Remove UI thread filtering of code actions

Closes #46431

src/EditorFeatures/Core.Wpf/Suggestions/SuggestedActionsSource.cs

@@ -301,9 +301,9 @@ static SuggestedActionSetPriority ConvertToSuggestedActionSetPriority(UnifiedSug
                 // See https://github.com/dotnet/roslyn/issues/29589
                 const bool includeSuppressionFixes = true;
 
-                return UnifiedSuggestedActionsSource.GetFilterAndOrderCodeFixes_MustBeCalledFromUIThread(
+                return UnifiedSuggestedActionsSource.GetFilterAndOrderCodeFixesAsync(
                     workspace, _owner._codeFixService, document, range.Span.ToTextSpan(),
-                    includeSuppressionFixes, isBlocking: true, addOperationScope, cancellationToken);
+                    includeSuppressionFixes, isBlocking: true, addOperationScope, cancellationToken).WaitAndGetResult(cancellationToken);
             }
 
             private static string GetFixCategory(DiagnosticSeverity severity)
@@ -352,9 +352,9 @@ private static string GetFixCategory(DiagnosticSeverity severity)
                 // then we want to filter out refactorings outside the selection span.
                 var filterOutsideSelection = !requestedActionCategories.Contains(PredefinedSuggestedActionCategoryNames.Refactoring);
 
-                return UnifiedSuggestedActionsSource.GetFilterAndOrderCodeRefactorings_MustBeCalledFromUIThread(
+                return UnifiedSuggestedActionsSource.GetFilterAndOrderCodeRefactoringsAsync(
                     workspace, _owner._codeRefactoringService, document, selection, isBlocking: true,
-                    addOperationScope, filterOutsideSelection, cancellationToken);
+                    addOperationScope, filterOutsideSelection, cancellationToken).WaitAndGetResult(cancellationToken);
             }
 
             public Task<bool> HasSuggestedActionsAsync(

src/EditorFeatures/Core/Implementation/CodeFixes/CodeFixService.cs

@@ -743,28 +743,10 @@ private async Task<IEnumerable<Diagnostic>> GetProjectDiagnosticsAsync(Project p
             foreach (var fixer in allFixers)
             {
                 await extensionManager.PerformActionAsync(fixer, () => fixer.RegisterCodeFixesAsync(context) ?? Task.CompletedTask).ConfigureAwait(false);
-                foreach (var fix in fixes)
-                {
-                    if (!fix.Action.PerformFinalApplicabilityCheck)
-                    {
+                if (fixes.Count > 0)
                     return true;
             }
 
-                    // Have to see if this fix is still applicable.  Jump to the foreground thread
-                    // to make that check.
-                    await ThreadingContext.JoinableTaskFactory.SwitchToMainThreadAsync(alwaysYield: true, cancellationToken);
-
-                    var applicable = fix.Action.IsApplicable(document.Project.Solution.Workspace);
-
-                    await TaskScheduler.Default;
-
-                    if (applicable)
-                    {
-                        return true;
-                    }
-                }
-            }
-
             return false;
         }
 

src/EditorFeatures/Test2/Diagnostics/AddImport/AddImportCrossLanguageTests.vb

@@ -224,7 +224,7 @@ End Namespace
             Await TestAsync(input, expected, codeActionIndex:=1)
         End Function
 
-        <Fact, Trait(Traits.Feature, Traits.Features.CodeActionsAddImport)>
+        <WpfFact, Trait(Traits.Feature, Traits.Features.CodeActionsAddImport)>
         Public Async Function AddProjectReference_CSharpToCSharp_Test() As Task
             Dim input =
                 <Workspace>
@@ -305,7 +305,7 @@ namespace CSAssembly2
             Await TestMissing(input)
         End Function
 
-        <Fact, Trait(Traits.Feature, Traits.Features.CodeActionsAddImport)>
+        <WpfFact, Trait(Traits.Feature, Traits.Features.CodeActionsAddImport)>
         Public Async Function TestAddProjectReference_CSharpToCSharp_WithProjectRenamed() As Task
             Dim input =
                 <Workspace>
@@ -358,7 +358,7 @@ namespace CSAssembly2
                             End Sub)
         End Function
 
-        <Fact, Trait(Traits.Feature, Traits.Features.CodeActionsAddImport)>
+        <WpfFact, Trait(Traits.Feature, Traits.Features.CodeActionsAddImport)>
         Public Async Function TestAddProjectReference_VBToVB() As Task
             Dim input =
                 <Workspace>
@@ -449,7 +449,7 @@ class C
             Await TestMissing(input)
         End Function
 
-        <Fact, Trait(Traits.Feature, Traits.Features.CodeActionsAddImport)>
+        <WpfFact, Trait(Traits.Feature, Traits.Features.CodeActionsAddImport)>
         <WorkItem(16022, "https://github.com/dotnet/roslyn/issues/16022")>
         Public Async Function TestAddProjectReference_EvenWithExistingUsing() As Task
             Dim input =

src/EditorFeatures/TestUtilities/Workspaces/TestWorkspace.cs

@@ -9,9 +9,11 @@
 using System.Collections.Immutable;
 using System.Linq;
 using System.Threading;
+using System.Threading.Tasks;
 using System.Windows.Threading;
 using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.Editor.Shared.Extensions;
+using Microsoft.CodeAnalysis.Editor.Shared.Utilities;
 using Microsoft.CodeAnalysis.Formatting;
 using Microsoft.CodeAnalysis.Host;
 using Microsoft.CodeAnalysis.MetadataAsSource;
@@ -707,6 +709,15 @@ public void ChangeSolution(Solution solution)
         public override bool CanApplyParseOptionChange(ParseOptions oldOptions, ParseOptions newOptions, Project project)
             => true;
 
+        internal override async Task<bool> CanAddProjectReferenceAsync(ProjectId referencingProject, ProjectId referencedProject, CancellationToken cancellationToken)
+        {
+            // VisualStudioWorkspace switches to the main thread for this call, so do the same thing here to catch tests
+            // that fail to account for this possibility.
+            var threadingContext = ExportProvider.GetExportedValue<IThreadingContext>();
+            await threadingContext.JoinableTaskFactory.SwitchToMainThreadAsync(alwaysYield: true, cancellationToken);
+            return true;
+        }
+
         internal ITextBuffer GetOrCreateBufferForPath(string? filePath, IContentType contentType, string languageName, string initialText)
         {
             // If we don't have a file path we'll just make something up for the purpose of this dictionary so all

src/Features/Core/Portable/AddImport/CodeActions/AssemblyReferenceCodeAction.cs

@@ -2,7 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
-using System;
+#nullable enable
+
 using System.Collections.Generic;
 using System.Threading;
 using System.Threading.Tasks;
@@ -16,47 +17,48 @@ internal abstract partial class AbstractAddImportFeatureService<TSimpleNameSynta
     {
         private class AssemblyReferenceCodeAction : AddImportCodeAction
         {
-            private readonly Lazy<string> _lazyResolvedPath;
-
             public AssemblyReferenceCodeAction(
                 Document originalDocument,
                 AddImportFixData fixData)
                 : base(originalDocument, fixData)
             {
                 Contract.ThrowIfFalse(fixData.Kind == AddImportFixKind.ReferenceAssemblySymbol);
-                _lazyResolvedPath = new Lazy<string>(ResolvePath);
             }
 
-            private string ResolvePath()
+            private Task<string?> ResolvePathAsync(CancellationToken cancellationToken)
             {
-                var assemblyResolverService = OriginalDocument.Project.Solution.Workspace.Services.GetService<IFrameworkAssemblyPathResolver>();
+                var assemblyResolverService = OriginalDocument.Project.Solution.Workspace.Services.GetRequiredService<IFrameworkAssemblyPathResolver>();
 
-                var assemblyPath = assemblyResolverService?.ResolveAssemblyPath(
+                return assemblyResolverService.ResolveAssemblyPathAsync(
                     OriginalDocument.Project.Id,
                     FixData.AssemblyReferenceAssemblyName,
-                    FixData.AssemblyReferenceFullyQualifiedTypeName);
-
-                return assemblyPath;
+                    FixData.AssemblyReferenceFullyQualifiedTypeName,
+                    cancellationToken);
             }
 
-            internal override bool PerformFinalApplicabilityCheck
-                => true;
+            protected override Task<IEnumerable<CodeActionOperation>> ComputePreviewOperationsAsync(CancellationToken cancellationToken)
+                => ComputeOperationsAsync(isPreview: true, cancellationToken);
 
-            internal override bool IsApplicable(Workspace workspace)
-                => !string.IsNullOrWhiteSpace(_lazyResolvedPath.Value);
+            protected override Task<IEnumerable<CodeActionOperation>> ComputeOperationsAsync(CancellationToken cancellationToken)
+                => ComputeOperationsAsync(isPreview: false, cancellationToken);
 
-            protected override async Task<IEnumerable<CodeActionOperation>> ComputeOperationsAsync(CancellationToken cancellationToken)
+            private async Task<IEnumerable<CodeActionOperation>> ComputeOperationsAsync(bool isPreview, CancellationToken cancellationToken)
             {
-                var service = OriginalDocument.Project.Solution.Workspace.Services.GetService<IMetadataService>();
-                var resolvedPath = _lazyResolvedPath.Value;
-                var reference = service.GetReference(resolvedPath, MetadataReferenceProperties.Assembly);
-
                 var newDocument = await GetUpdatedDocumentAsync(cancellationToken).ConfigureAwait(false);
+                var newProject = newDocument.Project;
 
                 // Now add the actual assembly reference.
-                var newProject = newDocument.Project;
+                if (!isPreview)
+                {
+                    var resolvedPath = await ResolvePathAsync(cancellationToken).ConfigureAwait(false);
+                    if (!string.IsNullOrWhiteSpace(resolvedPath))
+                    {
+                        var service = OriginalDocument.Project.Solution.Workspace.Services.GetRequiredService<IMetadataService>();
+                        var reference = service.GetReference(resolvedPath, MetadataReferenceProperties.Assembly);
                         newProject = newProject.WithMetadataReferences(
                             newProject.MetadataReferences.Concat(reference));
+                    }
+                }
 
                 var operation = new ApplyChangesOperation(newProject.Solution);
                 return SpecializedCollections.SingletonEnumerable<CodeActionOperation>(operation);

src/Features/Core/Portable/AddImport/CodeActions/MetadataSymbolReferenceCodeAction.cs

@@ -3,6 +3,8 @@
 // See the LICENSE file in the project root for more information.
 
 using System.Linq;
+using System.Threading;
+using System.Threading.Tasks;
 using Roslyn.Utilities;
 
 namespace Microsoft.CodeAnalysis.AddImport
@@ -17,14 +19,14 @@ public MetadataSymbolReferenceCodeAction(Document originalDocument, AddImportFix
                 Contract.ThrowIfFalse(fixData.Kind == AddImportFixKind.MetadataSymbol);
             }
 
-            protected override Project UpdateProject(Project project)
+            protected override Task<Project> UpdateProjectAsync(Project project, bool isPreview, CancellationToken cancellationToken)
             {
                 var projectWithReference = project.Solution.GetProject(FixData.PortableExecutableReferenceProjectId);
                 var reference = projectWithReference.MetadataReferences
                                                     .OfType<PortableExecutableReference>()
                                                     .First(pe => pe.FilePath == FixData.PortableExecutableReferenceFilePathToAdd);
 
-                return project.AddMetadataReference(reference);
+                return Task.FromResult(project.AddMetadataReference(reference));
             }
         }
     }

src/Features/Core/Portable/AddImport/CodeActions/ProjectSymbolReferenceCodeAction.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System.Threading;
+using System.Threading.Tasks;
 using Roslyn.Utilities;
 
 namespace Microsoft.CodeAnalysis.AddImport
@@ -27,19 +29,22 @@ private class ProjectSymbolReferenceCodeAction : SymbolReferenceCodeAction
             private bool ShouldAddProjectReference()
                 => FixData.ProjectReferenceToAdd != null && FixData.ProjectReferenceToAdd != OriginalDocument.Project.Id;
 
-            internal override bool PerformFinalApplicabilityCheck
-                => ShouldAddProjectReference();
-
-            internal override bool IsApplicable(Workspace workspace)
-                => ShouldAddProjectReference() &&
-                   workspace.CanAddProjectReference(
-                    OriginalDocument.Project.Id, FixData.ProjectReferenceToAdd);
+            protected override async Task<Project> UpdateProjectAsync(Project project, bool isPreview, CancellationToken cancellationToken)
+            {
+                if (!ShouldAddProjectReference())
+                {
+                    return project;
+                }
 
-            protected override Project UpdateProject(Project project)
+                if (!isPreview)
                 {
-                return ShouldAddProjectReference()
-                    ? project.AddProjectReference(new ProjectReference(FixData.ProjectReferenceToAdd))
-                    : project;
+                    if (!await project.Solution.Workspace.CanAddProjectReferenceAsync(OriginalDocument.Project.Id, FixData.ProjectReferenceToAdd, cancellationToken).ConfigureAwait(false))
+                    {
+                        return project;
+                    }
+                }
+
+                return project.AddProjectReference(new ProjectReference(FixData.ProjectReferenceToAdd));
             }
         }
     }

src/Features/Core/Portable/AddImport/CodeActions/SymbolReference.SymbolReferenceCodeAction.cs

@@ -2,8 +2,11 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System;
+using System.Collections.Generic;
 using System.Threading;
 using System.Threading.Tasks;
+using Microsoft.CodeAnalysis.CodeActions;
 
 namespace Microsoft.CodeAnalysis.AddImport
 {
@@ -24,18 +27,34 @@ private abstract class SymbolReferenceCodeAction : AddImportCodeAction
             {
             }
 
-            protected override async Task<Solution> GetChangedSolutionAsync(CancellationToken cancellationToken)
+            protected override async Task<IEnumerable<CodeActionOperation>> ComputePreviewOperationsAsync(CancellationToken cancellationToken)
+            {
+                var changedSolution = await GetChangedSolutionAsync(isPreview: true, cancellationToken).ConfigureAwait(false);
+                if (changedSolution == null)
+                {
+                    return Array.Empty<CodeActionOperation>();
+                }
+
+                return new CodeActionOperation[] { new ApplyChangesOperation(changedSolution) };
+            }
+
+            protected override Task<Solution> GetChangedSolutionAsync(CancellationToken cancellationToken)
+            {
+                return GetChangedSolutionAsync(isPreview: false, cancellationToken);
+            }
+
+            private async Task<Solution> GetChangedSolutionAsync(bool isPreview, CancellationToken cancellationToken)
             {
                 var updatedDocument = await GetUpdatedDocumentAsync(cancellationToken).ConfigureAwait(false);
 
                 // Defer to subtype to add any p2p or metadata refs as appropriate.
-                var updatedProject = UpdateProject(updatedDocument.Project);
+                var updatedProject = await UpdateProjectAsync(updatedDocument.Project, isPreview, cancellationToken).ConfigureAwait(false);
 
                 var updatedSolution = updatedProject.Solution;
                 return updatedSolution;
             }
 
-            protected abstract Project UpdateProject(Project project);
+            protected abstract Task<Project> UpdateProjectAsync(Project project, bool isPreview, CancellationToken cancellationToken);
         }
     }
 }

src/Features/Core/Portable/CodeRefactorings/SyncNamespace/AbstractSyncNamespaceCodeRefactoringProvider.MoveFileCodeAction.cs

@@ -38,8 +38,6 @@ public MoveFileCodeAction(State state, ImmutableArray<string> newFolders)
                 _newfolders = newFolders;
             }
 
-            internal override bool PerformFinalApplicabilityCheck => true;
-
             protected override async Task<IEnumerable<CodeActionOperation>> ComputeOperationsAsync(CancellationToken cancellationToken)
             {
                 var id = _state.Document.Id;

src/Features/Core/Portable/UnifiedSuggestions/UnifiedSuggestedActionsSource.cs

@@ -29,10 +29,9 @@ namespace Microsoft.CodeAnalysis.UnifiedSuggestions
     internal class UnifiedSuggestedActionsSource
     {
         /// <summary>
-        /// Gets, filters, and orders code fixes. Must be called from the UI thread
-        /// due to the eventual call to <see cref="CodeAction.IsApplicable(Workspace)"/>.
+        /// Gets, filters, and orders code fixes.
         /// </summary>
-        public static ImmutableArray<UnifiedSuggestedActionSet> GetFilterAndOrderCodeFixes_MustBeCalledFromUIThread(
+        public static async Task<ImmutableArray<UnifiedSuggestedActionSet>> GetFilterAndOrderCodeFixesAsync(
             Workspace workspace,
             ICodeFixService codeFixService,
             Document document,
@@ -46,30 +45,25 @@ internal class UnifiedSuggestedActionsSource
             // it. However, it's deliberate.  We want to make sure that the code runs on 
             // the background so that no one takes an accidentally dependency on running on 
             // the UI thread.
-            var fixes = Task.Run(
+            var fixes = await Task.Run(
                 () => codeFixService.GetFixesAsync(
                 document, selection, includeSuppressionFixes, isBlocking,
-                addOperationScope, cancellationToken), cancellationToken).WaitAndGetResult(cancellationToken);
+                addOperationScope, cancellationToken), cancellationToken).ConfigureAwait(false);
 
-            var filteredFixes = FilterOnUIThread(fixes, workspace);
+            var filteredFixes = FilterOnAnyThread(fixes);
             var organizedFixes = OrganizeFixes(workspace, filteredFixes, includeSuppressionFixes);
 
             return organizedFixes;
         }
 
-        private static ImmutableArray<CodeFixCollection> FilterOnUIThread(
-            ImmutableArray<CodeFixCollection> collections,
-            Workspace workspace)
+        private static ImmutableArray<CodeFixCollection> FilterOnAnyThread(ImmutableArray<CodeFixCollection> collections)
         {
-            return collections.Select(
-                c => FilterIndividuallyOnUIThread(c, workspace)).WhereNotNull().ToImmutableArray();
+            return collections.Select(c => FilterIndividuallyOnAnyThread(c)).WhereNotNull().ToImmutableArray();
         }
 
-        private static CodeFixCollection? FilterIndividuallyOnUIThread(
-            CodeFixCollection collection,
-            Workspace workspace)
+        private static CodeFixCollection? FilterIndividuallyOnAnyThread(CodeFixCollection collection)
         {
-            var applicableFixes = collection.Fixes.WhereAsArray(f => IsApplicable(f.Action, workspace));
+            var applicableFixes = collection.Fixes;
             return applicableFixes.Length == 0
                 ? null
                 : applicableFixes.Length == collection.Fixes.Length
@@ -411,10 +405,9 @@ private static bool IsBulkConfigurationAction(CodeAction action)
             => (action as AbstractConfigurationActionWithNestedActions)?.IsBulkConfigurationAction == true;
 
         /// <summary>
-        /// Gets, filters, and orders code refactorings. Must be called from the UI thread
-        /// due to the eventual call to <see cref="CodeAction.IsApplicable(Workspace)"/>.
+        /// Gets, filters, and orders code refactorings.
         /// </summary>
-        public static ImmutableArray<UnifiedSuggestedActionSet> GetFilterAndOrderCodeRefactorings_MustBeCalledFromUIThread(
+        public static async Task<ImmutableArray<UnifiedSuggestedActionSet>> GetFilterAndOrderCodeRefactoringsAsync(
             Workspace workspace,
             ICodeRefactoringService codeRefactoringService,
             Document document,
@@ -428,30 +421,27 @@ private static bool IsBulkConfigurationAction(CodeAction action)
             // it. However, it's deliberate.  We want to make sure that the code runs on 
             // the background so that no one takes an accidentally dependency on running on 
             // the UI thread.
-            var refactorings = Task.Run(
+            var refactorings = await Task.Run(
                 () => codeRefactoringService.GetRefactoringsAsync(
                     document, selection, isBlocking, addOperationScope,
-                    cancellationToken), cancellationToken).WaitAndGetResult(cancellationToken);
+                    cancellationToken), cancellationToken).ConfigureAwait(false);
 
-            var filteredRefactorings = FilterOnUIThread(refactorings, selection, filterOutsideSelection, workspace);
+            var filteredRefactorings = FilterOnAnyThread(refactorings, selection, filterOutsideSelection);
 
             return filteredRefactorings.SelectAsArray(
                 r => OrganizeRefactorings(workspace, r));
         }
 
-        private static ImmutableArray<CodeRefactoring> FilterOnUIThread(
+        private static ImmutableArray<CodeRefactoring> FilterOnAnyThread(
             ImmutableArray<CodeRefactoring> refactorings,
             TextSpan selection,
-            bool filterOutsideSelection,
-            Workspace workspace)
-            => refactorings.Select(r => FilterOnUIThread(
-                r, selection, filterOutsideSelection, workspace)).WhereNotNull().ToImmutableArray();
+            bool filterOutsideSelection)
+            => refactorings.Select(r => FilterOnAnyThread(r, selection, filterOutsideSelection)).WhereNotNull().ToImmutableArray();
 
-        private static CodeRefactoring? FilterOnUIThread(
+        private static CodeRefactoring? FilterOnAnyThread(
             CodeRefactoring refactoring,
             TextSpan selection,
-            bool filterOutsideSelection,
-            Workspace workspace)
+            bool filterOutsideSelection)
         {
             var actions = refactoring.CodeActions.WhereAsArray(IsActionAndSpanApplicable);
             return actions.Length == 0
@@ -462,11 +452,6 @@ private static bool IsBulkConfigurationAction(CodeAction action)
 
             bool IsActionAndSpanApplicable((CodeAction action, TextSpan? applicableSpan) actionAndSpan)
             {
-                if (!IsApplicable(actionAndSpan.action, workspace))
-                {
-                    return false;
-                }
-
                 if (filterOutsideSelection)
                 {
                     // Filter out refactorings with applicable span outside the selection span.
@@ -539,19 +524,6 @@ private static UnifiedSuggestedActionSet OrganizeRefactorings(Workspace workspac
                 applicableToSpan: refactoring.CodeActions.FirstOrDefault().applicableToSpan);
         }
 
-        private static bool IsApplicable(CodeAction action, Workspace workspace)
-        {
-            if (!action.PerformFinalApplicabilityCheck)
-            {
-                // If we don't even need to perform the final applicability check,
-                // then the code action is applicable.
-                return true;
-            }
-
-            // Otherwise, defer to the action to make the decision.
-            return action.IsApplicable(workspace);
-        }
-
         private static UnifiedSuggestedActionSetPriority GetUnifiedSuggestedActionSetPriority(CodeActionPriority key)
             => key switch
             {
@@ -564,8 +536,8 @@ private static UnifiedSuggestedActionSetPriority GetUnifiedSuggestedActionSetPri
 
         /// <summary>
         /// Filters and orders the code fix sets and code refactoring sets amongst each other.
-        /// Should be called with the results from <see cref="GetFilterAndOrderCodeFixes_MustBeCalledFromUIThread"/>
-        /// and <see cref="GetFilterAndOrderCodeRefactorings_MustBeCalledFromUIThread"/>.
+        /// Should be called with the results from <see cref="GetFilterAndOrderCodeFixesAsync"/>
+        /// and <see cref="GetFilterAndOrderCodeRefactoringsAsync"/>.
         /// </summary>
         public static ImmutableArray<UnifiedSuggestedActionSet>? FilterAndOrderActionSets(
             ImmutableArray<UnifiedSuggestedActionSet> fixes,

src/Features/LanguageServer/Protocol/Handler/CodeActions/CodeActionHelpers.cs

@@ -11,7 +11,6 @@
 using Microsoft.CodeAnalysis.CodeActions;
 using Microsoft.CodeAnalysis.CodeFixes;
 using Microsoft.CodeAnalysis.CodeRefactorings;
-using Microsoft.CodeAnalysis.Editor.Shared.Utilities;
 using Microsoft.CodeAnalysis.PooledObjects;
 using Microsoft.CodeAnalysis.UnifiedSuggestions;
 using Microsoft.VisualStudio.LanguageServer.Protocol;
@@ -35,12 +34,11 @@ internal static class CodeActionHelpers
             Document document,
             ICodeFixService codeFixService,
             ICodeRefactoringService codeRefactoringService,
-            IThreadingContext threadingContext,
             LSP.Range selection,
             CancellationToken cancellationToken)
         {
             var actionSets = await GetActionSetsAsync(
-                document, codeFixService, codeRefactoringService, threadingContext, selection, cancellationToken).ConfigureAwait(false);
+                document, codeFixService, codeRefactoringService, selection, cancellationToken).ConfigureAwait(false);
             if (!actionSets.HasValue)
             {
                 return Array.Empty<VSCodeAction>();
@@ -108,12 +106,11 @@ internal static class CodeActionHelpers
             Document document,
             ICodeFixService codeFixService,
             ICodeRefactoringService codeRefactoringService,
-            IThreadingContext threadingContext,
             LSP.Range selection,
             CancellationToken cancellationToken)
         {
             var actionSets = await GetActionSetsAsync(
-                document, codeFixService, codeRefactoringService, threadingContext, selection, cancellationToken).ConfigureAwait(false);
+                document, codeFixService, codeRefactoringService, selection, cancellationToken).ConfigureAwait(false);
             if (!actionSets.HasValue)
             {
                 return ImmutableArray<CodeAction>.Empty;
@@ -165,23 +162,19 @@ private static CodeAction GetNestedActionsFromActionSet(IUnifiedSuggestedAction
             Document document,
             ICodeFixService codeFixService,
             ICodeRefactoringService codeRefactoringService,
-            IThreadingContext threadingContext,
             LSP.Range selection,
             CancellationToken cancellationToken)
         {
             var text = await document.GetTextAsync(cancellationToken).ConfigureAwait(false);
             var textSpan = ProtocolConversions.RangeToTextSpan(selection, text);
 
-            // The logic to filter code actions requires the UI thread
-            await threadingContext.JoinableTaskFactory.SwitchToMainThreadAsync(cancellationToken);
-
-            var codeFixes = UnifiedSuggestedActionsSource.GetFilterAndOrderCodeFixes_MustBeCalledFromUIThread(
+            var codeFixes = await UnifiedSuggestedActionsSource.GetFilterAndOrderCodeFixesAsync(
                 document.Project.Solution.Workspace, codeFixService, document, textSpan, includeSuppressionFixes: true,
-                isBlocking: false, addOperationScope: _ => null, cancellationToken);
+                isBlocking: false, addOperationScope: _ => null, cancellationToken).ConfigureAwait(false);
 
-            var codeRefactorings = UnifiedSuggestedActionsSource.GetFilterAndOrderCodeRefactorings_MustBeCalledFromUIThread(
+            var codeRefactorings = await UnifiedSuggestedActionsSource.GetFilterAndOrderCodeRefactoringsAsync(
                 document.Project.Solution.Workspace, codeRefactoringService, document, textSpan, isBlocking: false,
-                addOperationScope: _ => null, filterOutsideSelection: false, cancellationToken);
+                addOperationScope: _ => null, filterOutsideSelection: false, cancellationToken).ConfigureAwait(false);
 
             var actionSets = UnifiedSuggestedActionsSource.FilterAndOrderActionSets(codeFixes, codeRefactorings, textSpan);
             return actionSets;

src/Features/LanguageServer/Protocol/Handler/CodeActions/CodeActionResolveHandler.cs

@@ -6,7 +6,6 @@
 
 using System;
 using System.Collections.Generic;
-using System.Collections.Immutable;
 using System.Composition;
 using System.Linq;
 using System.Threading;
@@ -14,7 +13,6 @@
 using Microsoft.CodeAnalysis.CodeActions;
 using Microsoft.CodeAnalysis.CodeFixes;
 using Microsoft.CodeAnalysis.CodeRefactorings;
-using Microsoft.CodeAnalysis.Editor.Shared.Utilities;
 using Microsoft.CodeAnalysis.Host.Mef;
 using Microsoft.CodeAnalysis.LanguageServer.Handler.CodeActions;
 using Microsoft.CodeAnalysis.PooledObjects;
@@ -37,20 +35,17 @@ internal class CodeActionResolveHandler : AbstractRequestHandler<LSP.VSCodeActio
     {
         private readonly ICodeFixService _codeFixService;
         private readonly ICodeRefactoringService _codeRefactoringService;
-        private readonly IThreadingContext _threadingContext;
 
         [ImportingConstructor]
         [Obsolete(MefConstruction.ImportingConstructorMessage, error: true)]
         public CodeActionResolveHandler(
             ICodeFixService codeFixService,
             ICodeRefactoringService codeRefactoringService,
-            IThreadingContext threadingContext,
             ILspSolutionProvider solutionProvider)
             : base(solutionProvider)
         {
             _codeFixService = codeFixService;
             _codeRefactoringService = codeRefactoringService;
-            _threadingContext = threadingContext;
         }
 
         public override async Task<LSP.VSCodeAction> HandleRequestAsync(LSP.VSCodeAction codeAction, RequestContext context, CancellationToken cancellationToken)
@@ -63,7 +58,6 @@ public override async Task<LSP.VSCodeAction> HandleRequestAsync(LSP.VSCodeAction
                 document,
                 _codeFixService,
                 _codeRefactoringService,
-                _threadingContext,
                 data.Range,
                 cancellationToken).ConfigureAwait(false);
 

src/Features/LanguageServer/Protocol/Handler/CodeActions/CodeActionsHandler.cs

@@ -10,7 +10,6 @@
 using System.Threading.Tasks;
 using Microsoft.CodeAnalysis.CodeFixes;
 using Microsoft.CodeAnalysis.CodeRefactorings;
-using Microsoft.CodeAnalysis.Editor.Shared.Utilities;
 using Microsoft.CodeAnalysis.Host.Mef;
 using Microsoft.CodeAnalysis.LanguageServer.Handler.CodeActions;
 using Microsoft.VisualStudio.LanguageServer.Protocol;
@@ -28,7 +27,6 @@ internal class CodeActionsHandler : AbstractRequestHandler<LSP.CodeActionParams,
     {
         private readonly ICodeFixService _codeFixService;
         private readonly ICodeRefactoringService _codeRefactoringService;
-        private readonly IThreadingContext _threadingContext;
 
         internal const string RunCodeActionCommandName = "Roslyn.RunCodeAction";
 
@@ -37,13 +35,11 @@ internal class CodeActionsHandler : AbstractRequestHandler<LSP.CodeActionParams,
         public CodeActionsHandler(
             ICodeFixService codeFixService,
             ICodeRefactoringService codeRefactoringService,
-            ILspSolutionProvider solutionProvider,
-            IThreadingContext threadingContext)
+            ILspSolutionProvider solutionProvider)
             : base(solutionProvider)
         {
             _codeFixService = codeFixService;
             _codeRefactoringService = codeRefactoringService;
-            _threadingContext = threadingContext;
         }
 
         public override async Task<LSP.VSCodeAction[]> HandleRequestAsync(LSP.CodeActionParams request, RequestContext context, CancellationToken cancellationToken)
@@ -55,8 +51,8 @@ public override async Task<LSP.VSCodeAction[]> HandleRequestAsync(LSP.CodeAction
             }
 
             var codeActions = await CodeActionHelpers.GetVSCodeActionsAsync(
-                request, document, _codeFixService, _codeRefactoringService, _threadingContext,
-                request.Range, cancellationToken).ConfigureAwait(false);
+                request, document, _codeFixService, _codeRefactoringService, request.Range,
+                cancellationToken).ConfigureAwait(false);
 
             return codeActions;
         }

src/Features/LanguageServer/Protocol/Handler/CodeActions/RunCodeActionHandler.cs

@@ -55,7 +55,7 @@ public async Task<object> HandleRequestAsync(LSP.ExecuteCommandParams request, R
             var document = _solutionProvider.GetDocument(runRequest.TextDocument);
             var codeActions = await CodeActionHelpers.GetCodeActionsAsync(
                 document, _codeFixService, _codeRefactoringService,
-                _threadingContext, runRequest.Range, cancellationToken).ConfigureAwait(false);
+                runRequest.Range, cancellationToken).ConfigureAwait(false);
 
             var actionToRun = CodeActionHelpers.GetCodeActionToResolve(runRequest.UniqueIdentifier, codeActions);
             Contract.ThrowIfNull(actionToRun);

src/VisualStudio/Core/Def/Implementation/ProjectSystem/MetadataReferences/VisualStudioFrameworkAssemblyPathResolverFactory.cs

@@ -2,11 +2,15 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System;
 using System.Composition;
 using System.IO;
 using System.Reflection;
 using System.Runtime.Versioning;
+using System.Threading;
+using System.Threading.Tasks;
 using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.Editor.Shared.Utilities;
 using Microsoft.CodeAnalysis.Host;
@@ -46,12 +50,13 @@ public Service(IThreadingContext threadingContext, VisualStudioWorkspace workspa
                 _serviceProvider = serviceProvider;
             }
 
-            public string ResolveAssemblyPath(
+            public async Task<string?> ResolveAssemblyPathAsync(
                 ProjectId projectId,
                 string assemblyName,
-                string fullyQualifiedTypeName = null)
+                string? fullyQualifiedTypeName,
+                CancellationToken cancellationToken)
             {
-                this.AssertIsForeground();
+                await ThreadingContext.JoinableTaskFactory.SwitchToMainThreadAsync(cancellationToken);
 
                 var assembly = ResolveAssembly(projectId, assemblyName);
                 if (assembly != null)

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

@@ -13,6 +13,7 @@
 using System.Linq;
 using System.Text;
 using System.Threading;
+using System.Threading.Tasks;
 using EnvDTE;
 using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.Diagnostics;
@@ -1351,9 +1352,10 @@ public void EnsureEditableDocuments(IEnumerable<DocumentId> documents)
         public void EnsureEditableDocuments(params DocumentId[] documents)
             => this.EnsureEditableDocuments((IEnumerable<DocumentId>)documents);
 
-        internal override bool CanAddProjectReference(ProjectId referencingProject, ProjectId referencedProject)
+        internal override async Task<bool> CanAddProjectReferenceAsync(ProjectId referencingProject, ProjectId referencedProject, CancellationToken cancellationToken)
         {
-            _foregroundObject.AssertIsForeground();
+            await _foregroundObject.ThreadingContext.JoinableTaskFactory.SwitchToMainThreadAsync(cancellationToken);
+
             if (!TryGetHierarchy(referencingProject, out var referencingHierarchy) ||
                 !TryGetHierarchy(referencedProject, out var referencedHierarchy))
             {

src/Workspaces/Core/Portable/CodeActions/CodeAction.cs

@@ -288,17 +288,6 @@ protected virtual Task<Document> PostProcessChangesAsync(Document document, Canc
             return document;
         }
 
-        internal virtual bool PerformFinalApplicabilityCheck => false;
-
-        /// <summary>
-        /// Called by the CodeActions on the UI thread to determine if the CodeAction is still
-        /// applicable and should be presented to the user.  CodeActions can override this if they
-        /// need to do any final checking that must be performed on the UI thread (for example
-        /// accessing and querying the Visual Studio DTE).
-        /// </summary>
-        internal virtual bool IsApplicable(Workspace workspace)
-            => true;
-
         #region Factories for standard code actions
 
         /// <summary>

src/Workspaces/Core/Portable/Workspace/Host/Metadata/FrameworkAssemblyPathResolverFactory.cs

@@ -2,9 +2,14 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
 using System;
 using System.Composition;
+using System.Threading;
+using System.Threading.Tasks;
 using Microsoft.CodeAnalysis.Host.Mef;
+using Roslyn.Utilities;
 
 namespace Microsoft.CodeAnalysis.Host
 {
@@ -31,10 +36,10 @@ public Service()
             //    return false;
             //}
 
-            public string ResolveAssemblyPath(ProjectId projectId, string assemblyName, string fullyQualifiedTypeName = null)
+            public Task<string?> ResolveAssemblyPathAsync(ProjectId projectId, string assemblyName, string? fullyQualifiedTypeName, CancellationToken cancellationToken)
             {
                 // Assembly path resolution not supported at the default workspace level.
-                return null;
+                return SpecializedTasks.Null<string>();
             }
         }
     }

src/Workspaces/Core/Portable/Workspace/Host/Metadata/IFrameworkAssemblyPathResolver.cs

@@ -2,6 +2,11 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#nullable enable
+
+using System.Threading;
+using System.Threading.Tasks;
+
 namespace Microsoft.CodeAnalysis.Host
 {
     /// <summary>
@@ -19,7 +24,7 @@ internal interface IFrameworkAssemblyPathResolver : IWorkspaceService
         /// exist in the assembly.</param>
         /// <param name="projectId">The project context to search within.</param>
         /// <param name="assemblyName">The name of the assembly to try to resolve.</param>
-        string ResolveAssemblyPath(ProjectId projectId, string assemblyName, string fullyQualifiedName = null);
+        Task<string?> ResolveAssemblyPathAsync(ProjectId projectId, string assemblyName, string? fullyQualifiedName, CancellationToken cancellationToken);
 
         // bool CanResolveType(ProjectId projectId, string assemblyName, string fullyQualifiedTypeName);
     }

src/Workspaces/Core/Portable/Workspace/Workspace.cs

@@ -1154,8 +1154,8 @@ public virtual bool CanApplyChange(ApplyChangesKind feature)
         /// Returns <see langword="true"/> if a reference to referencedProject can be added to
         /// referencingProject.  <see langword="false"/> otherwise.
         /// </summary>
-        internal virtual bool CanAddProjectReference(ProjectId referencingProject, ProjectId referencedProject)
-            => false;
+        internal virtual Task<bool> CanAddProjectReferenceAsync(ProjectId referencingProject, ProjectId referencedProject, CancellationToken cancellationToken)
+            => SpecializedTasks.False;
 
         /// <summary>
         /// Apply changes made to a solution back to the workspace.