Update code for clarity and add error handling test

src/EditorFeatures/Core/Implementation/MetadataAsSource/MetadataAsSourceFileService.cs

@@ -139,11 +139,14 @@ public async Task<MetadataAsSourceFile> GetGeneratedFileAsync(Project project, I
                         }
                         catch (Exception e) when (FatalError.ReportWithoutCrashUnlessCanceled(e))
                         {
+                            // Insert the exception message at the top of the fallback Metadata As Source document to
+                            // help with diagnosing failures. When the generated source is added to the document, this
+                            // comment will end up at the bottom.
                             var failureText = e.ToString();
-                            var failureTextAsComment = "// " + failureText.Replace("\n", "\n// ") + "\n";
+                            var failureTextAsComment = "// " + failureText.Replace("\n", "\n// ") + Environment.NewLine;
 
                             var sourceText = await temporaryDocument.GetTextAsync(cancellationToken).ConfigureAwait(false);
-                            temporaryDocument = temporaryDocument.WithText(sourceText.Replace(default, failureTextAsComment));
+                            temporaryDocument = temporaryDocument.WithText(sourceText.Replace(new TextSpan(0, 0), failureTextAsComment));
 
                             useDecompiler = false;
                         }

src/EditorFeatures/Test/MetadataAsSource/AbstractMetadataAsSourceTests.TestContext.cs

@@ -2,13 +2,15 @@
 
 using System;
 using System.Collections.Generic;
+using System.Globalization;
 using System.IO;
 using System.Linq;
 using System.Security;
 using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.CodeAnalysis.Editor.UnitTests.Workspaces;
 using Microsoft.CodeAnalysis.Text;
+using Microsoft.VisualStudio.Composition;
 using Microsoft.VisualStudio.Text;
 using Roslyn.Test.Utilities;
 using Roslyn.Utilities;
@@ -33,7 +35,8 @@ public static TestContext Create(
                 bool includeXmlDocComments = false,
                 string sourceWithSymbolReference = null,
                 string languageVersion = null,
-                string metadataLanguageVersion = null)
+                string metadataLanguageVersion = null,
+                ExportProvider exportProvider = null)
             {
                 projectLanguage ??= LanguageNames.CSharp;
                 metadataSources ??= SpecializedCollections.EmptyEnumerable<string>();
@@ -43,7 +46,8 @@ public static TestContext Create(
 
                 var workspace = CreateWorkspace(
                     projectLanguage, metadataSources, includeXmlDocComments,
-                    sourceWithSymbolReference, languageVersion, metadataLanguageVersion);
+                    sourceWithSymbolReference, languageVersion, metadataLanguageVersion,
+                    exportProvider);
                 return new TestContext(workspace);
             }
 
@@ -95,22 +99,23 @@ private static string GetSpaceSeparatedTokens(string source)
                 return string.Join(" ", tokens);
             }
 
-            public void VerifyResult(MetadataAsSourceFile file, string expected)
+            public void VerifyResult(MetadataAsSourceFile file, Lazy<string> expected)
             {
                 var actual = File.ReadAllText(file.FilePath).Trim();
                 var actualSpan = file.IdentifierLocation.SourceSpan;
+                var expectedText = expected.Value;
 
                 // Compare exact texts and verify that the location returned is exactly that
                 // indicated by expected
-                MarkupTestFile.GetSpan(expected, out expected, out var expectedSpan);
-                AssertEx.EqualOrDiff(expected, actual);
+                MarkupTestFile.GetSpan(expectedText, out expectedText, out var expectedSpan);
+                AssertEx.EqualOrDiff(expectedText, actual);
                 Assert.Equal(expectedSpan.Start, actualSpan.Start);
                 Assert.Equal(expectedSpan.End, actualSpan.End);
             }
 
-            public async Task GenerateAndVerifySourceAsync(string symbolMetadataName, string expected, Project project = null)
+            public async Task GenerateAndVerifySourceAsync(string symbolMetadataName, Lazy<string> expected, Project project = null, bool allowDecompilation = false)
             {
-                var result = await GenerateSourceAsync(symbolMetadataName, project);
+                var result = await GenerateSourceAsync(symbolMetadataName, project, allowDecompilation);
                 VerifyResult(result, expected);
             }
 
@@ -215,7 +220,8 @@ private static string DeduceLanguageString(string input)
             private static TestWorkspace CreateWorkspace(
                 string projectLanguage, IEnumerable<string> metadataSources,
                 bool includeXmlDocComments, string sourceWithSymbolReference,
-                string languageVersion, string metadataLanguageVersion)
+                string languageVersion, string metadataLanguageVersion,
+                ExportProvider exportProvider)
             {
                 var languageVersionAttribute = languageVersion is null ? "" : $@" LanguageVersion=""{languageVersion}""";
 
@@ -252,7 +258,7 @@ private static TestWorkspace CreateWorkspace(
     </Project>
 </Workspace>");
 
-                return TestWorkspace.Create(xmlString);
+                return TestWorkspace.Create(xmlString, exportProvider: exportProvider);
             }
 
             internal Document GetDocument(MetadataAsSourceFile file)

src/EditorFeatures/Test/MetadataAsSource/AbstractMetadataAsSourceTests.cs

@@ -1,10 +1,12 @@
 // Copyright (c) Microsoft.  All Rights Reserved.  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.Threading.Tasks;
 using Microsoft.CodeAnalysis.CSharp;
 using Microsoft.CodeAnalysis.Test.Utilities;
+using Microsoft.VisualStudio.Composition;
 using Roslyn.Utilities;
 using Xunit;
 
@@ -14,10 +16,10 @@ namespace Microsoft.CodeAnalysis.Editor.UnitTests.MetadataAsSource
     public abstract partial class AbstractMetadataAsSourceTests
     {
         internal static async Task GenerateAndVerifySourceAsync(
-            string metadataSource, string symbolName, string projectLanguage, string expected, bool includeXmlDocComments = false, string languageVersion = null, string metadataLanguageVersion = null)
+            string metadataSource, string symbolName, string projectLanguage, Lazy<string> expected, bool allowDecompilation = false, bool includeXmlDocComments = false, string languageVersion = null, string metadataLanguageVersion = null, ExportProvider exportProvider = null)
         {
-            using var context = TestContext.Create(projectLanguage, SpecializedCollections.SingletonEnumerable(metadataSource), includeXmlDocComments, languageVersion: languageVersion, metadataLanguageVersion: metadataLanguageVersion);
-            await context.GenerateAndVerifySourceAsync(symbolName, expected);
+            using var context = TestContext.Create(projectLanguage, SpecializedCollections.SingletonEnumerable(metadataSource), includeXmlDocComments, languageVersion: languageVersion, metadataLanguageVersion: metadataLanguageVersion, exportProvider: exportProvider);
+            await context.GenerateAndVerifySourceAsync(symbolName, expected, allowDecompilation: allowDecompilation);
         }
 
         internal static async Task GenerateAndVerifySourceLineAsync(string source, string language, string expected)

src/EditorFeatures/Test/MetadataAsSource/MetadataAsSourceTests.VisualBasic.cs

@@ -1,5 +1,6 @@
 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
+using System;
 using System.Threading.Tasks;
 using Microsoft.CodeAnalysis.MetadataAsSource;
 using Microsoft.CodeAnalysis.Test.Utilities;
@@ -21,15 +22,15 @@ Module M
     Public Class D
     End Class
 End Module";
-                await GenerateAndVerifySourceAsync(metadataSource, "M+D", LanguageNames.VisualBasic, $@"#Region ""{FeaturesResources.Assembly} ReferencedAssembly, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null""
+                await GenerateAndVerifySourceAsync(metadataSource, "M+D", LanguageNames.VisualBasic, new Lazy<string>(() => $@"#Region ""{FeaturesResources.Assembly} ReferencedAssembly, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null""
 ' {CodeAnalysisResources.InMemoryAssembly}
 #End Region
 
 Friend Module M
     Public Class [|D|]
         Public Sub New()
     End Class
-End Module");
+End Module"));
             }
 
             // This test depends on the version of mscorlib used by the TestWorkspace and may 
@@ -65,7 +66,7 @@ End Class
 End Namespace";
 
                 using var context = TestContext.Create(LanguageNames.VisualBasic);
-                await context.GenerateAndVerifySourceAsync("System.ObsoleteAttribute", expected);
+                await context.GenerateAndVerifySourceAsync("System.ObsoleteAttribute", new Lazy<string>(() => expected));
             }
 
             [Fact, Trait(Traits.Feature, Traits.Features.MetadataAsSource)]