/// <summary>
        /// Handles the <see cref="IActiveConfiguredProjectSubscriptionService"/> callback on the active configured project's
        /// design-time build change notification service.
        /// </summary>
        private async Task ProjectBuildRuleBlock_ChangedAsync(IProjectVersionedValue <IProjectSubscriptionUpdate> e)
        {
            await ThreadingService.SwitchToUIThread();

            using (ProjectAsynchronousTasksService.LoadedProject())
            {
                foreach (var resolvedReferenceChange in e.Value.ProjectChanges.Values)
                {
                    if (!WatchedDesignTimeBuildRules.Contains(resolvedReferenceChange.After.RuleName))
                    {
                        // This is an evaluation rule.
                        continue;
                    }

                    foreach (string resolvedReferencePath in resolvedReferenceChange.Difference.AddedItems)
                    {
                        // If this is a resolved project reference, we need to treat it specially.
                        string originalItemSpec = resolvedReferenceChange.After.Items[resolvedReferencePath]["OriginalItemSpec"];
                        if (!string.IsNullOrEmpty(originalItemSpec))
                        {
                            if (e.Value.CurrentState[ProjectReference.SchemaName].Items.ContainsKey(originalItemSpec))
                            {
                                string originalFullPath = UnconfiguredProject.MakeRooted(originalItemSpec);
                                ProjectReferenceState state;
                                if (!_projectReferenceFullPaths.TryGetValue(originalFullPath, out state))
                                {
                                    _projectReferenceFullPaths = _projectReferenceFullPaths.Add(originalFullPath, state = new ProjectReferenceState());
                                }

                                state.ResolvedPath = resolvedReferencePath;

                                // Be careful to not add assembly references that overlap with project references.
                                if (state.AsProjectReference)
                                {
                                    continue;
                                }
                            }
                        }

                        Marshal.ThrowExceptionForHR(_intellisenseEngine.AddAssemblyReference(resolvedReferencePath));
                    }

                    foreach (string resolvedReferencePath in resolvedReferenceChange.Difference.RemovedItems)
                    {
                        Marshal.ThrowExceptionForHR(_intellisenseEngine.RemoveAssemblyReference(resolvedReferencePath));
                    }

                    Marshal.ThrowExceptionForHR(_intellisenseEngine.StartIntellisenseEngine());
                }
            }
        }
        /// <summary>
        /// Handles the <see cref="IActiveConfiguredProjectSubscriptionService"/> callback event on the active configured project's
        /// change notification service.
        /// </summary>
        private async System.Threading.Tasks.Task ProjectRuleBlock_ChangedAsync(IProjectVersionedValue <IProjectSubscriptionUpdate> e)
        {
            await ThreadingService.SwitchToUIThread();

            await ProjectAsynchronousTasksService.LoadedProjectAsync(async delegate
            {
                var sourceFiles       = e.Value.ProjectChanges[CSharp.SchemaName];
                var projectReferences = e.Value.ProjectChanges[ProjectReference.SchemaName];
#pragma warning disable CA2007 // Do not directly await a Task (see https://github.com/dotnet/roslyn/issues/6770)
                var treeUpdate = await ProjectTreeService.PublishLatestTreeAsync(blockDuringLoadingTree: true);
#pragma warning restore CA2007 // Do not directly await a Task
                var tree = treeUpdate.Tree;

                foreach (var sourceUnit in sourceFiles.Difference.AddedItems.Select(item => SourceFileToLanguageServiceUnit(item, tree)).Where(u => u != null))
                {
                    Marshal.ThrowExceptionForHR(_intellisenseEngine.AddFile(sourceUnit.Item1, sourceUnit.Item2));
                }

                foreach (var sourceUnit in sourceFiles.Difference.RemovedItems.Select(item => SourceFileToLanguageServiceUnit(item, tree)).Where(u => u != null))
                {
                    Marshal.ThrowExceptionForHR(_intellisenseEngine.RemoveFile(sourceUnit.Item1, sourceUnit.Item2));
                }

                foreach (KeyValuePair <string, string> sourceFileNames in sourceFiles.Difference.RenamedItems)
                {
                    var newSourceUnit = SourceFileToLanguageServiceUnit(sourceFileNames.Value, tree);
                    if (newSourceUnit != null)
                    {
                        string beforeAbsolutePath = UnconfiguredProject.MakeRooted(sourceFileNames.Key);
                        Marshal.ThrowExceptionForHR(_intellisenseEngine.RenameFile(beforeAbsolutePath, newSourceUnit.Item1, newSourceUnit.Item2));
                    }
                }

                foreach (string projectReferencePath in projectReferences.Difference.AddedItems)
                {
                    string projectReferenceFullPath = UnconfiguredProject.MakeRooted(projectReferencePath);
                    ProjectReferenceState state;
                    if (!_projectReferenceFullPaths.TryGetValue(projectReferenceFullPath, out state))
                    {
                        _projectReferenceFullPaths = _projectReferenceFullPaths.Add(projectReferenceFullPath, state = new ProjectReferenceState());
                    }

                    IVsIntellisenseProject intellisenseProject;
                    if (LanguageServiceRegister.TryGetIntellisenseProject(projectReferenceFullPath, out intellisenseProject))
                    {
                        if (state.ResolvedPath != null && !state.AsProjectReference)
                        {
                            Marshal.ThrowExceptionForHR(_intellisenseEngine.RemoveAssemblyReference(state.ResolvedPath));
                        }

                        state.AsProjectReference = true;
                        Marshal.ThrowExceptionForHR(_intellisenseEngine.AddP2PReference(intellisenseProject));
                    }
                }

                foreach (string projectReferencePath in projectReferences.Difference.RemovedItems)
                {
                    string projectReferenceFullPath = UnconfiguredProject.MakeRooted(projectReferencePath);
                    _projectReferenceFullPaths      = _projectReferenceFullPaths.Remove(projectReferenceFullPath);

                    IVsIntellisenseProject intellisenseProject;
                    if (LanguageServiceRegister.TryGetIntellisenseProject(projectReferencePath, out intellisenseProject))
                    {
                        Marshal.ThrowExceptionForHR(_intellisenseEngine.RemoveP2PReference(_intellisenseEngine));
                    }

                    ProjectReferenceState state;
                    if (_projectReferenceFullPaths.TryGetValue(projectReferenceFullPath, out state))
                    {
                        state.AsProjectReference = false;
                    }
                }

                Marshal.ThrowExceptionForHR(_intellisenseEngine.StartIntellisenseEngine());
            });
        }