Don't explicitly downcast types.

src/Features/Core/Portable/CodeFixes/Suppression/AbstractSuppressionCodeFixProvider.FixAllProvider.cs

@@ -22,7 +22,6 @@ private SuppressionFixAllProvider()
             public async override Task<CodeAction> GetFixAsync(FixAllContext fixAllContext)
             {
                 var batchFixer = (BatchFixAllProvider)WellKnownFixAllProviders.BatchFixer;
-                var fixMultipleContext = fixAllContext as FixMultipleContext;
                 var suppressionFixer = (AbstractSuppressionCodeFixProvider)((WrapperCodeFixProvider)fixAllContext.CodeFixProvider).SuppressionFixProvider;
                 var isGlobalSuppression = NestedSuppressionCodeAction.IsEquivalenceKeyForGlobalSuppression(fixAllContext.CodeActionEquivalenceKey);
                 if (!isGlobalSuppression)
@@ -38,9 +37,14 @@ public async override Task<CodeAction> GetFixAsync(FixAllContext fixAllContext)
                 var title = fixAllContext.CodeActionEquivalenceKey;
                 if (fixAllContext.Document != null)
                 {
+#if false
                     var documentsAndDiagnosticsToFixMap = fixMultipleContext != null ?
                         fixMultipleContext.DocumentDiagnosticsToFix :
                         await batchFixer.GetDocumentDiagnosticsToFixAsync(fixAllContext).ConfigureAwait(false);
+#endif
+
+                    var documentsAndDiagnosticsToFixMap = 
+                        await fixAllContext.GetDocumentDiagnosticsToFixAsync(batchFixer).ConfigureAwait(false);
 
                     return !isGlobalSuppression ?
                         await batchFixer.GetFixAsync(documentsAndDiagnosticsToFixMap, fixAllContext).ConfigureAwait(false) :
@@ -48,9 +52,13 @@ public async override Task<CodeAction> GetFixAsync(FixAllContext fixAllContext)
                 }
                 else
                 {
+#if false
                     var projectsAndDiagnosticsToFixMap = fixMultipleContext != null ?
                         fixMultipleContext.ProjectDiagnosticsToFix :
                         await batchFixer.GetProjectDiagnosticsToFixAsync(fixAllContext).ConfigureAwait(false);
+#endif
+                    var projectsAndDiagnosticsToFixMap =
+                        await fixAllContext.GetProjectDiagnosticsToFixAsync(batchFixer).ConfigureAwait(false);
 
                     return !isGlobalSuppression ?
                         await batchFixer.GetFixAsync(projectsAndDiagnosticsToFixMap, fixAllContext).ConfigureAwait(false) :

src/Workspaces/Core/Portable/CodeFixes/FixAllOccurrences/FixAllContext.DiagnosticProvider.cs

@@ -3,6 +3,8 @@
 using System.Collections.Generic;
 using System.Threading.Tasks;
 using System.Threading;
+using System;
+using System.Collections.Immutable;
 
 namespace Microsoft.CodeAnalysis.CodeFixes
 {
@@ -32,5 +34,15 @@ public abstract class DiagnosticProvider
             /// </summary>
             public abstract Task<IEnumerable<Diagnostic>> GetAllDiagnosticsAsync(Project project, CancellationToken cancellationToken);
         }
+
+        internal virtual Task<ImmutableDictionary<Document, ImmutableArray<Diagnostic>>> GetDocumentDiagnosticsToFixAsync(BatchFixAllProvider batchFixer)
+        {
+            return batchFixer.GetDocumentDiagnosticsToFixAsync(this);
+        }
+
+        internal virtual Task<ImmutableDictionary<Project, ImmutableArray<Diagnostic>>> GetProjectDiagnosticsToFixAsync(BatchFixAllProvider batchFixer)
+        {
+            return batchFixer.GetProjectDiagnosticsToFixAsync(this);
+        }
     }
 }

src/Workspaces/Core/Portable/CodeFixes/FixAllOccurrences/FixMultipleContext.cs

@@ -96,19 +96,6 @@ private static ImmutableHashSet<string> GetDiagnosticsIds(IEnumerable<ImmutableA
             return uniqueIds.ToImmutable();
         }
 
-        public ImmutableDictionary<Document, ImmutableArray<Diagnostic>> DocumentDiagnosticsToFix => _diagnosticProvider.DocumentDiagnosticsToFix;
-        public ImmutableDictionary<Project, ImmutableArray<Diagnostic>> ProjectDiagnosticsToFix => _diagnosticProvider.ProjectDiagnosticsToFix;
-
-        public Diagnostic GetTriggerDiagnostic()
-        {
-            if (Document != null)
-            {
-                return DocumentDiagnosticsToFix[Document].First();
-            }
-
-            return ProjectDiagnosticsToFix[Project].First();
-        }
-
         public new FixAllContext WithCancellationToken(CancellationToken cancellationToken)
         {
             if (this.CancellationToken == cancellationToken)
@@ -120,5 +107,15 @@ public new FixAllContext WithCancellationToken(CancellationToken cancellationTok
                 new FixMultipleContext(Document, CodeFixProvider, CodeActionEquivalenceKey, DiagnosticIds, _diagnosticProvider, cancellationToken) :
                 new FixMultipleContext(Project, CodeFixProvider, CodeActionEquivalenceKey, DiagnosticIds, _diagnosticProvider, cancellationToken);
         }
+
+        internal override Task<ImmutableDictionary<Document, ImmutableArray<Diagnostic>>> GetDocumentDiagnosticsToFixAsync(BatchFixAllProvider batchFixer)
+        {
+            return Task.FromResult(_diagnosticProvider.DocumentDiagnosticsToFix);
+        }
+
+        internal override Task<ImmutableDictionary<Project, ImmutableArray<Diagnostic>>> GetProjectDiagnosticsToFixAsync(BatchFixAllProvider batchFixer)
+        {
+            return Task.FromResult(_diagnosticProvider.ProjectDiagnosticsToFix);
+        }
     }
 }