Make immediate window buffer writable before applying completion edit

src/EditorFeatures/Core/IDebuggerTextView.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 Microsoft.VisualStudio.Language.Intellisense;
 
 namespace Microsoft.CodeAnalysis.Editor
@@ -9,5 +10,8 @@ internal interface IDebuggerTextView
         bool IsImmediateWindow { get; }
 
         void HACK_StartCompletionSession(IIntellisenseSession editorSessionOpt);
+
+        uint StartBufferUpdate();
+        void EndBufferUpdate(uint cookie);
     }
 }

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

@@ -114,6 +114,16 @@ private CompletionProvider GetCompletionProvider(CompletionItem item)
                     var adjustedNewText = AdjustForVirtualSpace(textChange);
                     var editOptions = GetEditOptions(mappedSpan, adjustedNewText);
 
+                    // The immediate window is always marked read-only and the language service is
+                    // responsible for asking the buffer to make itself writable. We'll have to do that for
+                    // commit, so we need to drag the IVsTextLines around, too.
+                    // We have to ask the buffer to make itself writable, if it isn't already
+                    uint immediateWindowBufferUpdateCookie = 0;
+                    if (_isImmediateWindow)
+                    {
+                        immediateWindowBufferUpdateCookie = ((IDebuggerTextView)TextView).StartBufferUpdate();
+                    }
+
                     // Now actually make the text change to the document.
                     using (var textEdit = this.SubjectBuffer.CreateEdit(editOptions, reiteratedVersionNumber: null, editTag: null))
                     {
@@ -121,6 +131,11 @@ private CompletionProvider GetCompletionProvider(CompletionItem item)
                         textEdit.ApplyAndLogExceptions();
                     }
 
+                    if (_isImmediateWindow)
+                    {
+                        ((IDebuggerTextView)TextView).EndBufferUpdate(immediateWindowBufferUpdateCookie);
+                    }
+
                     // If the completion change requested a new position for the caret to go,
                     // then set the caret to go directly to that point.
                     if (completionChange.NewPosition.HasValue)

src/VisualStudio/Core/Def/Implementation/DebuggerIntelliSense/AbstractDebuggerIntelliSenseContext.cs

@@ -195,7 +195,7 @@ internal bool TryInitialize()
 
             var bufferGraph = _bufferGraphFactoryService.CreateBufferGraph(_projectionBuffer);
 
-            _debuggerTextView = new DebuggerTextView(_textView, bufferGraph, this.InImmediateWindow);
+            _debuggerTextView = new DebuggerTextView(_textView, bufferGraph, _debuggerTextLines, InImmediateWindow);
             return true;
         }
 

src/VisualStudio/Core/Def/Implementation/DebuggerIntelliSense/DebuggerTextView.cs

@@ -12,6 +12,7 @@
 using Microsoft.VisualStudio.Text.Formatting;
 using Microsoft.VisualStudio.Text.Outlining;
 using Microsoft.VisualStudio.Text.Projection;
+using Microsoft.VisualStudio.TextManager.Interop;
 using Microsoft.VisualStudio.Utilities;
 using Roslyn.Utilities;
 
@@ -23,15 +24,18 @@ internal partial class DebuggerTextView : IWpfTextView, IDebuggerTextView
         /// The actual debugger view of the watch or immediate window that we're wrapping
         /// </summary>
         private readonly IWpfTextView _innerTextView;
+        private readonly IVsTextLines _debuggerTextLines;
 
         public DebuggerTextView(
             IWpfTextView innerTextView,
             IBufferGraph bufferGraph,
+            IVsTextLines debuggerTextLines,
             bool isImmediateWindow)
         {
             _innerTextView = innerTextView;
-            this.BufferGraph = bufferGraph;
-            this.IsImmediateWindow = isImmediateWindow;
+            _debuggerTextLines = debuggerTextLines;
+            BufferGraph = bufferGraph;
+            IsImmediateWindow = isImmediateWindow;
         }
 
         /// <summary>
@@ -60,6 +64,19 @@ public bool IsImmediateWindow
             get;
         }
 
+        public uint StartBufferUpdate()
+        {
+            _debuggerTextLines.GetStateFlags(out var bufferFlags);
+            _debuggerTextLines.SetStateFlags((uint)((BUFFERSTATEFLAGS)bufferFlags & ~BUFFERSTATEFLAGS.BSF_USER_READONLY));
+
+            return bufferFlags;
+        }
+
+        public void EndBufferUpdate(uint cookie)
+        {
+            _debuggerTextLines.SetStateFlags(cookie);
+        }
+
         public ITextCaret Caret
         {
             get { return _innerTextView.Caret; }