For a shared item that is a contained document, use the correct itemId and...

For a shared item that is a contained document, use the correct itemId and hierarchy to get a document moniker from running document table.

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

@@ -152,5 +152,23 @@ private string GetActiveIntellisenseProjectContextInternal(IVsHierarchy hierarch
                 ? intellisenseProjectName as string
                 : null;
         }
+
+        public static bool TryGetItemIdInSharedHierarchy(IVsHierarchy hierarchy, uint itemId, IVsHierarchy sharedHierarchy, out uint itemIdInSharedHierarchy)
+        {
+            string fullPath;
+            int found;
+            VSDOCUMENTPRIORITY[] priority = new VSDOCUMENTPRIORITY[1];
+
+            if (ErrorHandler.Succeeded(((IVsProject)hierarchy).GetMkDocument(itemId, out fullPath))
+                && ErrorHandler.Succeeded(((IVsProject)sharedHierarchy).IsDocumentInProject(fullPath, out found, priority, out itemIdInSharedHierarchy))
+                && found != 0
+                && itemIdInSharedHierarchy != (uint)VSConstants.VSITEMID.Nil)
+            {
+                return true;
+            }
+
+            itemIdInSharedHierarchy = (uint)VSConstants.VSITEMID.Nil;
+            return false;
+        }
     }
 }

src/VisualStudio/Core/Def/Implementation/Utilities/VsHierarchyExtensions.cs

@@ -2,7 +2,6 @@
 
 using System;
 using System.Runtime.InteropServices;
-using Microsoft.VisualStudio;
 using Microsoft.VisualStudio.Shell.Interop;
 
 namespace Microsoft.VisualStudio.LanguageServices.Implementation.Utilities
@@ -53,6 +52,11 @@ public static string GetMonikerForHierarchyAndItemId(this IVsRunningDocumentTabl
 
             // Uh, OK, that's probably not good that we're supposedly an open file but not in the RDT. We'll fall back
             // to the IVsHierarchy's name property for this item.
+            return GetDocumentNameForHierarchyAndItemId(hierarchy, itemid);
+        }
+
+        public static string GetDocumentNameForHierarchyAndItemId(this IVsHierarchy hierarchy, uint itemid)
+        {
             object property;
             Marshal.ThrowExceptionForHR(hierarchy.GetProperty(itemid, (int)__VSHPROPID.VSHPROPID_Name, out property));
             return (string)property;

src/VisualStudio/Core/Def/Implementation/Venus/ContainedDocument.cs

@@ -98,10 +98,38 @@ internal sealed class ContainedDocument : ForegroundThreadAffinitizedObject, IVi
             _optionService = _workspace.Services.GetService<IOptionService>();
             _hostType = GetHostType();
 
+            this.SharedHierarchy = hierarchy == null ? null : LinkedFileUtilities.GetSharedHierarchyForItem(hierarchy, itemId);
+
             var rdt = (IVsRunningDocumentTable)componentModel.GetService<SVsServiceProvider>().GetService(typeof(SVsRunningDocumentTable));
-            var filePath = rdt.GetMonikerForHierarchyAndItemId(hierarchy, itemId);
 
-            this.SharedHierarchy = hierarchy == null ? null : LinkedFileUtilities.GetSharedHierarchyForItem(hierarchy, itemId);
+            string filePath = null;
+            if (this.SharedHierarchy == null)
+            {
+                // this is not a shared item, look up the hierarchy/itemid pair in the RDT
+                filePath = rdt.GetMonikerForHierarchyAndItemId(hierarchy, itemId);
+            }
+            else
+            {
+                // this *is* a shared item, map from itemId to shared itemId and look up the 
+                // sharedHierarchy/sharedItemId pair in the RDT.
+                uint itemIdInSharedHierarchy;
+                if (LinkedFileUtilities.TryGetItemIdInSharedHierarchy(hierarchy, itemId, this.SharedHierarchy, out itemIdInSharedHierarchy))
+                {
+                    filePath = rdt.GetMonikerForHierarchyAndItemId(this.SharedHierarchy, itemIdInSharedHierarchy);
+                }
+
+                // This is a shared item. But we either couldn't to map an itemId in its shared hierarchy or 
+                // we couldn't look up the document moniker in RDT for a shared hierarchy/item pair
+                // Since we only use this moniker as a key, we could fall back to something else, like the document name,
+                // which is the same fall back we have for a non-shared item. we use the headHierarchy/itemId pair for that.
+                if (filePath == null)
+                {
+                    Debug.Assert(false,"Could not get itemId in SharedHierarchy or could not find the document moniker for a shared item in its SharedHierarchy.");
+
+                    filePath = hierarchy.GetDocumentNameForHierarchyAndItemId(itemId);
+                }
+            }
+
 
             if (Project.Hierarchy != null)
             {