Allow library code to detect the JoinableTaskContext is not associated with Main thread

src/Microsoft.VisualStudio.Threading/JoinableTaskContext.cs

@@ -245,6 +245,15 @@ public bool IsWithinJoinableTask
         get { return this.AmbientTask is object; }
     }
 
+    /// <summary>
+    /// Gets a value indicating whether this instance is not associated with any main thread
+    /// (e.g. created with <see cref="CreateNoOpContext" />).
+    /// </summary>
+    /// <remarks>
+    /// This allows library code to skip some additional work in the environments that do not have a main thread.
+    /// </remarks>
+    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();