Add SynchronizationContext.GetAwaiter extension method

src/Microsoft.VisualStudio.Threading/AwaitExtensions.cs

@@ -31,6 +31,20 @@ public static TaskSchedulerAwaiter GetAwaiter(this TaskScheduler scheduler)
             return new TaskSchedulerAwaiter(scheduler);
         }
 
+        /// <summary>
+        /// Gets an awaiter that schedules continuations on the specified <see cref="SynchronizationContext"/>.
+        /// </summary>
+        /// <param name="synchronizationContext">The synchronization context used to execute continuations.</param>
+        /// <returns>An awaitable.</returns>
+        /// <remarks>
+        /// The awaiter that is returned will <em>always</em> result in yielding, even if already executing within the specified <paramref name="synchronizationContext"/>.
+        /// </remarks>
+        public static SynchronizationContextAwaiter GetAwaiter(this SynchronizationContext synchronizationContext)
+        {
+            Requires.NotNull(synchronizationContext, nameof(synchronizationContext));
+            return new SynchronizationContextAwaiter(synchronizationContext);
+        }
+
         /// <summary>
         /// Gets an awaitable that schedules continuations on the specified scheduler.
         /// </summary>
@@ -442,6 +456,71 @@ public void GetResult()
             }
         }
 
+        /// <summary>
+        /// An awaiter returned from <see cref="GetAwaiter(SynchronizationContext)"/>.
+        /// </summary>
+        public readonly struct SynchronizationContextAwaiter : ICriticalNotifyCompletion
+        {
+            private static readonly SendOrPostCallback SyncContextDelegate = s => ((Action)s!)();
+
+            /// <summary>
+            /// The context for continuations.
+            /// </summary>
+            private readonly SynchronizationContext syncContext;
+
+            /// <summary>
+            /// Initializes a new instance of the <see cref="SynchronizationContextAwaiter"/> struct.
+            /// </summary>
+            /// <param name="syncContext">The context for continuations.</param>
+            public SynchronizationContextAwaiter(SynchronizationContext syncContext)
+            {
+                this.syncContext = syncContext;
+            }
+
+            /// <summary>
+            /// Gets a value indicating whether no yield is necessary.
+            /// </summary>
+            /// <value>Always returns <see langword="false"/>.</value>
+            public bool IsCompleted => false;
+
+            /// <summary>
+            /// Schedules a continuation to execute using the specified <see cref="SynchronizationContext"/>.
+            /// </summary>
+            /// <param name="continuation">The delegate to invoke.</param>
+            public void OnCompleted(Action continuation) => this.syncContext.Post(SyncContextDelegate, continuation);
+
+            /// <summary>
+            /// Schedules a continuation to execute using the specified <see cref="SynchronizationContext"/>
+            /// without capturing the <see cref="ExecutionContext"/>.
+            /// </summary>
+            /// <param name="continuation">The action.</param>
+            public void UnsafeOnCompleted(Action continuation)
+            {
+#if NETFRAMEWORK // Only bother suppressing flow on .NET Framework where the perf would improve from doing so.
+                if (ExecutionContext.IsFlowSuppressed())
+                {
+                    this.syncContext.Post(SyncContextDelegate, continuation);
+                }
+                else
+                {
+                    using (ExecutionContext.SuppressFlow())
+                    {
+                        this.syncContext.Post(SyncContextDelegate, continuation);
+                    }
+                }
+#else
+                this.syncContext.Post(SyncContextDelegate, continuation);
+#endif
+            }
+
+            /// <summary>
+            /// Does nothing.
+            /// </summary>
+            public void GetResult()
+            {
+            }
+        }
+
         /// <summary>
         /// An awaitable that will always lead the calling async method to yield,
         /// then immediately resume, possibly on the original <see cref="SynchronizationContext"/>.

src/Microsoft.VisualStudio.Threading/net472/PublicAPI.Unshipped.txt

@@ -1,9 +1,16 @@
 Microsoft.VisualStudio.Threading.AsyncQueue<T>.ToArray() -> T[]!
 Microsoft.VisualStudio.Threading.AsyncReaderWriterLock.AsyncReaderWriterLock(Microsoft.VisualStudio.Threading.JoinableTaskContext? joinableTaskContext, bool captureDiagnostics = false) -> void
 Microsoft.VisualStudio.Threading.AsyncReaderWriterResourceLock<TMoniker, TResource>.AsyncReaderWriterResourceLock(Microsoft.VisualStudio.Threading.JoinableTaskContext? joinableTaskContext, bool captureDiagnostics) -> void
+Microsoft.VisualStudio.Threading.AwaitExtensions.SynchronizationContextAwaiter
+Microsoft.VisualStudio.Threading.AwaitExtensions.SynchronizationContextAwaiter.GetResult() -> void
+Microsoft.VisualStudio.Threading.AwaitExtensions.SynchronizationContextAwaiter.IsCompleted.get -> bool
+Microsoft.VisualStudio.Threading.AwaitExtensions.SynchronizationContextAwaiter.OnCompleted(System.Action! continuation) -> void
+Microsoft.VisualStudio.Threading.AwaitExtensions.SynchronizationContextAwaiter.SynchronizationContextAwaiter(System.Threading.SynchronizationContext! syncContext) -> void
+Microsoft.VisualStudio.Threading.AwaitExtensions.SynchronizationContextAwaiter.UnsafeOnCompleted(System.Action! continuation) -> void
 Microsoft.VisualStudio.Threading.JoinableTaskContext.IsMainThreadMaybeBlocked() -> bool
 Microsoft.VisualStudio.Threading.SemaphoreFaultedException
 Microsoft.VisualStudio.Threading.SemaphoreFaultedException.SemaphoreFaultedException() -> void
 Microsoft.VisualStudio.Threading.IllegalSemaphoreUsageException
 Microsoft.VisualStudio.Threading.IllegalSemaphoreUsageException.IllegalSemaphoreUsageException(string! message) -> void
+static Microsoft.VisualStudio.Threading.AwaitExtensions.GetAwaiter(this System.Threading.SynchronizationContext! synchronizationContext) -> Microsoft.VisualStudio.Threading.AwaitExtensions.SynchronizationContextAwaiter
 virtual Microsoft.VisualStudio.Threading.AsyncReaderWriterLock.DeadlockCheckTimeout.get -> System.TimeSpan

src/Microsoft.VisualStudio.Threading/netcoreapp3.1/PublicAPI.Unshipped.txt

@@ -1,9 +1,16 @@
 Microsoft.VisualStudio.Threading.AsyncQueue<T>.ToArray() -> T[]!
 Microsoft.VisualStudio.Threading.AsyncReaderWriterLock.AsyncReaderWriterLock(Microsoft.VisualStudio.Threading.JoinableTaskContext? joinableTaskContext, bool captureDiagnostics = false) -> void
 Microsoft.VisualStudio.Threading.AsyncReaderWriterResourceLock<TMoniker, TResource>.AsyncReaderWriterResourceLock(Microsoft.VisualStudio.Threading.JoinableTaskContext? joinableTaskContext, bool captureDiagnostics) -> void
+Microsoft.VisualStudio.Threading.AwaitExtensions.SynchronizationContextAwaiter
+Microsoft.VisualStudio.Threading.AwaitExtensions.SynchronizationContextAwaiter.GetResult() -> void
+Microsoft.VisualStudio.Threading.AwaitExtensions.SynchronizationContextAwaiter.IsCompleted.get -> bool
+Microsoft.VisualStudio.Threading.AwaitExtensions.SynchronizationContextAwaiter.OnCompleted(System.Action! continuation) -> void
+Microsoft.VisualStudio.Threading.AwaitExtensions.SynchronizationContextAwaiter.SynchronizationContextAwaiter(System.Threading.SynchronizationContext! syncContext) -> void
+Microsoft.VisualStudio.Threading.AwaitExtensions.SynchronizationContextAwaiter.UnsafeOnCompleted(System.Action! continuation) -> void
 Microsoft.VisualStudio.Threading.JoinableTaskContext.IsMainThreadMaybeBlocked() -> bool
 Microsoft.VisualStudio.Threading.SemaphoreFaultedException
 Microsoft.VisualStudio.Threading.SemaphoreFaultedException.SemaphoreFaultedException() -> void
 Microsoft.VisualStudio.Threading.IllegalSemaphoreUsageException
 Microsoft.VisualStudio.Threading.IllegalSemaphoreUsageException.IllegalSemaphoreUsageException(string! message) -> void
+static Microsoft.VisualStudio.Threading.AwaitExtensions.GetAwaiter(this System.Threading.SynchronizationContext! synchronizationContext) -> Microsoft.VisualStudio.Threading.AwaitExtensions.SynchronizationContextAwaiter
 virtual Microsoft.VisualStudio.Threading.AsyncReaderWriterLock.DeadlockCheckTimeout.get -> System.TimeSpan

src/Microsoft.VisualStudio.Threading/netstandard2.0/PublicAPI.Unshipped.txt

@@ -1,9 +1,16 @@
 Microsoft.VisualStudio.Threading.AsyncQueue<T>.ToArray() -> T[]!
 Microsoft.VisualStudio.Threading.AsyncReaderWriterLock.AsyncReaderWriterLock(Microsoft.VisualStudio.Threading.JoinableTaskContext? joinableTaskContext, bool captureDiagnostics = false) -> void
 Microsoft.VisualStudio.Threading.AsyncReaderWriterResourceLock<TMoniker, TResource>.AsyncReaderWriterResourceLock(Microsoft.VisualStudio.Threading.JoinableTaskContext? joinableTaskContext, bool captureDiagnostics) -> void
+Microsoft.VisualStudio.Threading.AwaitExtensions.SynchronizationContextAwaiter
+Microsoft.VisualStudio.Threading.AwaitExtensions.SynchronizationContextAwaiter.GetResult() -> void
+Microsoft.VisualStudio.Threading.AwaitExtensions.SynchronizationContextAwaiter.IsCompleted.get -> bool
+Microsoft.VisualStudio.Threading.AwaitExtensions.SynchronizationContextAwaiter.OnCompleted(System.Action! continuation) -> void
+Microsoft.VisualStudio.Threading.AwaitExtensions.SynchronizationContextAwaiter.SynchronizationContextAwaiter(System.Threading.SynchronizationContext! syncContext) -> void
+Microsoft.VisualStudio.Threading.AwaitExtensions.SynchronizationContextAwaiter.UnsafeOnCompleted(System.Action! continuation) -> void
 Microsoft.VisualStudio.Threading.JoinableTaskContext.IsMainThreadMaybeBlocked() -> bool
 Microsoft.VisualStudio.Threading.SemaphoreFaultedException
 Microsoft.VisualStudio.Threading.SemaphoreFaultedException.SemaphoreFaultedException() -> void
 Microsoft.VisualStudio.Threading.IllegalSemaphoreUsageException
 Microsoft.VisualStudio.Threading.IllegalSemaphoreUsageException.IllegalSemaphoreUsageException(string! message) -> void
+static Microsoft.VisualStudio.Threading.AwaitExtensions.GetAwaiter(this System.Threading.SynchronizationContext! synchronizationContext) -> Microsoft.VisualStudio.Threading.AwaitExtensions.SynchronizationContextAwaiter
 virtual Microsoft.VisualStudio.Threading.AsyncReaderWriterLock.DeadlockCheckTimeout.get -> System.TimeSpan

test/Microsoft.VisualStudio.Threading.Tests/AwaitExtensionsTests.cs

@@ -566,6 +566,53 @@ public async Task ConfigureAwaitForAggregateException_InnerCanceledAndFaulted()
         }
     }
 
+    [Fact]
+    public void GetAwaiter_SynchronizationContext_ValidatesArgs()
+    {
+        Assert.Throws<ArgumentNullException>(() => AwaitExtensions.GetAwaiter((SynchronizationContext)null!));
+    }
+
+    [Fact]
+    public async Task SyncContext_Awaiter()
+    {
+        TaskCompletionSource<SingleThreadedSynchronizationContext> syncContextSource = new();
+        SingleThreadedSynchronizationContext.Frame frame = new();
+        Thread? otherThread = null;
+        Task otherThreadTask = Task.Run(delegate
+        {
+            SingleThreadedSynchronizationContext syncContext;
+            try
+            {
+                syncContext = new();
+                otherThread = Thread.CurrentThread;
+                syncContextSource.SetResult(syncContext);
+            }
+            catch (Exception ex)
+            {
+                syncContextSource.SetException(ex);
+                throw;
+            }
+
+            syncContext.PushFrame(frame);
+        });
+
+        try
+        {
+            SynchronizationContext context = await syncContextSource.Task;
+            Assert.NotSame(Thread.CurrentThread, otherThread);
+            await context;
+            Assert.Same(Thread.CurrentThread, otherThread);
+            await Task.Yield();
+            Assert.Same(Thread.CurrentThread, otherThread);
+            await TaskScheduler.Default.SwitchTo(alwaysYield: true);
+            Assert.NotSame(Thread.CurrentThread, otherThread);
+        }
+        finally
+        {
+            frame.Continue = false;
+        }
+    }
+
     [SkippableFact]
     public async Task AwaitRegKeyChange()
     {