolutePath("/Home/Page.cshtml", pagePath);
 
             // Assert
             Assert.Equal(expectedPagePath, result);
@@ -1358,14 +1358,14 @@ namespace Microsoft.AspNet.Mvc.Razor.Test
         [InlineData("Page.cshtml")]
         [InlineData("Folder/Page.cshtml")]
         [InlineData("../../Folder1/Folder2/Page.cshtml")]
-        public void MakePathAbsolute_ResolvesRelativeToAppRoot_IfNoPageExecuting(string pagePath)
+        public void GetAbsolutePath_ResolvesRelativeToAppRoot_IfNoPageExecuting(string pagePath)
         {
             // Arrange
             var expectedPagePath = "/" + pagePath;
             var viewEngine = CreateViewEngine();
 
             // Act
-            var result = viewEngine.MakePathAbsolute(executingFilePath: null, pagePath: pagePath);
+            var result = viewEngine.GetAbsolutePath(executingFilePath: null, pagePath: pagePath);
 
             // Assert
             Assert.Equal(expectedPagePath, result);
@@ -1649,7 +1649,7 @@ namespace Microsoft.AspNet.Mvc.Razor.Test
             }
         }
 
-        // Return RazorViewEngine with factories that always successfully create instances.
+        // Return RazorViewEngine with a page factory provider that is always successful.
         private RazorViewEngine CreateSuccessfulViewEngine()
         {
             var pageFactory = new Mock<IRazorPageFactoryProvider>(MockBehavior.Strict);

test/Microsoft.AspNet.Mvc.Razor.Test/RazorViewTest.cs

@@ -135,7 +135,7 @@ namespace Microsoft.AspNet.Mvc.Razor
             var activator = Mock.Of<IRazorPageActivator>();
             var viewEngine = new Mock<IRazorViewEngine>();
             viewEngine
-                .Setup(p => p.MakePathAbsolute("_ViewStart", LayoutPath))
+                .Setup(p => p.GetAbsolutePath("_ViewStart", LayoutPath))
                 .Returns(LayoutPath);
             viewEngine
                 .Setup(v => v.GetPage(pagePath, LayoutPath, /*isPartial*/ true))
@@ -345,10 +345,10 @@ namespace Microsoft.AspNet.Mvc.Razor
 
             var viewEngine = new Mock<IRazorViewEngine>(MockBehavior.Strict);
             viewEngine
-                .Setup(engine => engine.MakePathAbsolute(/*executingFilePath*/ null, "/fake-layout-path"))
+                .Setup(engine => engine.GetAbsolutePath(/*executingFilePath*/ null, "/fake-layout-path"))
                 .Returns("/fake-layout-path");
             viewEngine
-                .Setup(engine => engine.MakePathAbsolute(/*executingFilePath*/ null, layoutPath))
+                .Setup(engine => engine.GetAbsolutePath(/*executingFilePath*/ null, layoutPath))
                 .Returns(layoutPath);
 
             var view = new RazorView(
@@ -1588,10 +1588,10 @@ namespace Microsoft.AspNet.Mvc.Razor
             });
             var viewEngine = new Mock<IRazorViewEngine>(MockBehavior.Strict);
             viewEngine
-                .Setup(engine => engine.MakePathAbsolute(/*executingFilePath*/ null, expectedViewStart))
+                .Setup(engine => engine.GetAbsolutePath(/*executingFilePath*/ null, expectedViewStart))
                 .Returns(expectedViewStart);
             viewEngine
-                .Setup(engine => engine.MakePathAbsolute(/*executingFilePath*/ null, expectedPage))
+                .Setup(engine => engine.GetAbsolutePath(/*executingFilePath*/ null, expectedPage))
                 .Returns(expectedPage);
 
             var view = new RazorView(
@@ -1646,10 +1646,10 @@ namespace Microsoft.AspNet.Mvc.Razor
 
             var viewEngine = new Mock<IRazorViewEngine>(MockBehavior.Strict);
             viewEngine
-                .Setup(engine => engine.MakePathAbsolute("~/_ViewStart.cshtml", "_Layout.cshtml"))
+                .Setup(engine => engine.GetAbsolutePath("~/_ViewStart.cshtml", "_Layout.cshtml"))
                 .Returns("~/_Layout.cshtml");
             viewEngine
-                .Setup(engine => engine.MakePathAbsolute("~/Home/_ViewStart.cshtml", "_Layout.cshtml"))
+                .Setup(engine => engine.GetAbsolutePath("~/Home/_ViewStart.cshtml", "_Layout.cshtml"))
                 .Returns("~/Home/_Layout.cshtml");
 
             var view = new RazorView(
@@ -1691,10 +1691,10 @@ namespace Microsoft.AspNet.Mvc.Razor
             });
             var viewEngine = new Mock<IRazorViewEngine>(MockBehavior.Strict);
             viewEngine
-                .Setup(engine => engine.MakePathAbsolute(/*executingFilePath*/ null, "Layout"))
+                .Setup(engine => engine.GetAbsolutePath(/*executingFilePath*/ null, "Layout"))
                 .Returns("Layout");
             viewEngine
-                .Setup(engine => engine.MakePathAbsolute(/*executingFilePath*/ null, /*pagePath*/ null))
+                .Setup(engine => engine.GetAbsolutePath(/*executingFilePath*/ null, /*pagePath*/ null))
                 .Returns<string>(null);
 
             var view = new RazorView(
@@ -1737,7 +1737,7 @@ namespace Microsoft.AspNet.Mvc.Razor
             });
             var viewEngine = new Mock<IRazorViewEngine>(MockBehavior.Strict);
             viewEngine
-                .Setup(p => p.MakePathAbsolute(/*executingFilePath*/ null, "/Layout.cshtml"))
+                .Setup(p => p.GetAbsolutePath(/*executingFilePath*/ null, "/Layout.cshtml"))
                 .Returns("/Layout.cshtml");
             viewEngine
                 .Setup(p => p.GetPage(/*executingFilePath*/ null, "/Layout.cshtml", /*isPartial*/ true))

test/Microsoft.AspNet.Mvc.ViewFeatures.Test/Rendering/HtmlHelperPartialExtensionsTest.cs

@@ -17,24 +17,62 @@ namespace Microsoft.AspNet.Mvc.Rendering
 {
     public class HtmlHelperPartialExtensionsTest
     {
-        public static TheoryData<Func<IHtmlHelper, IHtmlContent>> PartialExtensionMethods
+        // Func<IHtmlHelper, IHtmlContent>, expected Model, expected ViewDataDictionary
+        public static TheoryData<Func<IHtmlHelper, IHtmlContent>, object, ViewDataDictionary> PartialExtensionMethods
         {
             get
             {
-                var vdd = new ViewDataDictionary(new EmptyModelMetadataProvider());
+                var viewData = new ViewDataDictionary(new EmptyModelMetadataProvider());
-                return new TheoryData<Func<IHtmlHelper, IHtmlContent>>
+                var model = new object();
+                return new TheoryData<Func<IHtmlHelper, IHtmlContent>, object, ViewDataDictionary>
                 {
-                    helper => helper.Partial("test"),
+                    { helper => helper.Partial("test"), null, null },
-                    helper => helper.Partial("test", new object()),
+                    { helper => helper.Partial("test", model), model, null },
-                    helper => helper.Partial("test", vdd),
+                    { helper => helper.Partial("test", viewData), null, viewData },
-                    helper => helper.Partial("test", new object(), vdd)
+                    { helper => helper.Partial("test", model, viewData), model, viewData },
                 };
             }
         }
 
         [Theory]
         [MemberData(nameof(PartialExtensionMethods))]
-        public void PartialMethods_DoesNotWrapThrownException(Func<IHtmlHelper, IHtmlContent> partialMethod)
+        public void PartialMethods_CallHtmlHelperWithExpectedArguments(
+            Func<IHtmlHelper, IHtmlContent> partialMethod,
+            object expectedModel,
+            ViewDataDictionary expectedViewData)
+        {
+            // Arrange
+            var htmlContent = Mock.Of<IHtmlContent>();
+            var helper = new Mock<IHtmlHelper>(MockBehavior.Strict);
+            if (expectedModel == null)
+            {
+                // Extension methods without model parameter use ViewData.Model to get Model.
+                var viewData = expectedViewData ?? new ViewDataDictionary(new EmptyModelMetadataProvider());
+                helper
+                    .SetupGet(h => h.ViewData)
+                    .Returns(viewData)
+                    .Verifiable();
+            }
+
+            helper
+                .Setup(h => h.PartialAsync("test", expectedModel, expectedViewData))
+                .Returns(Task.FromResult(htmlContent))
+                .Verifiable();
+
+            // Act
+            var result = partialMethod(helper.Object);
+
+            // Assert
+            Assert.Same(htmlContent, result);
+            helper.VerifyAll();
+        }
+
+        [Theory]
+        [MemberData(nameof(PartialExtensionMethods))]
+        public void PartialMethods_DoesNotWrapThrownException(
+            Func<IHtmlHelper, IHtmlContent> partialMethod,
+            object unusedModel,
+            ViewDataDictionary unusedViewData)
         {
             // Arrange
             var expected = new InvalidOperationException();
@@ -54,6 +92,103 @@ namespace Microsoft.AspNet.Mvc.Rendering
             Assert.Same(expected, actual);
         }
 
+        // Func<IHtmlHelper, IHtmlContent>, expected Model, expected ViewDataDictionary
+        public static TheoryData<Func<IHtmlHelper, Task<IHtmlContent>>, object, ViewDataDictionary> PartialAsyncExtensionMethods
+        {
+            get
+            {
+                var viewData = new ViewDataDictionary(new EmptyModelMetadataProvider());
+                var model = new object();
+                return new TheoryData<Func<IHtmlHelper, Task<IHtmlContent>>, object, ViewDataDictionary>
+                {
+                    { helper => helper.PartialAsync("test"), null, null },
+                    { helper => helper.PartialAsync("test", model), model, null },
+                    { helper => helper.PartialAsync("test", viewData), null, viewData },
+                };
+            }
+        }
+
+        [Theory]
+        [MemberData(nameof(PartialAsyncExtensionMethods))]
+        public async Task PartialAsyncMethods_CallHtmlHelperWithExpectedArguments(
+            Func<IHtmlHelper, Task<IHtmlContent>> partialAsyncMethod,
+            object expectedModel,
+            ViewDataDictionary expectedViewData)
+        {
+            // Arrange
+            var htmlContent = Mock.Of<IHtmlContent>();
+            var helper = new Mock<IHtmlHelper>(MockBehavior.Strict);
+            if (expectedModel == null)
+            {
+                // Extension methods without model parameter use ViewData.Model to get Model.
+                var viewData = expectedViewData ?? new ViewDataDictionary(new EmptyModelMetadataProvider());
+                helper
+                    .SetupGet(h => h.ViewData)
+                    .Returns(viewData)
+                    .Verifiable();
+            }
+
+            helper
+                .Setup(h => h.PartialAsync("test", expectedModel, expectedViewData))
+                .Returns(Task.FromResult(htmlContent))
+                .Verifiable();
+
+            // Act
+            var result = await partialAsyncMethod(helper.Object);
+
+            // Assert
+            Assert.Same(htmlContent, result);
+            helper.VerifyAll();
+        }
+
+        // Func<IHtmlHelper, IHtmlContent>, expected Model, expected ViewDataDictionary
+        public static TheoryData<Func<IHtmlHelper, Task>, object, ViewDataDictionary> RenderPartialAsyncExtensionMethods
+        {
+            get
+            {
+                var viewData = new ViewDataDictionary(new EmptyModelMetadataProvider());
+                var model = new object();
+                return new TheoryData<Func<IHtmlHelper, Task>, object, ViewDataDictionary>
+                {
+                    { helper => helper.RenderPartialAsync("test"), null, null },
+                    { helper => helper.RenderPartialAsync("test", model), model, null },
+                    { helper => helper.RenderPartialAsync("test", viewData), null, viewData },
+                };
+            }
+        }
+
+        [Theory]
+        [MemberData(nameof(RenderPartialAsyncExtensionMethods))]
+        public async Task RenderPartialAsyncMethods_CallHtmlHelperWithExpectedArguments(
+            Func<IHtmlHelper, Task> renderPartialAsyncMethod,
+            object expectedModel,
+            ViewDataDictionary expectedViewData)
+        {
+            // Arrange
+            var htmlContent = Mock.Of<IHtmlContent>();
+            var helper = new Mock<IHtmlHelper>(MockBehavior.Strict);
+            if (expectedModel == null)
+            {
+                // Extension methods without model parameter use ViewData.Model to get Model.
+                var viewData = expectedViewData ?? new ViewDataDictionary(new EmptyModelMetadataProvider());
+                helper
+                    .SetupGet(h => h.ViewData)
+                    .Returns(viewData)
+                    .Verifiable();
+            }
+
+            helper
+                .Setup(h => h.RenderPartialAsync("test", expectedModel, expectedViewData))
+                .Returns(Task.FromResult(true))
+                .Verifiable();
+
+            // Act
+            await renderPartialAsyncMethod(helper.Object);
+
+            // Assert
+            helper.VerifyAll();
+        }
+
         [Fact]
         public void Partial_InvokesPartialAsyncWithCurrentModel()
         {