Ensure ViewData is set on PageResult after a handler method executes

[Fixes #7665] Initializing PageResult.Page as part of result execution breaks Identity UI

src/Microsoft.AspNetCore.Mvc.RazorPages/Internal/PageActionInvoker.cs

@@ -128,7 +128,6 @@ namespace Microsoft.AspNetCore.Mvc.RazorPages.Internal
                 {
                     _page = (PageBase)CacheEntry.PageFactory(_pageContext, _viewContext);
                 }
-
                 pageResult.Page = _page;
                 pageResult.ViewData = pageResult.ViewData ?? _pageContext.ViewData;
             }
@@ -278,6 +277,13 @@ namespace Microsoft.AspNetCore.Mvc.RazorPages.Internal
             {
                 _result = new PageResult();
             }
+
+            // Ensure ViewData is set on PageResult for backwards compatibility (For example, Identity UI accesses
+            // ViewData in a PageFilter's PageHandlerExecutedMethod)
+            if (_result is PageResult pageResult)
+            {
+                pageResult.ViewData = pageResult.ViewData ?? _pageContext.ViewData;
+            }
         }
 
         private Task Next(ref State next, ref Scope scope, ref object state, ref bool isCompleted)

test/Microsoft.AspNetCore.Mvc.FunctionalTests/RazorPagesTest.cs

@@ -1295,6 +1295,16 @@ Microsoft.AspNetCore.Mvc.ViewFeatures.ViewDataDictionary`1[AspNetCore.InjectedPa
             Assert.Equal("From ShortCircuitPageAtPageFilter.cshtml", content);
         }
 
+        [Fact]
+        public async Task ViewDataAvaialableInPageFilter_AfterHandlerMethod_ReturnsPageResult()
+        {
+            // Act
+            var content = await Client.GetStringAsync("http://localhost/Pages/ViewDataAvailableAfterHandlerExecuted");
+
+            // Assert
+            Assert.Equal("ViewData: Bar", content);
+        }
+
         private async Task AddAntiforgeryHeaders(HttpRequestMessage request)
         {
             var getResponse = await Client.GetAsync(request.RequestUri);

test/Microsoft.AspNetCore.Mvc.RazorPages.Test/Internal/PageActionInvokerTest.cs

@@ -473,6 +473,33 @@ namespace Microsoft.AspNetCore.Mvc.RazorPages.Internal
 
         #region Page Filters
 
+        [Fact]
+        public async Task ViewDataIsSet_AfterHandlerMethodIsExecuted()
+        {
+            // Arrange
+            var pageHandlerExecutedCalled = false;
+            var pageFilter = new Mock<IPageFilter>();
+            AllowSelector(pageFilter);
+            pageFilter
+                .Setup(f => f.OnPageHandlerExecuted(It.IsAny<PageHandlerExecutedContext>()))
+                .Callback<PageHandlerExecutedContext>(c =>
+                {
+                    pageHandlerExecutedCalled = true;
+                    var result = c.Result;
+                    var pageResult = Assert.IsType<PageResult>(result);
+                    Assert.IsType<ViewDataDictionary<TestPage>>(pageResult.ViewData);
+                    Assert.IsType<TestPage>(pageResult.Model);
+                    Assert.Null(pageResult.Page);
+                });
+            var invoker = CreateInvoker(new IFilterMetadata[] { pageFilter.Object }, result: new PageResult());
+
+            // Act
+            await invoker.InvokeAsync();
+
+            // Assert
+            Assert.True(pageHandlerExecutedCalled);
+        }
+
         [Fact]
         public async Task InvokeAction_InvokesPageFilter()
         {

test/WebSites/RazorPagesWebSite/Pages/ViewDataAvailableAfterHandlerExecuted.cshtml

@@ -0,0 +1,5 @@
+@page
+@model ViewDataAvailableAfterHandlerExecutedModel
+@{
+}
+ViewData: @ViewData["Foo"]

test/WebSites/RazorPagesWebSite/Pages/ViewDataAvailableAfterHandlerExecuted.cshtml.cs

@@ -0,0 +1,40 @@
+// 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;
+using Microsoft.AspNetCore.Mvc.Filters;
+using Microsoft.AspNetCore.Mvc.RazorPages;
+
+namespace RazorPagesWebSite.Pages
+{
+    [TestPageFilter]
+    public class ViewDataAvailableAfterHandlerExecutedModel : PageModel
+    {
+        public IActionResult OnGet()
+        {
+            return Page();
+        }
+
+        private class TestPageFilterAttribute : Attribute, IPageFilter
+        {
+            public void OnPageHandlerExecuted(PageHandlerExecutedContext context)
+            {
+                // This usage mimics Identity UI where it sets data into ViewData in a PageFilters's
+                // PageHandlerExecuted method.
+                if (context.Result is PageResult pageResult)
+                {
+                    pageResult.ViewData["Foo"] = "Bar";
+                }
+            }
+
+            public void OnPageHandlerExecuting(PageHandlerExecutingContext context)
+            {
+            }
+
+            public void OnPageHandlerSelected(PageHandlerSelectedContext context)
+            {
+            }
+        }
+    }
+}