Fix small issue in Identity UI (#12607)

The logic for selecting the views assembly had a small bug.
Added an E2E test to cover the scenario.

src/Identity/UI/src/IdentityBuilderUIExtensions.cs

@@ -53,22 +53,22 @@ namespace Microsoft.AspNetCore.Identity
 
         private static void AddRelatedParts(IdentityBuilder builder)
         {
-            // We try to resolve the UI framework that was used by looking at the entry assembly.
-            // When an app runs, the entry assembly will point to the built app. In some rare cases
-            // (functional testing) the app assembly will be different, and we'll try to locate it through
-            // the same mechanism that MVC uses today.
-            // Finally, if for some reason we aren't able to find the assembly, we'll use our default value
-            // (Bootstrap4)
-            if (!TryResolveUIFramework(Assembly.GetEntryAssembly(), out var framework) ||
-                !TryResolveUIFramework(GetApplicationAssembly(builder), out framework))
-            {
-                framework = default;
-            }
-
             var mvcBuilder = builder.Services
                 .AddMvc()
                 .ConfigureApplicationPartManager(partManager =>
                 {
+                    // We try to resolve the UI framework that was used by looking at the entry assembly.
+                    // When an app runs, the entry assembly will point to the built app. In some rare cases
+                    // (functional testing) the app assembly will be different, and we'll try to locate it through
+                    // the same mechanism that MVC uses today.
+                    // Finally, if for some reason we aren't able to find the assembly, we'll use our default value
+                    // (Bootstrap4)
+                    if (!TryResolveUIFramework(Assembly.GetEntryAssembly(), out var framework) &&
+                        !TryResolveUIFramework(GetApplicationAssembly(builder), out framework))
+                    {
+                        framework = default;
+                    }
+
                     var thisAssembly = typeof(IdentityBuilderUIExtensions).Assembly;
                     var relatedAssemblies = RelatedAssemblyAttribute.GetRelatedAssemblies(thisAssembly, throwOnError: true);
                     var relatedParts = relatedAssemblies.ToDictionary(
@@ -149,7 +149,7 @@ namespace Microsoft.AspNetCore.Identity
             }
 
             // If we find the metadata there must be a valid framework here.
-            if (!Enum.TryParse<UIFramework>(metadata, ignoreCase: true, out var uIFramework))
+            if (!Enum.TryParse<UIFramework>(metadata, ignoreCase: true, out uiFramework))
             {
                 var enumValues = string.Join(", ", Enum.GetNames(typeof(UIFramework)).Select(v => $"'{v}'"));
                 throw new InvalidOperationException(

src/Identity/test/Identity.FunctionalTests/Bootstrap3Tests/UIFramewrokAttributeTest.cs

@@ -0,0 +1,47 @@
+// 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.Linq;
+using Identity.DefaultUI.WebSite;
+using Microsoft.AspNetCore.Hosting;
+using Microsoft.AspNetCore.Mvc.ApplicationParts;
+using Microsoft.AspNetCore.Mvc.Testing;
+using Microsoft.Extensions.DependencyInjection;
+using Xunit;
+
+namespace Microsoft.AspNetCore.Identity.FunctionalTests.Bootstrap3Tests
+{
+    public class UIFramewrokAttributeTest : IClassFixture<WebApplicationFactory<Startup>>
+    {
+        public UIFramewrokAttributeTest(WebApplicationFactory<Startup> factory)
+        {
+            Factory = factory;
+        }
+
+        public WebApplicationFactory<Startup> Factory { get; }
+
+        [Fact]
+        public void DefaultWebSite_DefaultsToBootstrap3()
+        {
+            var hasV3Part = false;
+            var hasV4Part = false;
+            var factory = Factory.WithWebHostBuilder(
+                whb => whb.UseStartup<Startup>().ConfigureServices(
+                    services => services.AddMvc().ConfigureApplicationPartManager(
+                        apm => (hasV3Part, hasV4Part) = (HasPart(apm, "V3"), HasPart(apm, "V4")))));
+
+            // Act
+            var client = factory.CreateClient();
+
+            // Assert
+            Assert.True(hasV3Part);
+            Assert.False(hasV4Part);
+        }
+
+        private static bool HasPart(ApplicationPartManager apm, string name)
+        {
+            return apm.ApplicationParts
+                .Any(p => p is CompiledRazorAssemblyPart cp && cp.Assembly.GetName().Name == "Microsoft.AspNetCore.Identity.UI.Views." + name);
+        }
+    }
+}

src/Identity/testassets/Identity.DefaultUI.WebSite/Identity.DefaultUI.WebSite.csproj

@@ -48,6 +48,13 @@
     <ResolveStaticWebAssetsInputsDependsOn>_SetBootstrapFrameworkVersion</ResolveStaticWebAssetsInputsDependsOn>
   </PropertyGroup>
 
+    <ItemGroup>
+    <AssemblyAttribute Include="Microsoft.AspNetCore.Identity.UI.UIFrameworkAttribute">
+      <_Parameter1>$(IdentityDefaultUIFramework)</_Parameter1>
+    </AssemblyAttribute>
+  </ItemGroup>
+
+
   <Target Name="_SetBootstrapFrameworkVersion">
     <ItemGroup>
       <_StaticWebAssetsProjectReference Condition="'%(FileName)' == 'Microsoft.AspNetCore.Identity.UI'">