Add `AdditionalCompilationReferences` option to `RazorViewEngineOptions`.

- `AdditionalCompilationReferences` is a purely additive option to add `MetadataReference` instances to be used for Razor compilation.
- Added a test to validate the compilation references get combined with the `ApplicationPartManager`s metadata references.

#4497

src/Microsoft.AspNetCore.Mvc.Razor/Internal/DefaultRoslynCompilationService.cs

@@ -32,10 +32,11 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Internal
         private readonly Action<RoslynCompilationContext> _compilationCallback;
         private readonly CSharpParseOptions _parseOptions;
         private readonly CSharpCompilationOptions _compilationOptions;
+        private readonly IList<MetadataReference> _additionalMetadataReferences;
         private readonly ILogger _logger;
-        private object _applicationReferencesLock = new object();
-        private bool _applicationReferencesInitialized;
-        private IList<MetadataReference> _applicationReferences;
+        private object _compilationReferencesLock = new object();
+        private bool _compilationReferencesInitialized;
+        private IList<MetadataReference> _compilationReferences;
 
         /// <summary>
         /// Initalizes a new instance of the <see cref="DefaultRoslynCompilationService"/> class.
@@ -55,18 +56,19 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Internal
             _compilationCallback = optionsAccessor.Value.CompilationCallback;
             _parseOptions = optionsAccessor.Value.ParseOptions;
             _compilationOptions = optionsAccessor.Value.CompilationOptions;
+            _additionalMetadataReferences = optionsAccessor.Value.AdditionalCompilationReferences;
             _logger = loggerFactory.CreateLogger<DefaultRoslynCompilationService>();
         }
 
-        private IList<MetadataReference> ApplicationReferences
+        private IList<MetadataReference> CompilationReferences
         {
             get
             {
                 return LazyInitializer.EnsureInitialized(
-                    ref _applicationReferences,
-                    ref _applicationReferencesInitialized,
-                    ref _applicationReferencesLock,
-                    GetApplicationReferences);
+                    ref _compilationReferences,
+                    ref _compilationReferencesInitialized,
+                    ref _compilationReferencesLock,
+                    GetCompilationReferences);
             }
         }
 
@@ -99,7 +101,7 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Internal
                 assemblyName,
                 options: _compilationOptions,
                 syntaxTrees: new[] { syntaxTree },
-                references: ApplicationReferences);
+                references: CompilationReferences);
 
             compilation = Rewrite(compilation);
 
@@ -118,7 +120,7 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Internal
 
                     if (!result.Success)
                     {
-                        if (!compilation.References.Any() && !ApplicationReferences.Any())
+                        if (!compilation.References.Any() && !CompilationReferences.Any())
                         {
                             // DependencyModel had no references specified and the user did not use the
                             // CompilationCallback to add extra references. It is likely that the user did not specify
@@ -153,11 +155,22 @@ namespace Microsoft.AspNetCore.Mvc.Razor.Internal
         /// Gets the sequence of <see cref="MetadataReference"/> instances used for compilation.
         /// </summary>
         /// <returns>The <see cref="MetadataReference"/> instances.</returns>
-        protected virtual IList<MetadataReference> GetApplicationReferences()
+        protected virtual IList<MetadataReference> GetCompilationReferences()
         {
             var feature = new MetadataReferenceFeature();
             _partManager.PopulateFeature(feature);
-            return feature.MetadataReferences;
+            var applicationReferences = feature.MetadataReferences;
+
+            if (_additionalMetadataReferences.Count == 0)
+            {
+                return applicationReferences;
+            }
+
+            var compilationReferences = new List<MetadataReference>(applicationReferences.Count + _additionalMetadataReferences.Count);
+            compilationReferences.AddRange(applicationReferences);
+            compilationReferences.AddRange(_additionalMetadataReferences);
+
+            return compilationReferences;
         }
 
         private Assembly LoadStream(MemoryStream assemblyStream, MemoryStream pdbStream)

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

@@ -4,6 +4,7 @@
 using System;
 using System.Collections.Generic;
 using Microsoft.AspNetCore.Mvc.Razor.Compilation;
+using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.CSharp;
 using Microsoft.Extensions.FileProviders;
 
@@ -90,6 +91,12 @@ namespace Microsoft.AspNetCore.Mvc.Razor
         /// </remarks>
         public IList<string> AreaViewLocationFormats { get; } = new List<string>();
 
+        /// <summary>
+        /// Gets the <see cref="MetadataReference" /> instances that should be included in Razor compilation, along with
+        /// those discovered by <see cref="MetadataReferenceFeatureProvider" />s.
+        /// </summary>
+        public IList<MetadataReference> AdditionalCompilationReferences { get; } = new List<MetadataReference>();
+
         /// <summary>
         /// Gets or sets the callback that is used to customize Razor compilation
         /// to change compilation settings you can update <see cref="RoslynCompilationContext.Compilation"/> property.

test/Microsoft.AspNetCore.Mvc.Razor.Test/Internal/DefaultRoslynCompilationServiceTest.cs

@@ -2,6 +2,8 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
+using System.Collections.Generic;
+using System.Linq;
 using System.Reflection;
 using Microsoft.AspNetCore.Mvc.ApplicationParts;
 using Microsoft.AspNetCore.Mvc.Razor.Compilation;
@@ -316,6 +318,34 @@ public class MyNonCustomDefinedClass {}
             Assert.NotNull(result.CompiledType);
         }
 
+        [Fact]
+        public void GetCompilationReferences_CombinesApplicationPartAndOptionMetadataReferences()
+        {
+            // Arrange
+            var options = new RazorViewEngineOptions();
+            var objectAssemblyLocation = typeof(object).GetTypeInfo().Assembly.Location;
+            var objectAssemblyMetadataReference = MetadataReference.CreateFromFile(objectAssemblyLocation);
+            options.AdditionalCompilationReferences.Add(objectAssemblyMetadataReference);
+            var applicationPartManager = GetApplicationPartManager();
+            var compilationService = new TestRoslynCompilationService(applicationPartManager, options);
+            var feature = new MetadataReferenceFeature();
+            applicationPartManager.PopulateFeature(feature);
+            var partReferences = feature.MetadataReferences;
+            var expectedReferences = new List<MetadataReference>();
+            expectedReferences.AddRange(partReferences);
+            expectedReferences.Add(objectAssemblyMetadataReference);
+            var expectedReferenceDisplays = expectedReferences.Select(reference => reference.Display);
+
+            // Act
+            var references = compilationService.GetCompilationReferencesPublic();
+            var referenceDisplays = references.Select(reference => reference.Display);
+
+            // Assert
+            Assert.NotNull(references);
+            Assert.NotEmpty(references);
+            Assert.Equal(expectedReferenceDisplays, referenceDisplays);
+        }
+
         private static DiagnosticDescriptor GetDiagnosticDescriptor(string messageFormat)
         {
             return new DiagnosticDescriptor(
@@ -375,5 +405,15 @@ public class MyNonCustomDefinedClass {}
                 GetFileProviderAccessor(fileProvider),
                 NullLoggerFactory.Instance);
         }
+
+        private class TestRoslynCompilationService : DefaultRoslynCompilationService
+        {
+            public TestRoslynCompilationService(ApplicationPartManager partManager, RazorViewEngineOptions options)
+                : base(partManager, GetAccessor(options), GetFileProviderAccessor(), NullLoggerFactory.Instance)
+            {
+            }
+
+            public IList<MetadataReference> GetCompilationReferencesPublic() => GetCompilationReferences();
+        }
     }
 }