From 5c1b45b2ecc91c9420ced9087cc193bcc5081482 Mon Sep 17 00:00:00 2001
From: Johan Laanstra <jlaans@microsoft.com>
Date: Fri, 14 Jan 2022 14:58:22 -0800
Subject: [PATCH 1/2] Support DispatcherQueueSynchronizationContext.Send.

---
 ...g.DispatcherQueueSynchronizationContext.cs | 87 +++++++++++--------
 1 file changed, 50 insertions(+), 37 deletions(-)

diff --git a/src/Projections/Reunion/Microsoft.UI.Dispatching.DispatcherQueueSynchronizationContext.cs b/src/Projections/Reunion/Microsoft.UI.Dispatching.DispatcherQueueSynchronizationContext.cs
index f1f5a7e5c..b5094c15c 100644
--- a/src/Projections/Reunion/Microsoft.UI.Dispatching.DispatcherQueueSynchronizationContext.cs
+++ b/src/Projections/Reunion/Microsoft.UI.Dispatching.DispatcherQueueSynchronizationContext.cs
@@ -1,38 +1,51 @@
-using System;
-using System.Threading;
-using Microsoft.System;
-
-namespace Microsoft.System
-{
-    /// <summary>
-    /// DispatcherQueueSyncContext allows developers to await calls and get back onto the
-    /// UI thread. Needs to be installed on the UI thread through DispatcherQueueSyncContext.SetForCurrentThread
-    /// </summary>
-    public class DispatcherQueueSynchronizationContext : SynchronizationContext
-    {
-        private readonly DispatcherQueue m_dispatcherQueue;
-
-        public DispatcherQueueSynchronizationContext(DispatcherQueue dispatcherQueue)
-        {
-            m_dispatcherQueue = dispatcherQueue;
-        }
-
-        public override void Post(SendOrPostCallback d, object state)
-        {
-            if (d == null)
-                throw new ArgumentNullException(nameof(d));
-
-            m_dispatcherQueue.TryEnqueue(() => d(state));
-        }
-
-        public override void Send(SendOrPostCallback d, object state)
-        {
-            throw new NotSupportedException("Send not supported");
-        }
-
-        public override SynchronizationContext CreateCopy()
-        {
-            return new DispatcherQueueSynchronizationContext(m_dispatcherQueue);
-        }
-    }
+using System;
+using System.Threading;
+using Microsoft.System;
+
+namespace Microsoft.System
+{
+    /// <summary>
+    /// DispatcherQueueSyncContext allows developers to await calls and get back onto the
+    /// UI thread. Needs to be installed on the UI thread through DispatcherQueueSyncContext.SetForCurrentThread
+    /// </summary>
+    public class DispatcherQueueSynchronizationContext : SynchronizationContext
+    {
+        private readonly DispatcherQueue m_dispatcherQueue;
+
+        public DispatcherQueueSynchronizationContext(DispatcherQueue dispatcherQueue)
+        {
+            m_dispatcherQueue = dispatcherQueue;
+        }
+
+        public override void Post(SendOrPostCallback d, object state)
+        {
+            if (d == null)
+                throw new ArgumentNullException(nameof(d));
+
+            m_dispatcherQueue.TryEnqueue(() => d(state));
+        }
+
+        public override void Send(SendOrPostCallback d, object state)
+        {
+            if (m_dispatcherQueue.HasThreadAccess)
+            {
+                d(state);
+            }
+            else
+            {
+                var m = new ManualResetEvent(false);
+                m_dispatcherQueue.TryEnqueue(() =>
+                {
+                    d(state);
+                    m.Set();
+                });
+                m.WaitOne();
+            }
+        }
+
+        public override SynchronizationContext CreateCopy()
+        {
+            return new DispatcherQueueSynchronizationContext(m_dispatcherQueue);
+        }
+    }
 }
\ No newline at end of file

From 7e8bc5c334d5b1782d9b200211e98ca2b179c7e6 Mon Sep 17 00:00:00 2001
From: Johan Laanstra <jlaans@microsoft.com>
Date: Fri, 14 Jan 2022 15:40:42 -0800
Subject: [PATCH 2/2] Ensure exceptions flwo back correctly.

---
 ...g.DispatcherQueueSynchronizationContext.cs | 21 +++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/src/Projections/Reunion/Microsoft.UI.Dispatching.DispatcherQueueSynchronizationContext.cs b/src/Projections/Reunion/Microsoft.UI.Dispatching.DispatcherQueueSynchronizationContext.cs
index b5094c15c..301da6b9e 100644
--- a/src/Projections/Reunion/Microsoft.UI.Dispatching.DispatcherQueueSynchronizationContext.cs
+++ b/src/Projections/Reunion/Microsoft.UI.Dispatching.DispatcherQueueSynchronizationContext.cs
@@ -1,4 +1,5 @@
 using System;
+using System.Runtime.ExceptionServices;
 using System.Threading;
 using Microsoft.System;
 
@@ -34,12 +35,28 @@ public override void Send(SendOrPostCallback d, object state)
             else
             {
                 var m = new ManualResetEvent(false);
+                ExceptionDispatchInfo edi = null;
+
                 m_dispatcherQueue.TryEnqueue(() =>
                 {
-                    d(state);
-                    m.Set();
+                    try
+                    {
+                        d(state);
+                    }
+                    catch (Exception ex)
+                    {
+                        edi = ExceptionDispatchInfo.Capture(ex);
+                    }
+                    finally
+                    {
+                        m.Set();
+                    }
                 });
                 m.WaitOne();
+
+#pragma warning disable CA1508 // Avoid dead conditional code
+                edi?.Throw();
+#pragma warning restore CA1508 // Avoid dead conditional code
             }
         }