added ability to clear all diagnostics reproted from a IDiagnosticUpd…

src/EditorFeatures/Core/Implementation/EditAndContinue/EditAndContinueDiagnosticUpdateSource.cs

@@ -40,6 +40,7 @@ public EditAndContinueDiagnosticUpdateSource(IDiagnosticUpdateSourceRegistration
         public bool SupportGetDiagnostics => false;
 
         public event EventHandler<DiagnosticsUpdatedArgs> DiagnosticsUpdated;
+        public event EventHandler Cleared { add { } remove { } }
 
         public ImmutableArray<DiagnosticData> GetDiagnostics(Workspace workspace, ProjectId projectId, DocumentId documentId, object id, bool includeSuppressedDiagnostics = false, CancellationToken cancellationToken = default)
         {

src/EditorFeatures/Test/Diagnostics/DiagnosticServiceTests.cs

@@ -90,6 +90,68 @@ public void TestGetDiagnostics2()
             }
         }
 
+        [Fact, Trait(Traits.Feature, Traits.Features.Diagnostics)]
+        public void TestCleared()
+        {
+            using (var workspace = new TestWorkspace(TestExportProvider.ExportProviderWithCSharpAndVisualBasic))
+            {
+                var set = new ManualResetEvent(false);
+                var document = workspace.CurrentSolution.AddProject("TestProject", "TestProject", LanguageNames.CSharp).AddDocument("TestDocument", string.Empty);
+                var document2 = document.Project.AddDocument("TestDocument2", string.Empty);
+
+                var diagnosticService = new DiagnosticService(AsynchronousOperationListenerProvider.NullProvider);
+
+                var source1 = new TestDiagnosticUpdateSource(support: false, diagnosticData: null);
+                diagnosticService.Register(source1);
+
+                var source2 = new TestDiagnosticUpdateSource(support: false, diagnosticData: null);
+                diagnosticService.Register(source2);
+
+                diagnosticService.DiagnosticsUpdated += MarkSet;
+
+                // add bunch of data to the service for both sources
+                RaiseDiagnosticEvent(set, source1, workspace, document.Project.Id, document.Id, Tuple.Create(workspace, document));
+                RaiseDiagnosticEvent(set, source1, workspace, document.Project.Id, document.Id, Tuple.Create(workspace, document.Project, document));
+                RaiseDiagnosticEvent(set, source1, workspace, document2.Project.Id, document2.Id, Tuple.Create(workspace, document2));
+
+                RaiseDiagnosticEvent(set, source2, workspace, document.Project.Id, null, Tuple.Create(workspace, document.Project));
+                RaiseDiagnosticEvent(set, source2, workspace, null, null, Tuple.Create(workspace));
+
+                // confirm data is there.
+                var data1 = diagnosticService.GetDiagnostics(workspace, null, null, null, false, CancellationToken.None);
+                Assert.Equal(5, data1.Count());
+
+                diagnosticService.DiagnosticsUpdated -= MarkSet;
+
+                // confirm clear for a source
+                set.Reset();
+                var count = 0;
+                diagnosticService.DiagnosticsUpdated += MarkCalled;
+
+                source1.RaiseClearedEvent();
+
+                set.WaitOne();
+
+                // confirm there are 2 data left
+                var data2 = diagnosticService.GetDiagnostics(workspace, null, null, null, false, CancellationToken.None);
+                Assert.Equal(2, data2.Count());
+
+                void MarkCalled(object sender, DiagnosticsUpdatedArgs args)
+                {
+                    // event is serialized. no concurrent call
+                    if (++count == 3)
+                    {
+                        set.Set();
+                    }
+                }
+
+                void MarkSet(object sender, DiagnosticsUpdatedArgs args)
+                {
+                    set.Set();
+                }
+            }
+        }
+
         private static DiagnosticData RaiseDiagnosticEvent(ManualResetEvent set, TestDiagnosticUpdateSource source, TestWorkspace workspace, ProjectId project, DocumentId document, object id)
         {
             set.Reset();
@@ -126,6 +188,7 @@ public TestDiagnosticUpdateSource(bool support, DiagnosticData[] diagnosticData)
 
             public bool SupportGetDiagnostics { get { return _support; } }
             public event EventHandler<DiagnosticsUpdatedArgs> DiagnosticsUpdated;
+            public event EventHandler Cleared;
 
             public ImmutableArray<DiagnosticData> GetDiagnostics(Workspace workspace, ProjectId projectId, DocumentId documentId, object id, bool includeSuppressedDiagnostics = false, CancellationToken cancellationToken = default)
             {
@@ -136,6 +199,11 @@ public void RaiseUpdateEvent(DiagnosticsUpdatedArgs args)
             {
                 DiagnosticsUpdated?.Invoke(this, args);
             }
+
+            public void RaiseClearedEvent()
+            {
+                Cleared?.Invoke(this, EventArgs.Empty);
+            }
         }
     }
 }

src/EditorFeatures/TestUtilities/Squiggles/AbstractSquiggleProducerTests.cs

@@ -89,6 +89,7 @@ public void RaiseDiagnosticsUpdated(DiagnosticsUpdatedArgs args)
             }
 
             public event EventHandler<DiagnosticsUpdatedArgs> DiagnosticsUpdated;
+            public event EventHandler Cleared { add { } remove { } }
 
             public bool SupportGetDiagnostics => false;
 

src/Features/Core/Portable/Diagnostics/AbstractHostDiagnosticUpdateSource.cs

@@ -28,6 +28,7 @@ public ImmutableArray<DiagnosticData> GetDiagnostics(Workspace workspace, Projec
         }
 
         public event EventHandler<DiagnosticsUpdatedArgs> DiagnosticsUpdated;
+        public event EventHandler Cleared { add { } remove { } }
 
         public void RaiseDiagnosticsUpdated(DiagnosticsUpdatedArgs args)
         {

src/Features/Core/Portable/Diagnostics/DefaultDiagnosticAnalyzerService.cs

@@ -40,6 +40,7 @@ public IIncrementalAnalyzer CreateIncrementalAnalyzer(Workspace workspace)
         }
 
         public event EventHandler<DiagnosticsUpdatedArgs> DiagnosticsUpdated;
+        public event EventHandler Cleared { add { } remove { } }
 
         // this only support push model, pull model will be provided by DiagnosticService by caching everything this one pushed
         public bool SupportGetDiagnostics => false;

src/Features/Core/Portable/Diagnostics/DiagnosticAnalyzerService_UpdateSource.cs

@@ -43,6 +43,19 @@ public event EventHandler<DiagnosticsUpdatedArgs> DiagnosticsUpdated
             }
         }
 
+        public event EventHandler Cleared
+        {
+            add
+            {
+                // don't do anything. this update source doesn't use cleared event
+            }
+
+            remove
+            {
+                // don't do anything. this update source doesn't use cleared event
+            }
+        }
+
         internal void RaiseDiagnosticsUpdated(DiagnosticsUpdatedArgs args)
         {
             // all diagnostics events are serialized.

src/Features/Core/Portable/Diagnostics/DiagnosticService.cs

@@ -79,6 +79,37 @@ private void RaiseDiagnosticsUpdated(object sender, DiagnosticsUpdatedArgs args)
             }).CompletesAsyncOperation(eventToken);
         }
 
+        private void RaiseDiagnosticsCleared(object sender)
+        {
+            Contract.ThrowIfNull(sender);
+            var source = (IDiagnosticUpdateSource)sender;
+
+            var ev = _eventMap.GetEventHandlers<EventHandler<DiagnosticsUpdatedArgs>>(DiagnosticsUpdatedEventName);
+            if (!RequireRunningEventTasks(source, ev))
+            {
+                return;
+            }
+
+            var eventToken = _listener.BeginAsyncOperation(DiagnosticsUpdatedEventName);
+            _eventQueue.ScheduleTask(() =>
+            {
+                using (var pooledObject = SharedPools.Default<List<DiagnosticsUpdatedArgs>>().GetPooledObject())
+                {
+                    var removed = pooledObject.Object;
+                    if (!ClearDiagnosticsFromDataMapFor(source, removed))
+                    {
+                        // there is no change, nothing to raise events for.
+                        return;
+                    }
+
+                    foreach (var args in removed)
+                    {
+                        ev.RaiseEvent(handler => handler(sender, args));
+                    }
+                }
+            }).CompletesAsyncOperation(eventToken);
+        }
+
         private bool RequireRunningEventTasks(
             IDiagnosticUpdateSource source, EventMap.EventHandlerSet<EventHandler<DiagnosticsUpdatedArgs>> ev)
         {
@@ -150,12 +181,48 @@ private bool UpdateDataMap(IDiagnosticUpdateSource source, DiagnosticsUpdatedArg
             }
         }
 
+        private bool ClearDiagnosticsFromDataMapFor(IDiagnosticUpdateSource source, List<DiagnosticsUpdatedArgs> removed)
+        {
+            // we expect source who uses this ability to have small number of diagnostics.
+            lock (_gate)
+            {
+                Debug.Assert(_updateSources.Contains(source));
+
+                // 2 different workspaces (ex, PreviewWorkspaces) can return same Args.Id, we need to
+                // distinguish them. so we separate diagnostics per workspace map.
+                if (!_map.TryGetValue(source, out var workspaceMap))
+                {
+                    return false;
+                }
+
+                foreach (var (workspace, map) in workspaceMap)
+                {
+                    foreach (var (id, data) in map)
+                    {
+                        removed.Add(DiagnosticsUpdatedArgs.DiagnosticsRemoved(id, data.Workspace, solution: null, data.ProjectId, data.DocumentId));
+                    }
+                }
+
+                // all diagnostics from the source is cleared
+                _map.Remove(source);
+                return true;
+            }
+        }
+
         private void OnDiagnosticsUpdated(object sender, DiagnosticsUpdatedArgs e)
         {
             AssertIfNull(e.Diagnostics);
+
+            // all events are serialized by async event handler
             RaiseDiagnosticsUpdated(sender, e);
         }
 
+        private void OnCleared(object sender, EventArgs e)
+        {
+            // all events are serialized by async event handler
+            RaiseDiagnosticsCleared(sender);
+        }
+
         public IEnumerable<DiagnosticData> GetDiagnostics(
             Workspace workspace, ProjectId projectId, DocumentId documentId, object id, bool includeSuppressedDiagnostics, CancellationToken cancellationToken)
         {

src/Features/Core/Portable/Diagnostics/DiagnosticService_UpdateSourceRegistrationService.cs

@@ -28,7 +28,9 @@ public void Register(IDiagnosticUpdateSource source)
                 }
 
                 _updateSources = _updateSources.Add(source);
+
                 source.DiagnosticsUpdated += OnDiagnosticsUpdated;
+                source.Cleared += OnCleared;
             }
         }
     }

src/Features/Core/Portable/Diagnostics/IDiagnosticUpdateSource.cs

@@ -16,6 +16,11 @@ internal interface IDiagnosticUpdateSource
         /// </summary>
         event EventHandler<DiagnosticsUpdatedArgs> DiagnosticsUpdated;
 
+        /// <summary>
+        /// Raise this when all diagnostics reported from this update source has cleared
+        /// </summary>
+        event EventHandler Cleared;
+
         /// <summary>
         /// Return true if the source supports GetDiagnostics API otherwise, return false so that the engine can cache data from DiagnosticsUpdated in memory
         /// </summary>

src/VisualStudio/Core/Def/Implementation/TaskList/ExternalErrorDiagnosticUpdateSource.cs

@@ -76,6 +76,7 @@ internal ExternalErrorDiagnosticUpdateSource(
 
         public event EventHandler<bool> BuildStarted;
         public event EventHandler<DiagnosticsUpdatedArgs> DiagnosticsUpdated;
+        public event EventHandler Cleared { add { } remove { } }
 
         public bool IsInProgress => BuildInprogressState != null;
 

src/VisualStudio/Core/Test/Diagnostics/ExternalDiagnosticUpdateSourceTests.vb

@@ -346,6 +346,7 @@ Namespace Microsoft.VisualStudio.LanguageServices.UnitTests.Diagnostics
             End Property
 
             Public Event DiagnosticsUpdated As EventHandler(Of DiagnosticsUpdatedArgs) Implements IDiagnosticUpdateSource.DiagnosticsUpdated
+            Public Event Cleared As EventHandler Implements IDiagnosticUpdateSource.Cleared
 
             Public Function GetDiagnostics(workspace As Microsoft.CodeAnalysis.Workspace, projectId As ProjectId, documentId As DocumentId, id As Object, includeSuppressedDiagnostics As Boolean, cancellationToken As CancellationToken) As ImmutableArray(Of DiagnosticData) Implements IDiagnosticUpdateSource.GetDiagnostics
                 Return If(includeSuppressedDiagnostics, _data, _data.WhereAsArray(Function(d) Not d.IsSuppressed))

src/VisualStudio/Core/Test/EditAndContinue/EditAndContinueTestHelper.vb

@@ -43,6 +43,7 @@ Namespace Microsoft.VisualStudio.LanguageServices.UnitTests.EditAndContinue
             End Property
 
             Public Event DiagnosticsUpdated As EventHandler(Of DiagnosticsUpdatedArgs) Implements IDiagnosticUpdateSource.DiagnosticsUpdated
+            Public Event Cleared As EventHandler Implements IDiagnosticUpdateSource.Cleared
 
             Public Function GetDiagnostics(workspace As Microsoft.CodeAnalysis.Workspace, projectId As ProjectId, documentId As DocumentId, id As Object, includeSuppressedDiagnostics As Boolean, cancellationToken As CancellationToken) As ImmutableArray(Of DiagnosticData) Implements IDiagnosticUpdateSource.GetDiagnostics
                 Return If(includeSuppressedDiagnostics, _data, _data.WhereAsArray(Function(d) Not d.IsSuppressed))

src/Workspaces/Core/Portable/Diagnostics/DiagnosticData.cs

@@ -444,7 +444,6 @@ private static ImmutableDictionary<string, string> GetProperties(
             return additionalProperties == null
                 ? properties
                 : properties.AddRange(additionalProperties);
-            throw new NotImplementedException();
         }
 
         /// <summary>