Remove magic string keys from TH builders.

- Replaced the magic strings with extension methods that produce the same behavior as the string keys did.
- Added tests to validate new extension methods.

#1307

src/Microsoft.AspNetCore.Mvc.Razor.Extensions/ViewComponentTagHelperPass.cs

@@ -29,7 +29,7 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Extensions
             {
                 GenerateVCTHClass(visitor.Class, tagHelper.Value);
 
-                var tagHelperTypeName = tagHelper.Value.Metadata[TagHelperDescriptorBuilder.TypeNameKey];
+                var tagHelperTypeName = tagHelper.Value.GetTypeName();
                 if (visitor.Fields.UsedTagHelperTypeNames.Remove(tagHelperTypeName))
                 {
                     visitor.Fields.UsedTagHelperTypeNames.Add(GetVCTHFullName(visitor.Namespace, visitor.Class, tagHelper.Value));
@@ -155,7 +155,7 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Extensions
             foreach (var attribute in descriptor.BoundAttributes)
             {
                 writer.WriteAutoPropertyDeclaration(
-                    "public", attribute.TypeName, attribute.Metadata[TagHelperBoundAttributeDescriptorBuilder.PropertyNameKey]);
+                    "public", attribute.TypeName, attribute.GetPropertyName());
 
                 if (attribute.IndexerTypeName != null)
                 {
@@ -203,7 +203,7 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Extensions
         private string[] GetMethodParameters(TagHelperDescriptor descriptor)
         {
             var propertyNames = descriptor.BoundAttributes.Select(
-                attribute => attribute.Metadata[TagHelperBoundAttributeDescriptorBuilder.PropertyNameKey]);
+                attribute => attribute.GetPropertyName());
             var joinedPropertyNames = string.Join(", ", propertyNames);
             var parametersString = $"new {{ { joinedPropertyNames } }}";
 

src/Microsoft.AspNetCore.Razor.Language/BoundAttributeDescriptorExtensions.cs

@@ -0,0 +1,20 @@
+// 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;
+
+namespace Microsoft.AspNetCore.Razor.Language
+{
+    public static class BoundAttributeDescriptorExtensions
+    {
+        public static string GetPropertyName(this BoundAttributeDescriptor descriptor)
+        {
+            descriptor.Metadata.TryGetValue(TagHelperBoundAttributeDescriptorBuilder.PropertyNameKey, out var propertyName);
+
+            return propertyName;
+        }
+
+        public static bool IsDefaultKind(this BoundAttributeDescriptor descriptor)
+            => string.Equals(descriptor.Kind, TagHelperBoundAttributeDescriptorBuilder.DescriptorKind, StringComparison.Ordinal);
+    }
+}

src/Microsoft.AspNetCore.Razor.Language/CodeGeneration/DesignTimeTagHelperWriter.cs

@@ -196,7 +196,7 @@ namespace Microsoft.AspNetCore.Razor.Language.CodeGeneration
             string attributeName,
             BoundAttributeDescriptor descriptor)
         {
-            var propertyAccessor = $"{tagHelperVariableName}.{descriptor.Metadata[TagHelperBoundAttributeDescriptorBuilder.PropertyNameKey]}";
+            var propertyAccessor = $"{tagHelperVariableName}.{descriptor.GetPropertyName()}";
 
             if (isIndexerNameMatch)
             {

src/Microsoft.AspNetCore.Razor.Language/CodeGeneration/PreallocatedAttributeTargetExtension.cs

@@ -94,7 +94,7 @@ namespace Microsoft.AspNetCore.Razor.Language.CodeGeneration
             string attributeName,
             BoundAttributeDescriptor descriptor)
         {
-            var propertyAccessor = $"{tagHelperVariableName}.{descriptor.Metadata[TagHelperBoundAttributeDescriptorBuilder.PropertyNameKey]}";
+            var propertyAccessor = $"{tagHelperVariableName}.{descriptor.GetPropertyName()}";
 
             if (isIndexerNameMatch)
             {

src/Microsoft.AspNetCore.Razor.Language/CodeGeneration/RuntimeTagHelperWriter.cs

@@ -310,7 +310,7 @@ namespace Microsoft.AspNetCore.Razor.Language.CodeGeneration
         {
             var tagHelperVariableName = GetTagHelperVariableName(node.TagHelperTypeName);
             var tagHelperRenderingContext = context.TagHelperRenderingContext;
-            var propertyName = node.Descriptor.Metadata[TagHelperBoundAttributeDescriptorBuilder.PropertyNameKey];
+            var propertyName = node.Descriptor.GetPropertyName();
 
             // Ensure that the property we're trying to set has initialized its dictionary bound properties.
             if (node.IsIndexerNameMatch &&
@@ -453,7 +453,7 @@ namespace Microsoft.AspNetCore.Razor.Language.CodeGeneration
             string attributeName,
             BoundAttributeDescriptor descriptor)
         {
-            var propertyAccessor = $"{tagHelperVariableName}.{descriptor.Metadata[TagHelperBoundAttributeDescriptorBuilder.PropertyNameKey]}";
+            var propertyAccessor = $"{tagHelperVariableName}.{descriptor.GetPropertyName()}";
 
             if (isIndexerNameMatch)
             {

src/Microsoft.AspNetCore.Razor.Language/DefaultRazorIRLoweringPhase.cs

@@ -533,7 +533,7 @@ namespace Microsoft.AspNetCore.Razor.Language
 
                 foreach (var descriptor in block.Binding.Descriptors)
                 {
-                    var typeName = descriptor.Metadata[TagHelperDescriptorBuilder.TypeNameKey];
+                    var typeName = descriptor.GetTypeName();
                     _tagHelperFields.UsedTagHelperTypeNames.Add(typeName);
                 }
             }
@@ -543,7 +543,7 @@ namespace Microsoft.AspNetCore.Razor.Language
                 var descriptors = tagHelperBinding.Descriptors;
                 foreach (var descriptor in descriptors)
                 {
-                    var typeName = descriptor.Metadata[TagHelperDescriptorBuilder.TypeNameKey];
+                    var typeName = descriptor.GetTypeName();
                     var createTagHelper = new CreateTagHelperIRNode()
                     {
                         TagHelperTypeName = typeName,
@@ -577,8 +577,8 @@ namespace Microsoft.AspNetCore.Razor.Language
                         {
                             var associatedAttributeDescriptor = associatedDescriptor.BoundAttributes.First(
                                 attributeDescriptor => TagHelperMatchingConventions.CanSatisfyBoundAttribute(attribute.Name, attributeDescriptor));
-                            var tagHelperTypeName = associatedDescriptor.Metadata[TagHelperDescriptorBuilder.TypeNameKey];
+                            var tagHelperTypeName = associatedDescriptor.GetTypeName();
-                            var attributePropertyName = associatedAttributeDescriptor.Metadata[TagHelperBoundAttributeDescriptorBuilder.PropertyNameKey];
+                            var attributePropertyName = associatedAttributeDescriptor.GetPropertyName();
 
                             var setTagHelperProperty = new SetTagHelperPropertyIRNode()
                             {

src/Microsoft.AspNetCore.Razor.Language/DefaultRazorTagHelperBinderPhase.cs

@@ -267,13 +267,13 @@ namespace Microsoft.AspNetCore.Razor.Language
                 return false;
             }
 
-            if (descriptor.Kind != TagHelperDescriptorBuilder.DescriptorKind)
+            if (!descriptor.IsDefaultKind())
             {
-                // We only understand TagHelperDescriptors generated from ITagHelpers.
+                // We only understand default TagHelperDescriptors.
                 return false;
             }
 
-            var descriptorTypeName = descriptor.Metadata[TagHelperDescriptorBuilder.TypeNameKey];
+            var descriptorTypeName = descriptor.GetTypeName();
 
             if (lookupInfo.TypePattern.EndsWith("*", StringComparison.Ordinal))
             {

src/Microsoft.AspNetCore.Razor.Language/Legacy/TagHelperParseTreeRewriter.cs

@@ -273,7 +273,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Legacy
 
                         if (invalidRule != null)
                         {
-                            var typeName = descriptor.Metadata[TagHelperDescriptorBuilder.TypeNameKey];
+                            var typeName = descriptor.GetTypeName();
 
                             // End tag TagHelper that states it shouldn't have an end tag.
                             errorSink.OnError(
@@ -564,8 +564,8 @@ namespace Microsoft.AspNetCore.Razor.Language.Legacy
                         // Can't have a set of TagHelpers that expect different structures.
                         if (baseStructure.HasValue && baseStructure != rule.TagStructure)
                         {
-                            var baseDescriptorTypeName = baseDescriptor.Metadata[TagHelperDescriptorBuilder.TypeNameKey];
+                            var baseDescriptorTypeName = baseDescriptor.GetTypeName();
-                            var descriptorTypeName = descriptor.Metadata[TagHelperDescriptorBuilder.TypeNameKey];
+                            var descriptorTypeName = descriptor.GetTypeName();
                             errorSink.OnError(
                                 tagBlock.Start,
                                 LegacyResources.FormatTagHelperParseTreeRewriter_InconsistentTagStructure(

src/Microsoft.AspNetCore.Razor.Language/TagHelperBoundAttributeDescriptorBuilder.cs

@@ -9,8 +9,8 @@ namespace Microsoft.AspNetCore.Razor.Language
 {
     public sealed class TagHelperBoundAttributeDescriptorBuilder
     {
-        public static readonly string DescriptorKind = "ITagHelper";
+        internal const string DescriptorKind = "ITagHelper";
-        public static readonly string PropertyNameKey = "ITagHelper.PropertyName";
+        internal const string PropertyNameKey = "ITagHelper.PropertyName";
 
         private static readonly IReadOnlyDictionary<string, string> PrimitiveDisplayTypeNameLookups = new Dictionary<string, string>(StringComparer.Ordinal)
         {

src/Microsoft.AspNetCore.Razor.Language/TagHelperDescriptorBuilder.cs

@@ -9,8 +9,8 @@ namespace Microsoft.AspNetCore.Razor.Language
 {
     public sealed class TagHelperDescriptorBuilder
     {
-        public static readonly string DescriptorKind = "ITagHelper";
+        internal const string DescriptorKind = "ITagHelper";
-        public static readonly string TypeNameKey = "ITagHelper.TypeName";
+        internal const string TypeNameKey = "ITagHelper.TypeName";
 
         private static ICollection<char> InvalidNonWhitespaceAllowedChildCharacters { get; } = new HashSet<char>(
             new[] { '@', '!', '<', '/', '?', '[', '>', ']', '=', '"', '\'', '*' });

src/Microsoft.AspNetCore.Razor.Language/TagHelperDescriptorExtensions.cs

@@ -0,0 +1,20 @@
+// 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;
+
+namespace Microsoft.AspNetCore.Razor.Language
+{
+    public static class TagHelperDescriptorExtensions
+    {
+        public static string GetTypeName(this TagHelperDescriptor descriptor)
+        {
+            descriptor.Metadata.TryGetValue(TagHelperDescriptorBuilder.TypeNameKey, out var typeName);
+
+            return typeName;
+        }
+
+        public static bool IsDefaultKind(this TagHelperDescriptor descriptor)
+            => string.Equals(descriptor.Kind, TagHelperDescriptorBuilder.DescriptorKind, StringComparison.Ordinal);
+    }
+}

test/Microsoft.AspNetCore.Razor.Language.Test/BoundAttributeDescriptorExtensionsTest.cs

@@ -0,0 +1,81 @@
+// 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.Razor.Language
+{
+    public class BoundAttributeDescriptorExtensionsTest
+    {
+        [Fact]
+        public void GetPropertyName_ReturnsPropertyName()
+        {
+            // Arrange
+            var expectedPropertyName = "IntProperty";
+            var descriptor = TagHelperBoundAttributeDescriptorBuilder.Create("TestTagHelper")
+                .Name("test")
+                .PropertyName(expectedPropertyName)
+                .TypeName(typeof(int).FullName)
+                .Build();
+
+            // Act
+            var propertyName = descriptor.GetPropertyName();
+
+            // Assert
+            Assert.Equal(expectedPropertyName, propertyName);
+        }
+
+        [Fact]
+        public void GetPropertyName_ReturnsNullIfNoPropertyName()
+        {
+            // Arrange
+            var descriptor = TagHelperBoundAttributeDescriptorBuilder.Create("TestTagHelper")
+                .Name("test")
+                .TypeName(typeof(int).FullName)
+                .Build();
+
+            // Act
+            var propertyName = descriptor.GetPropertyName();
+
+            // Assert
+            Assert.Null(propertyName);
+        }
+
+        [Fact]
+        public void IsDefaultKind_ReturnsTrueIfFromDefaultBuilder()
+        {
+            // Arrange
+            var descriptor = TagHelperBoundAttributeDescriptorBuilder.Create("TestTagHelper")
+                .Name("test")
+                .PropertyName("IntProperty")
+                .TypeName(typeof(int).FullName)
+                .Build();
+
+            // Act
+            var isDefault = descriptor.IsDefaultKind();
+
+            // Assert
+            Assert.True(isDefault);
+        }
+
+        [Fact]
+        public void IsDefaultKind_ReturnsFalseIfFromCustomBuilder()
+        {
+            // Arrange
+            var descriptor = new CustomBoundAttributeDescriptor();
+
+            // Act
+            var isDefault = descriptor.IsDefaultKind();
+
+            // Assert
+            Assert.False(isDefault);
+        }
+
+        private class CustomBoundAttributeDescriptor : BoundAttributeDescriptor
+        {
+            public CustomBoundAttributeDescriptor() : base("custom")
+            {
+            }
+        }
+    }
+}

test/Microsoft.AspNetCore.Razor.Language.Test/TagHelperDescriptorExtensionsTest.cs

@@ -0,0 +1,72 @@
+// 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.Collections.Generic;
+using Xunit;
+
+namespace Microsoft.AspNetCore.Razor.Language
+{
+    public class TagHelperDescriptorExtensionsTest
+    {
+        [Fact]
+        public void GetTypeName_ReturnsTypeName()
+        {
+            // Arrange
+            var expectedTypeName = "TestTagHelper";
+            var descriptor = TagHelperDescriptorBuilder.Create(expectedTypeName, "TestAssembly").Build();
+
+            // Act
+            var typeName = descriptor.GetTypeName();
+
+            // Assert
+            Assert.Equal(expectedTypeName, typeName);
+        }
+
+        [Fact]
+        public void GetTypeName_ReturnsNullIfNoTypeName()
+        {
+            // Arrange
+            var descriptor = new CustomTagHelperDescriptor();
+
+            // Act
+            var typeName = descriptor.GetTypeName();
+
+            // Assert
+            Assert.Null(typeName);
+        }
+
+        [Fact]
+        public void IsDefaultKind_ReturnsTrueIfFromDefaultBuilder()
+        {
+            // Arrange
+            var descriptor = TagHelperDescriptorBuilder.Create("TestTagHelper", "TestAssembly").Build();
+
+            // Act
+            var isDefault = descriptor.IsDefaultKind();
+
+            // Assert
+            Assert.True(isDefault);
+        }
+
+        [Fact]
+        public void IsDefaultKind_ReturnsFalseIfFromCustomBuilder()
+        {
+            // Arrange
+            var descriptor = new CustomTagHelperDescriptor();
+
+            // Act
+            var isDefault = descriptor.IsDefaultKind();
+
+            // Assert
+            Assert.False(isDefault);
+        }
+
+        private class CustomTagHelperDescriptor : TagHelperDescriptor
+        {
+            public CustomTagHelperDescriptor() : base("custom")
+            {
+                Metadata = new Dictionary<string, string>();
+            }
+        }
+    }
+}

tooling/Microsoft.VisualStudio.RazorExtension/RazorInfo/TagHelperViewModel.cs

@@ -20,7 +20,7 @@ namespace Microsoft.VisualStudio.RazorExtension.RazorInfo
 
         public string TargetElement => string.Join(", ", _descriptor.TagMatchingRules.Select(rule => rule.TagName));
 
-        public string TypeName => _descriptor.Metadata[TagHelperDescriptorBuilder.TypeNameKey];
+        public string TypeName => _descriptor.GetTypeName();
     }
 }
 #endif