Allow library code to detect the JoinableContext is not associated with main thread

It allows library code to skip certain work under such conditions. For example to build an extensive dependency graph for dataflow dependencies, or dependencies in work queues.

Author: lifengl

src/Microsoft.VisualStudio.Threading/JoinableTaskContext.cs

@@ -245,6 +245,12 @@ public bool IsWithinJoinableTask
         get { return this.AmbientTask is object; }
     }
 
+    /// <summary>
+    /// Gets a value indicating whether gets a value indicating the JoinableTask is not associated with any main thread.
+    /// This allows library code to skip some additional work in the environments that do not have a main thread.
+    /// </summary>
+    public bool IsNoOpContext => this.UnderlyingSynchronizationContext is null;
+
     /// <summary>
     /// Gets a value indicating whether the main thread is blocked by any joinable task.
     /// </summary>

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

@@ -1,4 +1,5 @@
 Microsoft.VisualStudio.Threading.AsyncBarrier.SignalAndWait(System.Threading.CancellationToken cancellationToken) -> System.Threading.Tasks.ValueTask
 Microsoft.VisualStudio.Threading.IPendingExecutionRequestState
 Microsoft.VisualStudio.Threading.IPendingExecutionRequestState.IsCompleted.get -> bool
+Microsoft.VisualStudio.Threading.JoinableTaskContext.IsNoOpContext.get -> bool
 static Microsoft.VisualStudio.Threading.JoinableTaskContext.CreateNoOpContext() -> Microsoft.VisualStudio.Threading.JoinableTaskContext!

src/Microsoft.VisualStudio.Threading/net8.0-windows/PublicAPI.Unshipped.txt

@@ -1,4 +1,5 @@
 Microsoft.VisualStudio.Threading.AsyncBarrier.SignalAndWait(System.Threading.CancellationToken cancellationToken) -> System.Threading.Tasks.ValueTask
 Microsoft.VisualStudio.Threading.IPendingExecutionRequestState
 Microsoft.VisualStudio.Threading.IPendingExecutionRequestState.IsCompleted.get -> bool
+Microsoft.VisualStudio.Threading.JoinableTaskContext.IsNoOpContext.get -> bool
 static Microsoft.VisualStudio.Threading.JoinableTaskContext.CreateNoOpContext() -> Microsoft.VisualStudio.Threading.JoinableTaskContext!

src/Microsoft.VisualStudio.Threading/net8.0/PublicAPI.Unshipped.txt

@@ -1,4 +1,5 @@
 Microsoft.VisualStudio.Threading.AsyncBarrier.SignalAndWait(System.Threading.CancellationToken cancellationToken) -> System.Threading.Tasks.ValueTask
 Microsoft.VisualStudio.Threading.IPendingExecutionRequestState
 Microsoft.VisualStudio.Threading.IPendingExecutionRequestState.IsCompleted.get -> bool
+Microsoft.VisualStudio.Threading.JoinableTaskContext.IsNoOpContext.get -> bool
 static Microsoft.VisualStudio.Threading.JoinableTaskContext.CreateNoOpContext() -> Microsoft.VisualStudio.Threading.JoinableTaskContext!

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

@@ -1,4 +1,5 @@
 Microsoft.VisualStudio.Threading.AsyncBarrier.SignalAndWait(System.Threading.CancellationToken cancellationToken) -> System.Threading.Tasks.ValueTask
 Microsoft.VisualStudio.Threading.IPendingExecutionRequestState
 Microsoft.VisualStudio.Threading.IPendingExecutionRequestState.IsCompleted.get -> bool
+Microsoft.VisualStudio.Threading.JoinableTaskContext.IsNoOpContext.get -> bool
 static Microsoft.VisualStudio.Threading.JoinableTaskContext.CreateNoOpContext() -> Microsoft.VisualStudio.Threading.JoinableTaskContext!

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

@@ -727,6 +727,7 @@ public void Ctor_ExplicitNullSyncContext()
             Thread mainThread = Thread.CurrentThread;
             Assumes.NotNull(SynchronizationContext.Current);
             JoinableTaskContext jtc = JoinableTaskContext.CreateNoOpContext();
+            Assert.True(jtc.IsNoOpContext);
             await TaskScheduler.Default.SwitchTo(alwaysYield: true); // Get off the main thread.
             Assert.NotSame(mainThread, Thread.CurrentThread);
 
@@ -745,6 +746,7 @@ public void Ctor_NullSyncContextArg_AmbientSyncContext()
             Thread mainThread = Thread.CurrentThread;
             Assumes.NotNull(SynchronizationContext.Current);
             JoinableTaskContext jtc = new(null, null);
+            Assert.False(jtc.IsNoOpContext);
             await TaskScheduler.Default.SwitchTo(alwaysYield: true); // Get off the main thread.
             Assert.NotSame(mainThread, Thread.CurrentThread);
 
@@ -762,6 +764,7 @@ public void Ctor_Default()
             Thread mainThread = Thread.CurrentThread;
             Assumes.NotNull(SynchronizationContext.Current);
             JoinableTaskContext jtc = new();
+            Assert.False(jtc.IsNoOpContext);
             await TaskScheduler.Default.SwitchTo(alwaysYield: true); // Get off the main thread.
             Assert.NotSame(mainThread, Thread.CurrentThread);
 
@@ -771,6 +774,28 @@ public void Ctor_Default()
         });
     }
 
+    [Fact]
+    public void Ctor_DefaultWithNoSyncContext()
+    {
+        this.SimulateUIThread(async delegate
+        {
+            await TaskScheduler.Default.SwitchTo(alwaysYield: true); // Get off the main thread.
+
+            Thread currentThread = Thread.CurrentThread;
+
+            Assumes.Null(SynchronizationContext.Current);
+            JoinableTaskContext jtc = new();
+            Assert.True(jtc.IsNoOpContext);
+
+            await TaskScheduler.Default.SwitchTo();
+            Assert.Same(currentThread, Thread.CurrentThread);
+
+            // Verify that switching to the main thread works.
+            await jtc.Factory.SwitchToMainThreadAsync(this.TimeoutToken);
+            Assert.Same(currentThread, Thread.CurrentThread);
+        });
+    }
+
     protected override JoinableTaskContext CreateJoinableTaskContext()
     {
         return new JoinableTaskContextDerived();