Insert traces for cases..

.. when completion can not have a session.

src/EditorFeatures/Core/Implementation/IntelliSense/Completion/Controller.cs

@@ -3,6 +3,7 @@
 using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
+using System.Diagnostics;
 using Microsoft.CodeAnalysis.Completion;
 using Microsoft.CodeAnalysis.Editor.Commands;
 using Microsoft.CodeAnalysis.Editor.Options;
@@ -146,6 +147,8 @@ private bool StartNewModelComputation(ICompletionService completionService, Comp
 
             if (this.TextView.Selection.Mode == TextSelectionMode.Box)
             {
+                Trace.WriteLine("Box selection, cannot have completion");
+
                 // No completion with multiple selection
                 return false;
             }
@@ -154,6 +157,8 @@ private bool StartNewModelComputation(ICompletionService completionService, Comp
             var caret = TextView.GetCaretPoint(SubjectBuffer);
             if (!caret.HasValue)
             {
+                Trace.WriteLine("Caret is not mappable to subject buffer, cannot have completion");
+
                 return false;
             }
 
@@ -197,6 +202,7 @@ private ICompletionService GetCompletionService()
             Workspace workspace;
             if (!Workspace.TryGetWorkspace(this.SubjectBuffer.AsTextContainer(), out workspace))
             {
+                Trace.WriteLine("Failed to get a workspace, cannot have a completion session.");
                 return null;
             }
 

src/EditorFeatures/Core/Implementation/IntelliSense/Completion/Controller_InvokeCompletionList.cs

 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
 using System;
+using System.Diagnostics;
 using Microsoft.CodeAnalysis.Editor.Commands;
 
 namespace Microsoft.CodeAnalysis.Editor.Implementation.IntelliSense.Completion
@@ -31,6 +32,7 @@ void ICommandHandler<InvokeCompletionListCommandArgs>.ExecuteCommand(InvokeCompl
             var completionService = this.GetCompletionService();
             if (completionService == null)
             {
+                Trace.WriteLine("Failed to get completion service, cannot have a completion session.");
                 return;
             }
 

src/EditorFeatures/Core/Implementation/IntelliSense/Completion/Controller_TypeChar.cs

@@ -2,6 +2,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.Diagnostics;
 using System.Linq;
 using Microsoft.CodeAnalysis.Completion;
 using Microsoft.CodeAnalysis.Editor.Commands;
@@ -27,6 +28,8 @@ CommandState ICommandHandler<TypeCharCommandArgs>.GetCommandState(TypeCharComman
 
         void ICommandHandler<TypeCharCommandArgs>.ExecuteCommand(TypeCharCommandArgs args, Action nextHandler)
         {
+            Trace.WriteLine("Entered completion command handler for typechar.");
+
             AssertIsForeground();
 
             var initialCaretPosition = GetCaretPointInViewBuffer();
@@ -71,6 +74,8 @@ void ICommandHandler<TypeCharCommandArgs>.ExecuteCommand(TypeCharCommandArgs arg
             // to proceed. 
             if (this.TextView.TypeCharWasHandledStrangely(this.SubjectBuffer, args.TypedChar))
             {
+                Trace.WriteLine("typechar was handled by someone else, cannot have a completion session.");
+
                 if (sessionOpt != null)
                 {
                     // If we're on a seam (razor) with a computation, and the user types a character 
@@ -79,6 +84,8 @@ void ICommandHandler<TypeCharCommandArgs>.ExecuteCommand(TypeCharCommandArgs arg
                     // since the caret is no longer in our buffer.
                     if (isOnSeam && this.IsCommitCharacter(args.TypedChar))
                     {
+                        Trace.WriteLine("typechar was on seam and a commit char, cannot have a completion session.");
+
                         this.CommitOnTypeChar(args.TypedChar);
                         return;
                     }
@@ -86,6 +93,8 @@ void ICommandHandler<TypeCharCommandArgs>.ExecuteCommand(TypeCharCommandArgs arg
                              this.SubjectBuffer.GetOption(InternalFeatureOnOffOptions.AutomaticPairCompletion) &&
                              this.IsCommitCharacter(args.TypedChar))
                     {
+                        Trace.WriteLine("typechar was brace completion char and a commit char, cannot have a completion session.");
+
                         // I don't think there is any better way than this. if typed char is one of auto brace completion char,
                         // we don't do multiple buffer change check
                         this.CommitOnTypeChar(args.TypedChar);
@@ -93,6 +102,8 @@ void ICommandHandler<TypeCharCommandArgs>.ExecuteCommand(TypeCharCommandArgs arg
                     }
                     else
                     {
+                        Trace.WriteLine("we stop model computation, cannot have a completion session.");
+
                         // If we were computing anything, we stop.  We only want to process a typechar
                         // if it was a normal character.
                         this.StopModelComputation();
@@ -105,6 +116,8 @@ void ICommandHandler<TypeCharCommandArgs>.ExecuteCommand(TypeCharCommandArgs arg
             var completionService = this.GetCompletionService();
             if (completionService == null)
             {
+                Trace.WriteLine("handling typechar, completion service is null, cannot have a completion session.");
+
                 return;
             }
 
@@ -122,6 +135,8 @@ void ICommandHandler<TypeCharCommandArgs>.ExecuteCommand(TypeCharCommandArgs arg
                 // computation and start computing the model in the background.
                 if (isTextuallyTriggered)
                 {
+                    Trace.WriteLine("no completion session yet and this is a trigger char, starting model computation.");
+
                     // First create the session that represents that we now have a potential
                     // completion list.  Then tell it to start computing.
                     StartNewModelComputation(completionService, triggerInfo, filterItems: true);
@@ -129,12 +144,16 @@ void ICommandHandler<TypeCharCommandArgs>.ExecuteCommand(TypeCharCommandArgs arg
                 }
                 else
                 {
+                    Trace.WriteLine("no completion session yet and this is NOT a trigger char, we won't have completion.");
+
                     // No need to do anything.  Just stay in the state where we have no session.
                     return;
                 }
             }
             else
             {
+                Trace.WriteLine("we have a completion session.");
+
                 sessionOpt.UpdateModelTrackingSpan(initialCaretPosition);
 
                 // If the session is up, it may be in one of many states.  It may know nothing
@@ -147,6 +166,8 @@ void ICommandHandler<TypeCharCommandArgs>.ExecuteCommand(TypeCharCommandArgs arg
                 {
                     if (isTextuallyTriggered)
                     {
+                        Trace.WriteLine("computing completion again and filtering...");
+
                         // The character typed was something like "a".  It can both filter a list if
                         // we have computed one, or it can trigger a new list.  Ask the computation
                         // to compute again. If nothing has been computed, then it will try to
@@ -177,6 +198,8 @@ void ICommandHandler<TypeCharCommandArgs>.ExecuteCommand(TypeCharCommandArgs arg
                     // we have computed the list of completions.
                     if (this.IsFilterCharacter(args.TypedChar))
                     {
+                        Trace.WriteLine("filtering the session...");
+
                         // Known to be a filter character for the currently selected item.  So just 
                         // filter the session.
                         sessionOpt.FilterModel(CompletionFilterReason.TypeChar);
@@ -190,12 +213,16 @@ void ICommandHandler<TypeCharCommandArgs>.ExecuteCommand(TypeCharCommandArgs arg
                     // Now, commit if it was a commit character.
                     if (this.IsCommitCharacter(args.TypedChar))
                     {
+                        Trace.WriteLine("committing the session...");
+
                         // Known to be a commit character for the currently selected item.  So just
                         // commit the session.
                         this.CommitOnTypeChar(args.TypedChar);
                     }
                     else
                     {
+                        Trace.WriteLine("dismissing the session...");
+
                         // Now dismiss the session.
                         this.StopModelComputation();
                     }
@@ -205,6 +232,8 @@ void ICommandHandler<TypeCharCommandArgs>.ExecuteCommand(TypeCharCommandArgs arg
 
                     if (isTextuallyTriggered)
                     {
+                        Trace.WriteLine("the char commit/dismiss -ed a session and is trigerring completion again. starting model computation.");
+
                         // First create the session that represents that we now have a potential
                         // completion list.
                         StartNewModelComputation(completionService, triggerInfo, filterItems: true);