Do not override default `Layout` value

- #3745

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

@@ -155,8 +155,12 @@ namespace Microsoft.AspNet.Mvc.Razor
                     var viewStart = ViewStartPages[i];
                     context.ExecutingFilePath = viewStart.Path;
 
-                    // Copy the layout value from the previous view start (if any) to the current.
+                    // If non-null, copy the layout value from the previous view start to the current. Otherwise leave
-                    viewStart.Layout = layout;
+                    // Layout default alone.
+                    if (layout != null)
+                    {
+                        viewStart.Layout = layout;
+                    }
 
                     await RenderPageCoreAsync(viewStart, context);
 
@@ -170,8 +174,11 @@ namespace Microsoft.AspNet.Mvc.Razor
                 context.ExecutingFilePath = oldFilePath;
             }
 
-            // Copy the layout value from the view start page(s) (if any) to the entry page.
+            // If non-null, copy the layout value from the view start page(s) to the entry page.
-            RazorPage.Layout = layout;
+            if (layout != null)
+            {
+                RazorPage.Layout = layout;
+            }
         }
 
         private async Task RenderLayoutAsync(

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

@@ -6,7 +6,6 @@ using System.Collections.Generic;
 using System.IO;
 using System.Linq;
 using System.Threading.Tasks;
-using Microsoft.AspNet.Http.Features;
 using Microsoft.AspNet.Http.Internal;
 using Microsoft.AspNet.Mvc.Abstractions;
 using Microsoft.AspNet.Mvc.ModelBinding;
@@ -359,6 +358,102 @@ namespace Microsoft.AspNet.Mvc.Razor
             activator.Verify();
         }
 
+        [Fact]
+        public async Task RenderAsync_ExecutesDefaultLayout()
+        {
+            // Arrange
+            var path = "/Views/Home/Index.cshtml";
+            var layoutPath = "/Views/_Shared/_Layout.cshtml";
+            var page = new TestableRazorPage(p => { })
+            {
+                Path = path,
+                // Initialize Layout property when instantiated.
+                Layout = layoutPath,
+            };
+            var layoutExecuted = false;
+            var layout = new TestableRazorPage(
+                p =>
+                {
+                    layoutExecuted = true;
+                    p.RenderBodyPublic();
+                })
+            {
+                Path = layoutPath,
+            };
+
+            var viewEngine = new Mock<IRazorViewEngine>(MockBehavior.Strict);
+            viewEngine
+                .Setup(engine => engine.GetPage(path, layoutPath))
+                .Returns(new RazorPageResult(layoutPath, layout));
+
+            var view = new RazorView(
+                viewEngine.Object,
+                Mock.Of<IRazorPageActivator>(),
+                new IRazorPage[0],
+                page,
+                new HtmlTestEncoder());
+            var context = CreateViewContext(view);
+
+            // Act
+            await view.RenderAsync(context);
+
+            // Assert
+            Assert.True(layoutExecuted);
+        }
+
+        [Fact]
+        public async Task RenderAsync_ExecutesDefaultLayout_WithViewStart()
+        {
+            // Arrange
+            var path = "/Views/Home/Index.cshtml";
+            var layoutPath = "/Views/_Shared/_Layout.cshtml";
+            var viewStartPath = "/Views/_ViewStart.cshtml";
+
+            var viewStart = new TestableRazorPage(p => { })
+            {
+                Path = viewStartPath,
+            };
+            var page = new TestableRazorPage(p => { })
+            {
+                Path = path,
+                // Initialize Layout property when instantiated.
+                Layout = layoutPath,
+            };
+
+            var layoutExecuted = false;
+            var layout = new TestableRazorPage(
+                p =>
+                {
+                    layoutExecuted = true;
+                    p.RenderBodyPublic();
+                })
+            {
+                Path = layoutPath,
+            };
+
+            var viewEngine = new Mock<IRazorViewEngine>(MockBehavior.Strict);
+            viewEngine
+                .Setup(engine => engine.GetAbsolutePath(viewStartPath, /* pagePath */ null))
+                .Returns<string>(null);
+            viewEngine
+                .Setup(engine => engine.GetPage(path, layoutPath))
+                .Returns(new RazorPageResult(layoutPath, layout));
+
+            var view = new RazorView(
+                viewEngine.Object,
+                Mock.Of<IRazorPageActivator>(),
+                new[] { viewStart },
+                page,
+                new HtmlTestEncoder());
+            var context = CreateViewContext(view);
+
+            // Act
+            await view.RenderAsync(context);
+
+            // Assert
+            Assert.True(layoutExecuted);
+        }
+
         [Fact]
         public async Task RenderAsync_ThrowsIfLayoutPageCannotBeFound_MessageUsesGetPageLocations()
         {