Use consistent calling convention for updates

src/Workspaces/Core/Portable/Workspace/Solution/ProjectDependencyGraph_AddProjectReference.cs

@@ -23,10 +23,7 @@ internal ProjectDependencyGraph WithAdditionalProjectReferences(ProjectId projec
 
             var newReferencesMap = ComputeNewReferencesMapForAdditionalProjectReferences(_referencesMap, projectId, referencedProjectIds);
 
-            var newReverseReferencesMap =
-                _lazyReverseReferencesMap != null
-                    ? ComputeNewReverseReferencesMapForAdditionalProjectReferences(_lazyReverseReferencesMap, projectId, referencedProjectIds)
-                    : null;
+            var newReverseReferencesMap = ComputeNewReverseReferencesMapForAdditionalProjectReferences(_lazyReverseReferencesMap, projectId, referencedProjectIds);
 
             var newTransitiveReferencesMap = ComputeNewTransitiveReferencesMapForAdditionalProjectReferences(_transitiveReferencesMap, projectId, referencedProjectIds);
 
@@ -64,13 +61,15 @@ internal ProjectDependencyGraph WithAdditionalProjectReferences(ProjectId projec
 
         /// <summary>
         /// Computes a new <see cref="_lazyReverseReferencesMap"/> for the addition of additional project references.
-        /// Must be called on a non-null map.
         /// </summary>
-        private static ImmutableDictionary<ProjectId, ImmutableHashSet<ProjectId>> ComputeNewReverseReferencesMapForAdditionalProjectReferences(
-            ImmutableDictionary<ProjectId, ImmutableHashSet<ProjectId>> existingReverseReferencesMap,
+        private static ImmutableDictionary<ProjectId, ImmutableHashSet<ProjectId>>? ComputeNewReverseReferencesMapForAdditionalProjectReferences(
+            ImmutableDictionary<ProjectId, ImmutableHashSet<ProjectId>>? existingReverseReferencesMap,
             ProjectId projectId,
             IReadOnlyList<ProjectId> referencedProjectIds)
         {
+            if (existingReverseReferencesMap is null)
+                return null;
+
             var builder = existingReverseReferencesMap.ToBuilder();
 
             foreach (var referencedProject in referencedProjectIds)

src/Workspaces/Core/Portable/Workspace/Solution/ProjectDependencyGraph_RemoveProject.cs

@@ -23,12 +23,10 @@ internal ProjectDependencyGraph WithProjectRemoved(ProjectId projectId)
 
             // The direct reverse references map is updated by removing the key for the project getting removed, and
             // also updating any direct references to the removed project.
-            var reverseReferencesMap = _lazyReverseReferencesMap is object
-                ? ComputeNewReverseReferencesMapForRemovedProject(
-                    existingForwardReferencesMap: _referencesMap,
-                    existingReverseReferencesMap: _lazyReverseReferencesMap,
-                    projectId)
-                : null;
+            var reverseReferencesMap = ComputeNewReverseReferencesMapForRemovedProject(
+                existingForwardReferencesMap: _referencesMap,
+                existingReverseReferencesMap: _lazyReverseReferencesMap,
+                projectId);
             var transitiveReferencesMap = ComputeNewTransitiveReferencesMapForRemovedProject(_transitiveReferencesMap, projectId);
             var reverseTransitiveReferencesMap = ComputeNewReverseTransitiveReferencesMapForRemovedProject(_reverseTransitiveReferencesMap, projectId);
             return new ProjectDependencyGraph(
@@ -84,11 +82,17 @@ internal ProjectDependencyGraph WithProjectRemoved(ProjectId projectId)
         /// <summary>
         /// Computes a new <see cref="_lazyReverseReferencesMap"/> for the removal of a project.
         /// </summary>
-        private static ImmutableDictionary<ProjectId, ImmutableHashSet<ProjectId>> ComputeNewReverseReferencesMapForRemovedProject(
+        private static ImmutableDictionary<ProjectId, ImmutableHashSet<ProjectId>>? ComputeNewReverseReferencesMapForRemovedProject(
             ImmutableDictionary<ProjectId, ImmutableHashSet<ProjectId>> existingForwardReferencesMap,
-            ImmutableDictionary<ProjectId, ImmutableHashSet<ProjectId>> existingReverseReferencesMap,
+            ImmutableDictionary<ProjectId, ImmutableHashSet<ProjectId>>? existingReverseReferencesMap,
             ProjectId projectId)
         {
+            if (existingReverseReferencesMap is null)
+            {
+                // The map was never calculated for the previous graph, so there is nothing to update.
+                return null;
+            }
+
             var builder = existingReverseReferencesMap.ToBuilder();
 
             // Iterate over each project referenced by 'projectId', which is now being removed. Update the reverse

src/Workspaces/Core/Portable/Workspace/Solution/ProjectDependencyGraph_RemoveProjectReference.cs

@@ -20,9 +20,7 @@ internal ProjectDependencyGraph WithProjectReferenceRemoved(ProjectId projectId,
 
             // Incrementally update the graph
             var referencesMap = _referencesMap.SetItem(projectId, _referencesMap[projectId].Remove(referencedProjectId));
-            var reverseReferencesMap = _lazyReverseReferencesMap is object
-                ? ComputeNewReverseReferencesMapForRemovedProjectReference(_lazyReverseReferencesMap, projectId, referencedProjectId)
-                : null;
+            var reverseReferencesMap = ComputeNewReverseReferencesMapForRemovedProjectReference(_lazyReverseReferencesMap, projectId, referencedProjectId);
             var transitiveReferencesMap = ComputeNewTransitiveReferencesMapForRemovedProjectReference(_transitiveReferencesMap, projectId, referencedProjectId);
             var reverseTransitiveReferencesMap = ComputeNewReverseTransitiveReferencesMapForRemovedProjectReference(_reverseTransitiveReferencesMap, projectId, referencedProjectId);
 
@@ -36,11 +34,14 @@ internal ProjectDependencyGraph WithProjectReferenceRemoved(ProjectId projectId,
                 dependencySets: default);
         }
 
-        private ImmutableDictionary<ProjectId, ImmutableHashSet<ProjectId>> ComputeNewReverseReferencesMapForRemovedProjectReference(
-            ImmutableDictionary<ProjectId, ImmutableHashSet<ProjectId>> existingReverseReferencesMap,
+        private ImmutableDictionary<ProjectId, ImmutableHashSet<ProjectId>>? ComputeNewReverseReferencesMapForRemovedProjectReference(
+            ImmutableDictionary<ProjectId, ImmutableHashSet<ProjectId>>? existingReverseReferencesMap,
             ProjectId projectId,
             ProjectId referencedProjectId)
         {
+            if (existingReverseReferencesMap is null)
+                return null;
+
             if (existingReverseReferencesMap.TryGetValue(referencedProjectId, out var referencingProjects))
             {
                 return existingReverseReferencesMap.SetItem(referencedProjectId, referencingProjects.Remove(projectId));