Add tests, changes per PR comments

Author: pranavkm

src/Microsoft.AspNetCore.Mvc.Razor/RazorFileHierarchy.cs

@@ -3,7 +3,6 @@
 
 using System;
 using System.Collections.Generic;
-using System.Diagnostics;
 using System.Text;
 
 namespace Microsoft.AspNetCore.Mvc.Razor
@@ -12,7 +11,7 @@ internal static class RazorFileHierarchy
     {
         private const string ViewStartFileName = "_ViewStart.cshtml";
 
-        public static IEnumerable<string> FindViewImports(string path)
+        public static IEnumerable<string> GetViewStartPaths(string path)
         {
             if (string.IsNullOrEmpty(path))
             {
@@ -24,37 +23,15 @@ public static IEnumerable<string> FindViewImports(string path)
                 throw new ArgumentException(Resources.RazorProject_PathMustStartWithForwardSlash, nameof(path));
             }
 
-            var basePath = "/";
-            Debug.Assert(!string.IsNullOrEmpty(path));
             if (path.Length == 1)
             {
                 yield break;
             }
 
-            if (!path.StartsWith(basePath, StringComparison.OrdinalIgnoreCase))
-            {
-                yield break;
-            }
-
-            StringBuilder builder;
-            var fileNameIndex = path.LastIndexOf('/');
-            var length = path.Length;
-            Debug.Assert(fileNameIndex != -1);
-            if (string.Compare(path, fileNameIndex + 1, ViewStartFileName, 0, ViewStartFileName.Length) == 0)
-            {
-                // If the specified path is for the file hierarchy being constructed, then the first file that applies
-                // to it is in a parent directory.
-                builder = new StringBuilder(path, 0, fileNameIndex, fileNameIndex + ViewStartFileName.Length);
-                length = fileNameIndex;
-            }
-            else
-            {
-                builder = new StringBuilder(path);
-            }
-
-            var maxDepth = 255;
-            var index = length;
-            while (maxDepth-- > 0 && index > basePath.Length && (index = path.LastIndexOf('/', index - 1)) != -1)
+            var builder = new StringBuilder(path);
+            var maxIterations = 255;
+            var index = path.Length;
+            while (maxIterations-- > 0 && index > 1 && (index = path.LastIndexOf('/', index - 1)) != -1)
             {
                 builder.Length = index + 1;
                 builder.Append(ViewStartFileName);

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

@@ -43,7 +43,7 @@ public class RazorViewEngine : IRazorViewEngine
         private readonly DiagnosticListener _diagnosticListener;
 
         /// <summary>
-        /// Initializes a new instance of the RazorViewEngine
+        /// Initializes a new instance of the <see cref="RazorViewEngine" />.
         /// </summary>
         public RazorViewEngine(
             IRazorPageFactoryProvider pageFactory,
@@ -435,7 +435,7 @@ private IReadOnlyList<ViewLocationCacheItem> GetViewStartPages(
         {
             var viewStartPages = new List<ViewLocationCacheItem>();
 
-            foreach (var filePath in RazorFileHierarchy.FindViewImports(path))
+            foreach (var filePath in RazorFileHierarchy.GetViewStartPaths(path))
             {
                 var result = _pageFactory.CreateFactory(filePath);
                 var viewDescriptor = result.ViewDescriptor;

src/Microsoft.AspNetCore.Mvc.RazorPages/Infrastructure/PageActionInvokerProvider.cs

@@ -211,7 +211,7 @@ internal List<Func<IRazorPage>> GetViewStartFactories(CompiledPageActionDescript
         {
             var viewStartFactories = new List<Func<IRazorPage>>();
             // Always pick up all _ViewStarts, including the ones outside the Pages root.
-            foreach (var filePath in RazorFileHierarchy.FindViewImports(descriptor.RelativePath))
+            foreach (var filePath in RazorFileHierarchy.GetViewStartPaths(descriptor.RelativePath))
             {
                 var factoryResult = _razorPageFactoryProvider.CreateFactory(filePath);
                 if (factoryResult.Success)

test/Microsoft.AspNetCore.Mvc.Razor.Test/RazorFileHierarchyTest.cs

@@ -0,0 +1,64 @@
+// 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 Xunit;
+
+namespace Microsoft.AspNetCore.Mvc.Razor
+{
+    public class RazorFileHierarchyTest
+    {
+        [Fact]
+        public void GetViewStartPaths_ForFileAtRoot()
+        {
+            // Arrange
+            var expected = new[] { "/_ViewStart.cshtml", };
+            var path = "/Home.cshtml";
+
+            // Act
+            var actual = RazorFileHierarchy.GetViewStartPaths(path);
+
+            // Assert
+            Assert.Equal(expected, actual);
+        }
+
+        [Fact]
+        public void GetViewStartPaths_ForForFileInViewsDirectory()
+        {
+            // Arrange
+            var expected = new[]
+            {
+                "/Views/Home/_ViewStart.cshtml",
+                "/Views/_ViewStart.cshtml",
+                "/_ViewStart.cshtml",
+            };
+            var path = "/Views/Home/Index.cshtml";
+
+            // Act
+            var actual = RazorFileHierarchy.GetViewStartPaths(path);
+
+            // Assert
+            Assert.Equal(expected, actual);
+        }
+
+        [Fact]
+        public void GetViewStartPaths_ForForFileInAreasDirectory()
+        {
+            // Arrange
+            var expected = new[]
+            {
+                "/Areas/Views/MyArea/Home/_ViewStart.cshtml",
+                "/Areas/Views/MyArea/_ViewStart.cshtml",
+                "/Areas/Views/_ViewStart.cshtml",
+                "/Areas/_ViewStart.cshtml",
+                "/_ViewStart.cshtml",
+            };
+            var path = "/Areas/Views/MyArea/Home/Index.cshtml";
+
+            // Act
+            var actual = RazorFileHierarchy.GetViewStartPaths(path);
+
+            // Assert
+            Assert.Equal(expected, actual);
+        }
+    }
+}