Allow Dispatcher to be implicitly cast to TaskScheduler

src/Avalonia.Base/Threading/Dispatcher.TaskScheduler.cs

@@ -0,0 +1,31 @@
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace Avalonia.Threading;
+
+public partial class Dispatcher
+{
+    private TaskScheduler? _taskScheduler;
+
+    public static implicit operator TaskScheduler(Dispatcher dispatcher)
+    {
+        lock (dispatcher.InstanceLock)
+        {
+            if (dispatcher._taskScheduler == null)
+            {
+                var prevContext = SynchronizationContext.Current;
+                SynchronizationContext.SetSynchronizationContext(dispatcher.GetContextWithPriority(DispatcherPriority.Default));
+                try
+                {
+                    dispatcher._taskScheduler = TaskScheduler.FromCurrentSynchronizationContext();
+                }
+                finally
+                {
+                    SynchronizationContext.SetSynchronizationContext(prevContext);
+                }
+            }
+        }
+
+        return dispatcher._taskScheduler;
+    }
+}

tests/Avalonia.Base.UnitTests/DispatcherTests.cs

@@ -764,4 +764,15 @@ public partial class DispatcherTests
         }
     }
 
+    [Fact]
+    public async Task Dispatcher_Can_Act_As_TaskScheduler()
+    {
+        var impl = new SimpleDispatcherImpl();
+        Dispatcher.InitializeUIThreadDispatcher(impl);
+        Thread? continuationThread = null;
+        _ = Task.CompletedTask.ContinueWith(t => continuationThread = Thread.CurrentThread, Dispatcher.UIThread);
+        Assert.True(impl.AskedForSignal);
+        impl.ExecuteSignal();
+        Assert.Equal(Dispatcher.UIThread.Thread, continuationThread);
+    }
 }