Check for ITagHelper in tag helper feature provider (#2602)

Fixes #2482

Author: pranavkm

Diff for: src/Microsoft.AspNetCore.Mvc.Razor.Extensions.Version1_X/ViewComponentTagHelperDescriptorProvider.cs

@@ -27,8 +27,16 @@ public void Execute(TagHelperDescriptorProviderContext context)
                 return;
             }
 
+            var vcAttribute = compilation.GetTypeByMetadataName(ViewComponentTypes.ViewComponentAttribute);
+            var nonVCAttribute = compilation.GetTypeByMetadataName(ViewComponentTypes.NonViewComponentAttribute);
+            if (vcAttribute == null || vcAttribute.TypeKind == TypeKind.Error)
+            {
+                // Could not find attributes we care about in the compilation. Nothing to do.
+                return;
+            }
+
             var types = new List<INamedTypeSymbol>();
-            var visitor = ViewComponentTypeVisitor.Create(compilation, types);
+            var visitor = new ViewComponentTypeVisitor(vcAttribute, nonVCAttribute, types);
 
             // We always visit the global namespace.
             visitor.Visit(compilation.Assembly.GlobalNamespace);

Diff for: src/Microsoft.AspNetCore.Mvc.Razor.Extensions.Version1_X/ViewComponentTypeVisitor.cs

@@ -16,13 +16,6 @@ internal class ViewComponentTypeVisitor : SymbolVisitor
         private readonly INamedTypeSymbol _nonViewComponentAttribute;
         private readonly List<INamedTypeSymbol> _results;
 
-        public static ViewComponentTypeVisitor Create(Compilation compilation, List<INamedTypeSymbol> results)
-        {
-            var vcAttribute = compilation.GetTypeByMetadataName(ViewComponentTypes.ViewComponentAttribute);
-            var nonVCAttribute = compilation.GetTypeByMetadataName(ViewComponentTypes.NonViewComponentAttribute);
-            return new ViewComponentTypeVisitor(vcAttribute, nonVCAttribute, results);
-        }
-
         public ViewComponentTypeVisitor(
             INamedTypeSymbol viewComponentAttribute,
             INamedTypeSymbol nonViewComponentAttribute,
@@ -31,12 +24,8 @@ public ViewComponentTypeVisitor(
             _viewComponentAttribute = viewComponentAttribute;
             _nonViewComponentAttribute = nonViewComponentAttribute;
             _results = results;
-
-            Enabled = _viewComponentAttribute != null;
         }
 
-        public bool Enabled { get; set; }
-
         public override void VisitNamedType(INamedTypeSymbol symbol)
         {
             if (IsViewComponent(symbol))
@@ -65,11 +54,6 @@ public override void VisitNamespace(INamespaceSymbol symbol)
 
         internal bool IsViewComponent(INamedTypeSymbol symbol)
         {
-            if (!Enabled)
-            {
-                return false;
-            }
-
             if (symbol.DeclaredAccessibility != Accessibility.Public ||
                 symbol.IsAbstract ||
                 symbol.IsGenericType ||

Diff for: src/Microsoft.AspNetCore.Mvc.Razor.Extensions/ViewComponentTagHelperDescriptorProvider.cs

@@ -27,8 +27,16 @@ public void Execute(TagHelperDescriptorProviderContext context)
                 return;
             }
 
+            var vcAttribute = compilation.GetTypeByMetadataName(ViewComponentTypes.ViewComponentAttribute);
+            var nonVCAttribute = compilation.GetTypeByMetadataName(ViewComponentTypes.NonViewComponentAttribute);
+            if (vcAttribute == null || vcAttribute.TypeKind == TypeKind.Error)
+            {
+                // Could not find attributes we care about in the compilation. Nothing to do.
+                return;
+            }
+
             var types = new List<INamedTypeSymbol>();
-            var visitor = ViewComponentTypeVisitor.Create(compilation, types);
+            var visitor = new ViewComponentTypeVisitor(vcAttribute, nonVCAttribute, types);
 
             // We always visit the global namespace.
             visitor.Visit(compilation.Assembly.GlobalNamespace);

Diff for: src/Microsoft.AspNetCore.Mvc.Razor.Extensions/ViewComponentTypeVisitor.cs

@@ -13,13 +13,6 @@ internal class ViewComponentTypeVisitor : SymbolVisitor
         private readonly INamedTypeSymbol _nonViewComponentAttribute;
         private readonly List<INamedTypeSymbol> _results;
 
-        public static ViewComponentTypeVisitor Create(Compilation compilation, List<INamedTypeSymbol> results)
-        {
-            var vcAttribute = compilation.GetTypeByMetadataName(ViewComponentTypes.ViewComponentAttribute);
-            var nonVCAttribute = compilation.GetTypeByMetadataName(ViewComponentTypes.NonViewComponentAttribute);
-            return new ViewComponentTypeVisitor(vcAttribute, nonVCAttribute, results);
-        }
-
         public ViewComponentTypeVisitor(
             INamedTypeSymbol viewComponentAttribute,
             INamedTypeSymbol nonViewComponentAttribute,

Diff for: src/Microsoft.CodeAnalysis.Razor/CompilationTagHelperFeature.cs

@@ -40,15 +40,11 @@ protected override void OnInitialized()
 
         internal static bool IsValidCompilation(Compilation compilation)
         {
-            var iTagHelper = compilation.GetTypeByMetadataName(TagHelperTypes.ITagHelper);
             var @string = compilation.GetSpecialType(SpecialType.System_String);
 
-            // Do some minimal tests to verify the compilation is valid. If symbols for ITagHelper or System.String 
-            // are missing or errored, the compilation may be missing references.
-            return iTagHelper != null &&
-                iTagHelper.TypeKind != TypeKind.Error &&
-                @string != null &&
-                @string.TypeKind != TypeKind.Error;
+            // Do some minimal tests to verify the compilation is valid. If symbols for System.String
+            // is missing or errored, the compilation may be missing references.
+            return @string != null && @string.TypeKind != TypeKind.Error;
         }
     }
 }

Diff for: src/Microsoft.CodeAnalysis.Razor/DefaultTagHelperDescriptorProvider.cs

@@ -31,8 +31,15 @@ public void Execute(TagHelperDescriptorProviderContext context)
                 return;
             }
 
+            var iTagHelper = compilation.GetTypeByMetadataName(TagHelperTypes.ITagHelper);
+            if (iTagHelper == null || iTagHelper.TypeKind == TypeKind.Error)
+            {
+                // Could not find attributes we care about in the compilation. Nothing to do.
+                return;
+            }
+
             var types = new List<INamedTypeSymbol>();
-            var visitor = TagHelperTypeVisitor.Create(compilation, types);
+            var visitor = new TagHelperTypeVisitor(iTagHelper, types);
 
             // We always visit the global namespace.
             visitor.Visit(compilation.Assembly.GlobalNamespace);

Diff for: src/Microsoft.CodeAnalysis.Razor/TagHelperTypeVisitor.cs

@@ -11,12 +11,6 @@ internal class TagHelperTypeVisitor : SymbolVisitor
         private INamedTypeSymbol _interface;
         private List<INamedTypeSymbol> _results;
 
-        public static TagHelperTypeVisitor Create(Compilation compilation, List<INamedTypeSymbol> results)
-        {
-            var @interface = compilation.GetTypeByMetadataName(TagHelperTypes.ITagHelper);
-            return new TagHelperTypeVisitor(@interface, results);
-        }
-
         public TagHelperTypeVisitor(INamedTypeSymbol @interface, List<INamedTypeSymbol> results)
         {
             _interface = @interface;

Diff for: test/Microsoft.CodeAnalysis.Razor.Test/CompilationTagHelperFeatureTest.cs

@@ -13,7 +13,7 @@ namespace Microsoft.CodeAnalysis.Razor
     public class CompilationTagHelperFeatureTest
     {
         [Fact]
-        public void IsValidCompilation_ReturnsFalseIfITagHelperInterfaceCannotBeFound()
+        public void IsValidCompilation_ReturnsTrueIfTagHelperInterfaceCannotBeFound()
         {
             // Arrange
             var references = new[]
@@ -26,7 +26,7 @@ public void IsValidCompilation_ReturnsFalseIfITagHelperInterfaceCannotBeFound()
             var result = CompilationTagHelperFeature.IsValidCompilation(compilation);
 
             // Assert
-            Assert.False(result);
+            Assert.True(result);
         }
 
         [Fact]