All migrated code compiles now.

src/Menees.VsTools/MainPackage.cs

@@ -24,7 +24,6 @@ public sealed partial class MainPackage : AsyncPackage, IDisposable
 	{
 		#region Private Data Members
 
-		private static MainPackage instance;
 		private static Options generalOptions;
 		private static BaseConverter.Options baseConverterOptions;
 
@@ -53,21 +52,6 @@ public MainPackage()
 
 		#region Internal Properties
 
-		internal static MainPackage Instance
-		{
-			get
-			{
-				ThreadHelper.ThrowIfNotOnUIThread();
-
-				if (instance == null)
-				{
-					ForceLoad();
-				}
-
-				return instance;
-			}
-		}
-
 		internal static Options GeneralOptions
 		{
 			get
@@ -196,7 +180,7 @@ protected override async System.Threading.Tasks.Task InitializeAsync(Cancellatio
 				LogMessage(string.Format("After {0}'s base.Initialize()", this.ToString()));
 
 				await ThreadHelper.JoinableTaskFactory.SwitchToMainThreadAsync();
-				instance = this;
+				ScanInfo.GetUserRegistryRoot = () => this.UserRegistryRoot;
 
 				// Ryan Molden from Microsoft says that GetDialogPage caches the result, so we're going to cache it too.
 				// I've also verified GetDialogPage's caching implementation in VS11 by looking at it with Reflector.

src/Menees.VsTools/Menees.VsTools.csproj

@@ -178,6 +178,7 @@
     <Compile Include="StatisticsDialog.xaml.cs">
       <DependentUpon>StatisticsDialog.xaml</DependentUpon>
     </Compile>
+    <Compile Include="Tasks\BackgroundOptions.cs" />
     <Compile Include="Tasks\CommentTask.cs" />
     <Compile Include="Tasks\CommentTaskProvider.cs" />
     <Compile Include="Tasks\CommentToken.cs" />

src/Menees.VsTools/Tasks/BackgroundOptions.cs

@@ -0,0 +1,66 @@
+namespace Menees.VsTools.Tasks
+{
+	#region Using Directives
+
+	using System;
+	using System.Collections.Generic;
+	using System.Linq;
+	using System.Text;
+	using System.Threading.Tasks;
+
+	#endregion
+
+	internal sealed class BackgroundOptions
+	{
+		#region Private Data Members
+
+		private readonly object locker = new object();
+
+		#endregion
+
+		#region Public Events
+
+		public event EventHandler Updated;
+
+		#endregion
+
+		#region Public Properties
+
+		public bool EnableCommentScans { get; private set; }
+
+		public string ExcludeFromCommentScans { get; private set; }
+
+		#endregion
+
+		#region Public Methods
+
+		public bool Update(Options options)
+		{
+			bool updated = false;
+
+			lock (this.locker)
+			{
+				if (this.EnableCommentScans != options.EnableCommentScans)
+				{
+					this.EnableCommentScans = options.EnableCommentScans;
+					updated = true;
+				}
+
+				if (this.ExcludeFromCommentScans != options.ExcludeFromCommentScans)
+				{
+					this.ExcludeFromCommentScans = options.ExcludeFromCommentScans;
+					updated = true;
+				}
+			}
+
+			if (updated)
+			{
+				this.Updated?.Invoke(this, EventArgs.Empty);
+			}
+
+			return updated;
+		}
+
+		#endregion
+	}
+}

src/Menees.VsTools/Tasks/CommentTaskProvider.cs

@@ -20,6 +20,7 @@ namespace Menees.VsTools.Tasks
 	using Microsoft.VisualStudio.Shell.Interop;
 	using Microsoft.VisualStudio.Text;
 	using Microsoft.VisualStudio.TextManager.Interop;
+	using Sys = System.Threading.Tasks;
 
 	#endregion
 
@@ -38,6 +39,7 @@ internal sealed class CommentTaskProvider : TaskProvider, IVsTaskListEvents
 		private readonly FileMonitor fileMonitor;
 		private readonly FileItemManager manager;
 		private readonly List<CommentToken> foregroundTokens = new List<CommentToken>();
+		private readonly BackgroundOptions backgroundOptions = new BackgroundOptions();
 
 		private bool disposed;
 		private int isBackgroundTimerExecuting;
@@ -55,7 +57,6 @@ public CommentTaskProvider(MainPackage package)
 			ThreadHelper.ThrowIfNotOnUIThread();
 
 			this.ServiceProvider = package.ServiceProvider;
-			MainPackage.GeneralOptions.Applied += this.Options_Applied;
 
 			// In some cases the task list may not be available (e.g., during devenv.exe /build).
 			if (!(this.ServiceProvider.GetService(typeof(SVsTaskList)) is IVsTaskList taskList))
@@ -64,6 +65,10 @@ public CommentTaskProvider(MainPackage package)
 			}
 			else
 			{
+				Options options = MainPackage.GeneralOptions;
+				options.Applied += this.Options_Applied;
+				this.backgroundOptions.Update(options);
+
 				// Register a custom category so Visual Studio will invoke our IVsTaskListEvents callbacks.
 				VSTASKCATEGORY[] assignedCategory = new VSTASKCATEGORY[1];
 				int hr = this.VsTaskList.RegisterCustomCategory(CategoryId, (uint)TaskCategory.Comments + 1, assignedCategory);
@@ -90,7 +95,7 @@ public CommentTaskProvider(MainPackage package)
 				this.solutionMonitor = new SolutionMonitor(this);
 				this.documentMonitor = new DocumentMonitor(this);
 				this.fileMonitor = new FileMonitor(this);
-				this.manager = new FileItemManager(this, this.fileMonitor, MainPackage.GeneralOptions);
+				this.manager = new FileItemManager(this, this.fileMonitor, this.backgroundOptions);
 
 				// Enable the timers last.  The BackgroundTimerCallback will fire after ScanDelay (on a worker thread),
 				// so we have to ensure that everything is initialized before its first callback.
@@ -206,27 +211,11 @@ private void BackgroundTimerCallback(object state)
 		{
 			if (Interlocked.CompareExchange(ref this.isBackgroundTimerExecuting, 1, 0) == 0)
 			{
-				// TODO: Get rid of these nested if blocks that are just here to get things to compile. [Bill, 4/19/2020]
-				if (this.appliedOptionsPending)
-				{
-					this.appliedOptionsPending = false;
-					if (this.foregroundTokensChanged)
-					{
-						this.foregroundTokensChanged = false;
-						if (this.backgroundTokens == null)
-						{
-							this.backgroundTokens = new List<CommentToken>();
-						}
-					}
-				}
-
-#if TODO // TODO: Rewrite this to work from a background thread. [Bill, 4/19/2020]
-			// Make sure only one thread at a time is running (in case this handler takes longer than the timer's interval).
+				// Make sure only one thread at a time is running (in case this handler takes longer than the timer's interval).
 				try
 				{
-					// TODO: Switch to UI thread (from BackgroundTimerCallback)? [Bill, 4/19/2020]
 					// If the user disables this in Options, then immediately stop scanning (without waiting for a restart).
-					if (MainPackage.GeneralOptions.EnableCommentScans)
+					if (this.backgroundOptions.EnableCommentScans)
 					{
 						bool updateAll = this.appliedOptionsPending;
 						this.appliedOptionsPending = false;
@@ -242,22 +231,10 @@ private void BackgroundTimerCallback(object state)
 
 						if (this.backgroundTokens.Count > 0 || updateAll)
 						{
-							IEnumerable<HierarchyItem> allHierarchyItems = this.solutionMonitor.GetChangedHierarchy();
-							if (allHierarchyItems != null)
-							{
-								Debug("Updating Hierarchy ({0})", allHierarchyItems.Count());
-								this.manager.UpdateHierarchy(allHierarchyItems);
-							}
-
-							IReadOnlyDictionary<string, DocumentItem> changedDocuments = this.documentMonitor.GetChangedDocuments();
-							if (changedDocuments != null)
-							{
-								Debug(
-									"Updating Documents ({0}): {1}",
-									changedDocuments.Count,
-									string.Join(", ", changedDocuments.Select(pair => Path.GetFileName(pair.Key) + '=' + (pair.Value != null))));
-								this.manager.UpdateDocuments(changedDocuments);
-							}
+							var foregroundUpdate = ThreadHelper.JoinableTaskFactory.RunAsync(this.ForegroundUpdateAsync);
+#pragma warning disable VSTHRD102 // Implement internal logic asynchronously. Some of the VS calls must be made from the foreground thread.
+							foregroundUpdate.Join();
+#pragma warning restore VSTHRD102 // Implement internal logic asynchronously
 
 							IReadOnlyDictionary<string, bool> changedFiles = this.fileMonitor.GetChangedFiles();
 							if (changedFiles != null)
@@ -284,7 +261,28 @@ private void BackgroundTimerCallback(object state)
 				{
 					Interlocked.Exchange(ref this.isBackgroundTimerExecuting, 0);
 				}
-#endif
+			}
+		}
+
+		private async Sys.Task ForegroundUpdateAsync()
+		{
+			await ThreadHelper.JoinableTaskFactory.SwitchToMainThreadAsync();
+
+			IEnumerable<HierarchyItem> allHierarchyItems = this.solutionMonitor.GetChangedHierarchy();
+			if (allHierarchyItems != null)
+			{
+				Debug("Updating Hierarchy ({0})", allHierarchyItems.Count());
+				this.manager.UpdateHierarchy(allHierarchyItems);
+			}
+
+			IReadOnlyDictionary<string, DocumentItem> changedDocuments = this.documentMonitor.GetChangedDocuments();
+			if (changedDocuments != null)
+			{
+				Debug(
+					"Updating Documents ({0}): {1}",
+					changedDocuments.Count,
+					string.Join(", ", changedDocuments.Select(pair => Path.GetFileName(pair.Key) + '=' + (pair.Value != null))));
+				this.manager.UpdateDocuments(changedDocuments);
 			}
 		}
 
@@ -337,9 +335,13 @@ private void CacheCommentTokens()
 
 		private void Options_Applied(object sender, EventArgs e)
 		{
-			this.appliedOptionsPending = true;
+			ThreadHelper.ThrowIfNotOnUIThread();
+			if (this.backgroundOptions.Update(MainPackage.GeneralOptions))
+			{
+				this.appliedOptionsPending = true;
+			}
 		}
 
-#endregion
+		#endregion
 	}
 }

src/Menees.VsTools/Tasks/FileItem.cs

@@ -41,8 +41,6 @@ internal sealed class FileItem
 
 		internal FileItem(CommentTaskProvider provider, string fileName)
 		{
-			ThreadHelper.ThrowIfNotOnUIThread();
-
 			// Note: The caller should check FileUtility.IsValidPath(fileName) before invoking this constructor.
 			this.provider = provider;
 			this.FileName = fileName;
@@ -85,8 +83,6 @@ public DocumentItem Document
 
 			set
 			{
-				ThreadHelper.ThrowIfNotOnUIThread();
-
 				if (this.document != value)
 				{
 					this.document = value;

src/Menees.VsTools/Tasks/FileItemManager.cs

@@ -23,21 +23,20 @@ internal sealed class FileItemManager
 		private readonly Dictionary<string, FileItem> files = new Dictionary<string, FileItem>(StringComparer.OrdinalIgnoreCase);
 		private readonly HashSet<FileItem> changedItems = new HashSet<FileItem>();
 		private readonly Dictionary<CommentTask, bool> changedTasks = new Dictionary<CommentTask, bool>();
-		private readonly Options options;
+		private readonly BackgroundOptions options;
 		private readonly Dictionary<string, Regex> excludePatternsCache = new Dictionary<string, Regex>();
 		private readonly List<Regex> backgroundExcludePatterns = new List<Regex>();
 
 		#endregion
 
 		#region Constructors
 
-		public FileItemManager(CommentTaskProvider provider, FileMonitor monitor, Options options)
+		public FileItemManager(CommentTaskProvider provider, FileMonitor monitor, BackgroundOptions options)
 		{
 			this.provider = provider;
 			this.monitor = monitor;
-
 			this.options = options;
-			this.options.Applied += this.Options_Applied;
+			this.options.Updated += this.Options_Updated;
 			this.RefreshExcludePatterns();
 		}
 
@@ -296,7 +295,7 @@ private void RefreshItem(FileItem item, RefreshAction action)
 			}
 		}
 
-		private void Options_Applied(object sender, EventArgs e)
+		private void Options_Updated(object sender, EventArgs e)
 		{
 			this.RefreshExcludePatterns();
 		}

src/Menees.VsTools/Tasks/ScanInfo.cs

@@ -75,6 +75,8 @@ private ScanInfo(IEnumerable<Delimiter> delimiters, IEnumerable<Language> langua
 
 		#region Public Properties
 
+		public static Func<RegistryKey> GetUserRegistryRoot { get; set; }
+
 		public bool IsScannable
 		{
 			get
@@ -90,8 +92,6 @@ public bool IsScannable
 
 		public static ScanInfo Get(FileItem file)
 		{
-			ThreadHelper.ThrowIfNotOnUIThread();
-
 			Cache cache = LazyCache.Value;
 			string extension = Path.GetExtension(file.FileName);
 			if (cache.TryGet(extension, out ScanInfo result))
@@ -217,7 +217,6 @@ private static ScanInfo Merge(ScanInfo scanInfo1, ScanInfo scanInfo2)
 
 		private static ScanInfo Infer(FileItem file, Cache cache)
 		{
-			ThreadHelper.ThrowIfNotOnUIThread();
 			ScanInfo result = Unscannable;
 
 			try
@@ -266,13 +265,11 @@ private static ScanInfo Infer(FileItem file, Cache cache)
 
 		private static bool TryGetCustomExtensionScanInfo(string extension, Cache cache, ref ScanInfo scanInfo)
 		{
-			ThreadHelper.ThrowIfNotOnUIThread();
 			bool result = false;
 
-			MainPackage package = MainPackage.Instance;
-			if (package != null && !string.IsNullOrEmpty(extension) && extension.Length > 1 && extension[0] == '.')
+			if (!string.IsNullOrEmpty(extension) && extension.Length > 1 && extension[0] == '.')
 			{
-				using (RegistryKey studioUserRoot = package.UserRegistryRoot)
+				using (RegistryKey studioUserRoot = GetUserRegistryRoot?.Invoke())
 				{
 					if (studioUserRoot != null)
 					{