Fix crash in DocHighlights when delegating to a different language.

src/Features/Core/Portable/DocumentHighlighting/AbstractDocumentHighlightsService.cs

@@ -76,8 +76,8 @@ internal abstract partial class AbstractDocumentHighlightsService : IDocumentHig
 
             // Get unique tags for referenced symbols
             return await GetTagsForReferencedSymbolAsync(
-                new SymbolAndProjectId(symbol, document.Project.Id), documentsToSearch,
-                solution, cancellationToken).ConfigureAwait(false);
+                new SymbolAndProjectId(symbol, document.Project.Id),
+                document, documentsToSearch, cancellationToken).ConfigureAwait(false);
         }
 
         private static async Task<ISymbol> GetSymbolToSearchAsync(Document document, int position, SemanticModel semanticModel, ISymbol symbol, CancellationToken cancellationToken)
@@ -95,8 +95,8 @@ private static async Task<ISymbol> GetSymbolToSearchAsync(Document document, int
 
         private async Task<ImmutableArray<DocumentHighlights>> GetTagsForReferencedSymbolAsync(
             SymbolAndProjectId symbolAndProjectId,
+            Document document,
             IImmutableSet<Document> documentsToSearch,
-            Solution solution,
             CancellationToken cancellationToken)
         {
             var symbol = symbolAndProjectId.Symbol;
@@ -106,11 +106,11 @@ private static async Task<ISymbol> GetSymbolToSearchAsync(Document document, int
                 var progress = new StreamingProgressCollector(
                     StreamingFindReferencesProgress.Instance);
                 await SymbolFinder.FindReferencesAsync(
-                    symbolAndProjectId, solution, progress,
+                    symbolAndProjectId, document.Project.Solution, progress,
                     documentsToSearch, cancellationToken).ConfigureAwait(false);
 
                 return await FilterAndCreateSpansAsync(
-                    progress.GetReferencedSymbols(), solution, documentsToSearch, 
+                    progress.GetReferencedSymbols(), document, documentsToSearch,
                     symbol, cancellationToken).ConfigureAwait(false);
             }
 
@@ -142,10 +142,12 @@ private static bool ShouldConsiderSymbol(ISymbol symbol)
         }
 
         private async Task<ImmutableArray<DocumentHighlights>> FilterAndCreateSpansAsync(
-            IEnumerable<ReferencedSymbol> references, Solution solution, 
-            IImmutableSet<Document> documentsToSearch, ISymbol symbol, 
+            IEnumerable<ReferencedSymbol> references, Document document,
+            IImmutableSet<Document> documentsToSearch, ISymbol symbol,
             CancellationToken cancellationToken)
         {
+            var solution = document.Project.Solution;
+
             references = references.FilterToItemsToShow();
             references = references.FilterNonMatchingMethodNames(solution, symbol);
             references = references.FilterToAliasMatches(symbol as IAliasSymbol);
@@ -157,13 +159,20 @@ private static bool ShouldConsiderSymbol(ISymbol symbol)
 
             var additionalReferences = new List<Location>();
 
-            foreach (var document in documentsToSearch)
+            foreach (var additionalDocument in documentsToSearch)
             {
-                additionalReferences.AddRange(await GetAdditionalReferencesAsync(document, symbol, cancellationToken).ConfigureAwait(false));
+                // 'documentsToSearch' may contain documents from languages other than our own
+                // (for example cshtml files when we're searching the cs document).  Since we're
+                // delegating to a virtual method for this language type, we have to make sure
+                // we only process the document if it's also our language.
+                if (additionalDocument.Project.Language == document.Project.Language)
+                {
+                    additionalReferences.AddRange(await GetAdditionalReferencesAsync(document, symbol, cancellationToken).ConfigureAwait(false));
+                }
             }
 
             return await CreateSpansAsync(
-                solution, symbol, references, additionalReferences, 
+                solution, symbol, references, additionalReferences,
                 documentsToSearch, cancellationToken).ConfigureAwait(false);
         }