diff --git a/Codist/Semantics/CSharpParser.cs b/Codist/Semantics/CSharpParser.cs
index 9c411a4c..ac1ec3e9 100644
--- a/Codist/Semantics/CSharpParser.cs
+++ b/Codist/Semantics/CSharpParser.cs
@@ -134,17 +134,17 @@ public TextBufferParser(CSharpParser container, ITextBuffer buffer) {
 
 			public event EventHandler<EventArgs<SemanticState>> StateUpdated;
 
-			internal bool IsDisposed => _Parser == null;
+			public bool IsDisposed => _Parser == null;
 			public ITextBuffer TextBuffer => _Buffer;
 
 			public void Ref() {
 				_Ref++;
-				Debug.WriteLine($"Ref+ {_Ref} {_Name}");
+				$"Ref+ {_Ref} {_Name}".Log();
 			}
 
 			internal void Release() {
 				if (Interlocked.Exchange(ref _Parser, null) != null) {
-					Debug.WriteLine($"{_Name} tagger released");
+					$"{_Name} tagger released".Log();
 					ReleaseResources();
 					_Ref = 0;
 				}
@@ -274,7 +274,7 @@ void UnsubscribeBufferEvents(ITextBuffer buffer) {
 			}
 
 			void WorkspaceChanged(object sender, WorkspaceChangeEventArgs args) {
-				Debug.WriteLine($"Workspace {args.Kind}: {args.DocumentId}");
+				$"Workspace {args.Kind}: {args.DocumentId}".Log();
 				var parser = _Parser;
 				if (parser == null) {
 					return;
@@ -308,17 +308,18 @@ void WorkspaceChanged(object sender, WorkspaceChangeEventArgs args) {
 			void TextBuffer_ContentTypeChanged(object sender, ContentTypeChangedEventArgs e) {
 				if (_IsInteractiveWindow == false
 					&& e.AfterContentType.IsOfType(Constants.CodeTypes.CSharp) == false) {
+					$"ContentType changed to {e.AfterContentType.DisplayName}".Log();
 					Dispose();
 				}
 			}
 
 			public void Dispose() {
 				if (--_Ref > 0) {
-					Debug.WriteLine($"Ref- {_Ref} {_Name}");
+					$"Ref- {_Ref} {_Name}".Log();
 					return;
 				}
 				if (Interlocked.Exchange(ref _Parser, null) != null) {
-					Debug.WriteLine($"{_Name} tagger disposed");
+					$"{_Name} tagger disposed".Log();
 					ReleaseResources();
 				}
 			}
@@ -402,7 +403,7 @@ async Task ParseAsync(Workspace workspace, Document document, ITextSnapshot snap
 					throw;
 				}
 				if (Interlocked.CompareExchange(ref _State, (int)ParserState.Completed, (int)ParserState.Working) == (int)ParserState.Working) {
-					Debug.WriteLine($"{snapshot.TextBuffer.GetDocument().GetDocId()} end parsing {snapshot.Version} on thread {Thread.CurrentThread.ManagedThreadId}");
+					$"{snapshot.TextBuffer.GetDocument().GetDocId()} end parsing {snapshot.Version} on thread {Thread.CurrentThread.ManagedThreadId}".Log();
 					_Callback(new SemanticState(workspace, model, snapshot, document));
 					Interlocked.CompareExchange(ref _State, (int)ParserState.Idle, (int)ParserState.Completed);
 				}
diff --git a/Codist/Semantics/ITextBufferParser.cs b/Codist/Semantics/ITextBufferParser.cs
index f407c929..991aa57c 100644
--- a/Codist/Semantics/ITextBufferParser.cs
+++ b/Codist/Semantics/ITextBufferParser.cs
@@ -11,6 +11,7 @@ namespace Codist
 	interface ITextBufferParser : IDisposable
 	{
 		ITextBuffer TextBuffer { get; }
+		bool IsDisposed { get; }
 
 		/// <summary>
 		/// Gets <see cref="SemanticState"/> from given <see cref="ITextSnapshot"/>. If the <paramref name="snapshot"/> is not the same as the <see cref="SemanticState.Snapshot"/> in the <paramref name="state"/>, a new parsing will be scheduled. After the parsing, the semantic state can be retrieved via the <see cref="StateUpdated"/> event.
diff --git a/Codist/Taggers/CSharpTagger.cs b/Codist/Taggers/CSharpTagger.cs
index 4a6ddde6..9ab04bc8 100644
--- a/Codist/Taggers/CSharpTagger.cs
+++ b/Codist/Taggers/CSharpTagger.cs
@@ -25,14 +25,18 @@ sealed class CSharpTagger : ITagger<IClassificationTag>, IDisposable
 
 		ConcurrentQueue<SnapshotSpan> _PendingSpans = new ConcurrentQueue<SnapshotSpan>();
 		CancellationTokenSource _RenderBreaker;
-
+		int _Ref;
 		ITextBufferParser _Parser;
 
 		public CSharpTagger(CSharpParser parser, ITextBuffer buffer) {
 			_Parser = parser.GetParser(buffer);
 			_Parser.StateUpdated += HandleParseResult;
+			Ref();
 		}
 
+		public bool Disabled { get; set; }
+		public event EventHandler<SnapshotSpanEventArgs> TagsChanged;
+
 		void HandleParseResult(object sender, EventArgs<SemanticState> result) {
 			var pendingSpans = _PendingSpans;
 			if (pendingSpans != null) {
@@ -48,9 +52,6 @@ void HandleParseResult(object sender, EventArgs<SemanticState> result) {
 			}
 		}
 
-		public bool Disabled { get; set; }
-		public event EventHandler<SnapshotSpanEventArgs> TagsChanged;
-
 		public IEnumerable<ITagSpan<IClassificationTag>> GetTags(NormalizedSnapshotSpanCollection spans) {
 			var p = _Parser;
 			if (p == null || Disabled) {
@@ -80,7 +81,14 @@ static IEnumerable<SnapshotSpan> MapToOldSpans(NormalizedSnapshotSpanCollection
 			}
 		}
 
+		public void Ref() {
+			++_Ref;
+		}
+
 		public void Dispose() {
+			if (_Ref-- > 0) {
+				return;
+			}
 			_PendingSpans = null;
 			SyncHelper.CancelAndDispose(ref _RenderBreaker, false);
 			ITextBufferParser t = _Parser;
diff --git a/Codist/Taggers/TaggerFactories.cs b/Codist/Taggers/TaggerFactories.cs
index 285d77cd..9efd6229 100644
--- a/Codist/Taggers/TaggerFactories.cs
+++ b/Codist/Taggers/TaggerFactories.cs
@@ -108,6 +108,7 @@ ITagger<IClassificationTag> GetTagger(ITextView textView, ITextBuffer buffer) {
 				return null;
 			}
 			if (_LastView == textView && _LastTextBuffer == buffer) {
+				_LastTagger.Ref();
 				return _LastTagger;
 			}