Don't use exceptions for flow control

src/EditorFeatures/CSharp/DecompiledSource/CSharpDecompiledSourceService.cs

@@ -41,14 +41,19 @@ public CSharpDecompiledSourceService()
         {
         }
 
-        public async Task<Document> AddSourceToAsync(Document document, Compilation symbolCompilation, ISymbol symbol, MetadataReference metadataReference, string assemblyLocation, SyntaxFormattingOptions formattingOptions, CancellationToken cancellationToken)
+        public async Task<Document?> AddSourceToAsync(Document document, Compilation symbolCompilation, ISymbol symbol, MetadataReference? metadataReference, string? assemblyLocation, SyntaxFormattingOptions formattingOptions, CancellationToken cancellationToken)
         {
             // Get the name of the type the symbol is in
             var containingOrThis = symbol.GetContainingTypeOrThis();
             var fullName = GetFullReflectionName(containingOrThis);
 
             // Decompile
-            document = PerformDecompilation(document, fullName, symbolCompilation, metadataReference, assemblyLocation);
+            var decompiledDocument = PerformDecompilation(document, fullName, symbolCompilation, metadataReference, assemblyLocation);
+
+            if (decompiledDocument is null)
+                return null;
+
+            document = decompiledDocument;
 
             document = await AddAssemblyInfoRegionAsync(document, symbol, cancellationToken).ConfigureAwait(false);
 
@@ -74,7 +79,7 @@ public static async Task<Document> FormatDocumentAsync(Document document, Syntax
             return formattedDoc;
         }
 
-        private static Document PerformDecompilation(Document document, string fullName, Compilation compilation, MetadataReference? metadataReference, string assemblyLocation)
+        private static Document? PerformDecompilation(Document document, string fullName, Compilation compilation, MetadataReference? metadataReference, string? assemblyLocation)
         {
             var logger = new StringBuilder();
             var resolver = new AssemblyResolver(compilation, logger);
@@ -84,12 +89,11 @@ private static Document PerformDecompilation(Document document, string fullName,
             if (metadataReference is not null)
                 file = resolver.TryResolve(metadataReference, PEStreamOptions.PrefetchEntireImage);
 
-            if (file is null && assemblyLocation is null)
-            {
-                throw new NotSupportedException(FeaturesResources.Cannot_navigate_to_the_symbol_under_the_caret);
-            }
+            if (file is null && assemblyLocation is not null)
+                file = new PEFile(assemblyLocation, PEStreamOptions.PrefetchEntireImage);
 
-            file ??= new PEFile(assemblyLocation, PEStreamOptions.PrefetchEntireImage);
+            if (file is null)
+                return null;
 
             // Initialize a decompiler with default settings.
             var decompiler = new CSharpDecompiler(file, resolver, new DecompilerSettings());

src/Features/Core/Portable/DecompiledSource/IDecompiledSourceService.cs

@@ -2,8 +2,6 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
-#nullable disable
-
 using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.CodeAnalysis.Formatting;
@@ -24,7 +22,7 @@ internal interface IDecompiledSourceService : ILanguageService
         /// <param name="metadataReference">The reference that contains the symbol</param>
         /// <param name="assemblyLocation">The location of the implementation assembly to decompile</param>
         /// <param name="cancellationToken">To cancel document operations</param>
-        /// <returns>The updated document</returns>
-        Task<Document> AddSourceToAsync(Document document, Compilation symbolCompilation, ISymbol symbol, MetadataReference metadataReference, string assemblyLocation, SyntaxFormattingOptions formattingOptions, CancellationToken cancellationToken);
+        /// <returns>The updated document, or null if the decompilation could not be performed</returns>
+        Task<Document?> AddSourceToAsync(Document document, Compilation symbolCompilation, ISymbol symbol, MetadataReference? metadataReference, string? assemblyLocation, SyntaxFormattingOptions formattingOptions, CancellationToken cancellationToken);
     }
 }

src/Features/Core/Portable/MetadataAsSource/DecompilationMetadataAsSourceFileProvider.cs

@@ -92,7 +92,15 @@ public DecompilationMetadataAsSourceFileProvider(IImplementationAssemblyLookupSe
 
                         if (decompiledSourceService != null)
                         {
-                            temporaryDocument = await decompiledSourceService.AddSourceToAsync(temporaryDocument, compilation, symbol, refInfo.metadataReference, refInfo.assemblyLocation, options.GenerationOptions.CleanupOptions.FormattingOptions, cancellationToken).ConfigureAwait(false);
+                            var decompilationDocument = await decompiledSourceService.AddSourceToAsync(temporaryDocument, compilation, symbol, refInfo.metadataReference, refInfo.assemblyLocation, options.GenerationOptions.CleanupOptions.FormattingOptions, cancellationToken).ConfigureAwait(false);
+                            if (decompilationDocument is not null)
+                            {
+                                temporaryDocument = decompilationDocument;
+                            }
+                            else
+                            {
+                                useDecompiler = false;
+                            }
                         }
                         else
                         {