Support having a file be both a document and additional file.

src/EditorFeatures/Core/Shared/Preview/PreviewWorkspace.cs

@@ -50,7 +50,7 @@ public override void OpenDocument(DocumentId documentId, bool activate = true)
         {
             if (this.CurrentSolution.ContainsAdditionalDocument(documentId))
             {
-                OpenAdditionalDocument(documentId, activate);
+                OpenAdditionalDocument(documentId);
                 return;
             }
 
@@ -60,6 +60,11 @@ public override void OpenDocument(DocumentId documentId, bool activate = true)
             this.OnDocumentOpened(documentId, text.Container);
         }
 
+        /// <summary>
+        /// Puts the specified additional document into the open state.
+        /// </summary>
+        /// <param name="documentId">The <see cref="DocumentId"/> to open.</param>
+        /// <param name="activate">Ignored - not necessary for additional documents.</param>
         public override void OpenAdditionalDocument(DocumentId documentId, bool activate = true)
         {
             var document = this.CurrentSolution.GetAdditionalDocument(documentId);

src/VisualStudio/Core/Def/Implementation/ProjectSystem/AbstractProject.cs

@@ -39,7 +39,7 @@ internal abstract partial class AbstractProject : ForegroundThreadAffinitizedObj
         private readonly List<ProjectReference> _projectReferences = new List<ProjectReference>();
         private readonly List<VisualStudioMetadataReference> _metadataReferences = new List<VisualStudioMetadataReference>();
         private readonly Dictionary<DocumentId, IVisualStudioHostDocument> _documents = new Dictionary<DocumentId, IVisualStudioHostDocument>();
-        private readonly Dictionary<string, IVisualStudioHostDocument> _documentMonikers = new Dictionary<string, IVisualStudioHostDocument>(StringComparer.OrdinalIgnoreCase);
+        private readonly Dictionary<string, (IVisualStudioHostDocument document, int refCount)> _documentMonikers = new Dictionary<string, (IVisualStudioHostDocument, int)>(StringComparer.OrdinalIgnoreCase);
         private readonly Dictionary<string, VisualStudioAnalyzer> _analyzers = new Dictionary<string, VisualStudioAnalyzer>(StringComparer.OrdinalIgnoreCase);
         private readonly Dictionary<DocumentId, IVisualStudioHostDocument> _additionalDocuments = new Dictionary<DocumentId, IVisualStudioHostDocument>();
 
@@ -410,8 +410,9 @@ public IVisualStudioHostDocument GetCurrentDocumentFromPath(string filePath)
         {
             lock (_gate)
             {
-                _documentMonikers.TryGetValue(filePath, out var document);
-                return document;
+                return _documentMonikers.TryGetValue(filePath, out var value)
+                    ? value.document
+                    : null;
             }
         }
 
@@ -892,7 +893,7 @@ private static void OnDocumentUpdatedOnDisk(object sender, EventArgs e)
             }
         }
 
-        private static void OnAdditionalDocumentOpened(object sender, bool isCurrentContext)
+        private static void OnAdditionalDocumentOpened(object sender, bool isCurrentContextIgnored)
         {
             IVisualStudioHostDocument document = (IVisualStudioHostDocument)sender;
             AbstractProject project = (AbstractProject)document.Project;
@@ -901,7 +902,7 @@ private static void OnAdditionalDocumentOpened(object sender, bool isCurrentCont
 
             if (project._pushingChangesToWorkspaceHosts)
             {
-                project.ProjectTracker.NotifyWorkspaceHosts(host => host.OnAdditionalDocumentOpened(document.Id, document.GetOpenTextBuffer(), isCurrentContext));
+                project.ProjectTracker.NotifyWorkspaceHosts(host => host.OnAdditionalDocumentOpened(document.Id, document.GetOpenTextBuffer()));
             }
             else
             {
@@ -951,6 +952,7 @@ protected void AddFile(
                 filePath: filename,
                 sourceCodeKind: sourceCodeKind,
                 getFolderNames: getFolderNames,
+                isAdditionalFile: false,
                 canUseTextBuffer: CanUseTextBuffer,
                 updatedOnDiskHandler: s_documentUpdatedOnDiskEventHandler,
                 openedHandler: s_documentOpenedEventHandler,
@@ -1014,7 +1016,7 @@ internal void AddDocument(IVisualStudioHostDocument document, bool isCurrentCont
                 lock (_gate)
                 {
                     _documents.Add(document.Id, document);
-                    _documentMonikers.Add(document.Key.Moniker, document);
+                    AddMoniker(document);
                 }
 
                 if (_pushingChangesToWorkspaceHosts)
@@ -1053,22 +1055,22 @@ internal void RemoveDocument(IVisualStudioHostDocument document)
                 lock (_gate)
                 {
                     _documents.Remove(document.Id);
-                    _documentMonikers.Remove(document.Key.Moniker);
+                    RemoveMoniker(document);
                 }
 
                 UninitializeDocument(document);
                 OnDocumentRemoved(document.Key.Moniker);
             }
         }
 
-        internal void AddAdditionalDocument(IVisualStudioHostDocument document, bool isCurrentContext)
+        internal void AddAdditionalDocument(IVisualStudioHostDocument document)
         {
             AssertIsForeground();
 
             lock (_gate)
             {
                 _additionalDocuments.Add(document.Id, document);
-                _documentMonikers.Add(document.Key.Moniker, document);
+                AddMoniker(document);
             }
 
             if (_pushingChangesToWorkspaceHosts)
@@ -1077,7 +1079,7 @@ internal void AddAdditionalDocument(IVisualStudioHostDocument document, bool isC
 
                 if (document.IsOpen)
                 {
-                    this.ProjectTracker.NotifyWorkspaceHosts(host => host.OnAdditionalDocumentOpened(document.Id, document.GetOpenTextBuffer(), isCurrentContext));
+                    this.ProjectTracker.NotifyWorkspaceHosts(host => host.OnAdditionalDocumentOpened(document.Id, document.GetOpenTextBuffer()));
                 }
             }
 
@@ -1096,12 +1098,48 @@ internal void RemoveAdditionalDocument(IVisualStudioHostDocument document)
             lock (_gate)
             {
                 _additionalDocuments.Remove(document.Id);
-                _documentMonikers.Remove(document.Key.Moniker);
+                RemoveMoniker(document);
             }
 
             UninitializeAdditionalDocument(document);
         }
 
+        private void AddMoniker(IVisualStudioHostDocument document)
+        {
+            var moniker = document.Key.Moniker;
+            if (_documentMonikers.TryGetValue(moniker, out var value))
+            {
+                value.refCount++;
+                _documentMonikers[moniker] = (value.document, value.refCount);
+            }
+            else
+            {
+                _documentMonikers.Add(moniker, (document, 1));
+            }
+        }
+
+        private void RemoveMoniker(IVisualStudioHostDocument document)
+        {
+            var moniker = document.Key.Moniker;
+            if (_documentMonikers.TryGetValue(moniker, out var value))
+            {
+                Debug.Assert(value.document.Equals(document));
+                value.refCount--;
+                if (value.refCount == 0)
+                {
+                    _documentMonikers.Remove(moniker);
+                }
+                else
+                {
+                    _documentMonikers[moniker] = (value.document, value.refCount);
+                }
+            }
+            else
+            {
+                Debug.Fail($"Couldn't find '{moniker}' in {nameof(_documentMonikers)} to remove it.");
+            }
+        }
+
         public virtual void Disconnect()
         {
             AssertIsForeground();

src/VisualStudio/Core/Def/Implementation/ProjectSystem/AbstractProject_Analyzers.cs

@@ -137,6 +137,7 @@ public void AddAdditionalFile(string additionalFilePath, Func<IVisualStudioHostD
                 sourceCodeKind: SourceCodeKind.Regular,
                 getFolderNames: _ => SpecializedCollections.EmptyReadOnlyList<string>(),
                 canUseTextBuffer: _ => true,
+                isAdditionalFile: true,
                 updatedOnDiskHandler: s_additionalDocumentUpdatedOnDiskEventHandler,
                 openedHandler: s_additionalDocumentOpenedEventHandler,
                 closingHandler: s_additionalDocumentClosingEventHandler);
@@ -146,7 +147,7 @@ public void AddAdditionalFile(string additionalFilePath, Func<IVisualStudioHostD
                 return;
             }
 
-            AddAdditionalDocument(document, isCurrentContext: getIsInCurrentContext(document));
+            AddAdditionalDocument(document);
         }
 
         public void RemoveAdditionalFile(string additionalFilePath)

src/VisualStudio/Core/Def/Implementation/ProjectSystem/DocumentKey.cs

@@ -1,6 +1,7 @@
 // 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;
 using Microsoft.CodeAnalysis.Text;
 using Microsoft.VisualStudio.Shell.Interop;
@@ -14,38 +15,47 @@ namespace Microsoft.VisualStudio.LanguageServices.Implementation.ProjectSystem
     /// 
     /// Immutable, since this object is used as a key into some dictionaries.
     /// </summary>
+    [DebuggerDisplay("{GetDebuggerDisplay(),nq")]
     internal class DocumentKey : IEquatable<DocumentKey>
     {
-        private readonly IVisualStudioHostProject _hostProject;
-        private readonly string _moniker;
+        public IVisualStudioHostProject HostProject { get; }
+        public string Moniker { get; }
+        public bool IsAdditionalFile { get; }
 
-        public IVisualStudioHostProject HostProject { get { return _hostProject; } }
-        public string Moniker { get { return _moniker; } }
-
-        public DocumentKey(IVisualStudioHostProject hostProject, string moniker)
+        public DocumentKey(IVisualStudioHostProject hostProject, string moniker, bool isAdditionalFile)
         {
             Contract.ThrowIfNull(hostProject);
             Contract.ThrowIfNull(moniker);
 
-            _hostProject = hostProject;
-            _moniker = moniker;
+            HostProject = hostProject;
+            Moniker = moniker;
+            IsAdditionalFile = isAdditionalFile;
         }
 
         public bool Equals(DocumentKey other)
         {
             return other != null &&
                 HostProject == other.HostProject &&
-                Moniker.Equals(other.Moniker, StringComparison.OrdinalIgnoreCase);
+                Moniker.Equals(other.Moniker, StringComparison.OrdinalIgnoreCase) &&
+                IsAdditionalFile.Equals(other.IsAdditionalFile);
         }
 
         public override int GetHashCode()
         {
-            return Hash.Combine(HostProject.GetHashCode(), StringComparer.OrdinalIgnoreCase.GetHashCode(Moniker));
+            return Hash.Combine(HostProject.GetHashCode(),
+                Hash.Combine(
+                    StringComparer.OrdinalIgnoreCase.GetHashCode(Moniker),
+                    IsAdditionalFile.GetHashCode()));
         }
 
         public override bool Equals(object obj)
         {
             return this.Equals(obj as DocumentKey);
         }
+
+        private string GetDebuggerDisplay()
+        {
+            return $"{Moniker} (additional: {IsAdditionalFile})";
+        }
     }
 }