Added UI thread sync placeholder obj & methods.

src/NTwain/DataSource.cs

@@ -44,7 +44,7 @@ namespace NTwain
         public ReturnCode ShowUI(IntPtr windowHandle, bool modal = false)
         {
             var rc = ReturnCode.Failure;
-            Session.Invoke(() =>
+            Session.InternalInvoke(() =>
             {
                 var ui = new TW_USERINTERFACE
                 {
@@ -72,7 +72,7 @@ namespace NTwain
         public ReturnCode Enable(bool showUI, IntPtr windowHandle, bool modal = false)
         {
             var rc = ReturnCode.Failure;
-            Session.Invoke(() =>
+            Session.InternalInvoke(() =>
             {
                 var ui = new TW_USERINTERFACE
                 {

src/NTwain/Threading/ActionItem.cs

@@ -25,6 +25,10 @@ namespace NTwain.Threading
             {
                 action();
             }
+            catch (Exception ex)
+            {
+                // TODO: do something
+            }
             finally
             {
                 waiter?.Set();

src/NTwain/Threading/IThreadContext.cs

@@ -0,0 +1,22 @@
+using System;
+
+namespace NTwain.Threading
+{
+    /// <summary>
+    /// Interface for running some work on an associated thread.
+    /// </summary>
+    interface IThreadContext
+    {
+        /// <summary>
+        /// Runs the action synchronously on the associated thread.
+        /// </summary>
+        /// <param name="action"></param>
+        void Invoke(Action action);
+
+        /// <summary>
+        /// Runs the action asynchronously on the associated thread.
+        /// </summary>
+        /// <param name="action"></param>
+        void BeginInvoke(Action action);
+    }
+}

src/NTwain/Threading/UIThreadContext.cs

@@ -0,0 +1,95 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Reflection;
+using System.Text;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace NTwain.Threading
+{
+    /// <summary>
+    /// A <see cref="IThreadContext"/> using <see cref="SynchronizationContext"/>
+    /// for running actions on an UI thread.
+    /// </summary>
+    class UIThreadContext : IThreadContext
+    {
+        private readonly SynchronizationContext context;
+
+        /// <summary>
+        /// Creates a new <see cref="UIThreadContext"/> using
+        /// the <see cref="SynchronizationContext.Current"/>.
+        /// </summary>
+        public UIThreadContext() : this(SynchronizationContext.Current)
+        {
+
+        }
+
+        /// <summary>
+        /// Creates a new <see cref="UIThreadContext"/> using
+        /// the specified <see cref="SynchronizationContext"/>.
+        /// </summary>
+        /// <param name="context"></param>
+        public UIThreadContext(SynchronizationContext context)
+        {
+            this.context = context ?? throw new ArgumentNullException(nameof(context));
+            ;
+        }
+
+        /// <summary>
+        /// Runs the action asynchronously on the <see cref="SynchronizationContext"/> thread.
+        /// </summary>
+        /// <param name="action"></param>
+        public void BeginInvoke(Action action)
+        {
+            if (action == null) return;
+
+            context.Post(o =>
+            {
+                try
+                {
+                    action();
+                }
+                catch (Exception ex)
+                {
+                    // TODO: do something
+                }
+            }, null);
+        }
+
+        /// <summary>
+        /// Runs the action synchronously on the <see cref="SynchronizationContext"/> thread.
+        /// </summary>
+        /// <param name="action"></param>
+        public void Invoke(Action action)
+        {
+            if (action == null) return;
+
+            Exception error = null;
+            context.Send(o =>
+            {
+                try
+                {
+                    action();
+                }
+                catch (Exception ex)
+                {
+                    error = ex;
+                }
+            }, null);
+
+            if (error != null) { Rethrow(error); }
+        }
+
+        /// <summary>
+        /// Rethrows the specified excetion while keeping stack trace.
+        /// </summary>
+        /// <param name="ex">The ex.</param>
+        static void Rethrow(Exception ex)
+        {
+            typeof(Exception).GetMethod("PrepForRemoting",
+                BindingFlags.NonPublic | BindingFlags.Instance)?.Invoke(ex, new object[0]);
+            throw ex;
+        }
+    }
+}

src/NTwain/Threading/WinMsgLoop.cs

@@ -5,6 +5,7 @@ using System.Collections.Generic;
 using System.ComponentModel;
 using System.Diagnostics;
 using System.Linq;
+using System.Reflection;
 using System.Runtime.InteropServices;
 using System.Security;
 using System.Text;
@@ -15,7 +16,7 @@ namespace NTwain.Threading
     /// <summary>
     /// Provides an internal message pump on Windows using a background thread.
     /// </summary>
-    class WinMsgLoop
+    class WinMsgLoop : IThreadContext
     {
         static ushort classAtom;
         static IntPtr hInstance;
@@ -143,10 +144,6 @@ namespace NTwain.Threading
                                 }
                             }
                         }
-                        catch
-                        {
-                            UnsafeNativeMethods.PostQuitMessage(0);
-                        }
                         finally
                         {
                             // clear queue
@@ -163,7 +160,7 @@ namespace NTwain.Threading
                 loopThread.Start();
                 startWaiter.Wait();
 
-                if (startErr != null) throw startErr;
+                if (startErr != null) Rethrow(startErr);
             }
         }
 
@@ -193,10 +190,7 @@ namespace NTwain.Threading
             return loopThread == Thread.CurrentThread || loopThread == null;
         }
 
-        /// <summary>
+
-        /// Runs the action synchronously on the internal message pump thread.
-        /// </summary>
-        /// <param name="action"></param>
         public void Invoke(Action action)
         {
             if (IsSameThread())
@@ -216,10 +210,7 @@ namespace NTwain.Threading
             }
         }
 
-        /// <summary>
+
-        /// Runs the action asynchronously on the internal message pump thread.
-        /// </summary>
-        /// <param name="action"></param>
         public void BeginInvoke(Action action)
         {
             if (hWnd == IntPtr.Zero) action();
@@ -231,6 +222,17 @@ namespace NTwain.Threading
         }
 
 
+        /// <summary>
+        /// Rethrows the specified excetion while keeping stack trace.
+        /// </summary>
+        /// <param name="ex">The ex.</param>
+        static void Rethrow(Exception ex)
+        {
+            typeof(Exception).GetMethod("PrepForRemoting",
+                BindingFlags.NonPublic | BindingFlags.Instance)?.Invoke(ex, new object[0]);
+            throw ex;
+        }
+
         [SuppressUnmanagedCodeSecurity]
         internal static class UnsafeNativeMethods
         {

src/NTwain/TwainSession.MsgHandling.cs

@@ -23,7 +23,7 @@ namespace NTwain
             DataGroups dg, DataArgumentType dat, Message msg, IntPtr data)
         {
             Debug.WriteLine($"Thread {Thread.CurrentThread.ManagedThreadId}: {nameof(Handle32BitCallback)}({dg}, {dat}, {msg}, {data})");
-            BeginInvoke(() =>
+            InternalBeginInvoke(() =>
             {
                 Debug.WriteLine($"Thread {Thread.CurrentThread.ManagedThreadId}: in BeginInvoke {nameof(Handle32BitCallback)}({dg}, {dat}, {msg}, {data})");
                 HandleSourceMsg(msg);

src/NTwain/TwainSession.Props.cs

@@ -5,6 +5,7 @@ using System.Collections.Generic;
 using System.ComponentModel;
 using System.Linq;
 using System.Text;
+using System.Threading;
 
 namespace NTwain
 {

src/NTwain/TwainSession.cs

@@ -20,7 +20,6 @@ namespace NTwain
     {
         internal readonly TwainConfig Config;
 
-        private IntPtr _hWnd;
         // cache generated twain sources so if you get same source from same session it'll return the same object
         readonly Dictionary<string, DataSource> _ownedSources = new Dictionary<string, DataSource>();
         // need to keep delegate around to prevent GC
@@ -28,6 +27,9 @@ namespace NTwain
         // for windows only
         readonly WinMsgLoop _winMsgLoop;
 
+        private IntPtr _hWnd;
+        private IThreadContext _extSyncContext;
+
 
         /// <summary>
         /// Constructs a new <see cref="TwainSession"/>.
@@ -49,27 +51,53 @@ namespace NTwain
         }
 
         /// <summary>
-        /// Synchronously invokes an action on the internal thread if possible.
+        /// Sets the optional synchronization context.
+        /// Because most TWAIN-related things are happening on a different thread,
+        /// this allows events to be raised on the thread associated with this context and
+        /// may be useful if you want to handle them in the UI thread.
+        /// </summary>
+        /// <param name="context">Usually you want to use <see cref="SynchronizationContext.Current"/> while on the UI thread.</param>
+        public void SetSynchronizationContext(SynchronizationContext context)
+        {
+            _extSyncContext = new UIThreadContext(context);
+        }
+
+        /// <summary>
+        /// Synchronously invokes an action on the external user thread if possible.
         /// </summary>
         /// <param name="action"></param>
-        /// <exception cref="ArgumentNullException"></exception>
+        void ExternalInvoke(Action action)
-        public void Invoke(Action action)
         {
-            if (action == null) throw new ArgumentNullException(nameof(action));
+            if (_extSyncContext != null) _extSyncContext.Invoke(action);
+            action();
+        }
 
+        /// <summary>
+        /// Asynchronously invokes an action on the external user thread if possible.
+        /// </summary>
+        /// <param name="action"></param>
+        void ExternalBeginInvoke(Action action)
+        {
+            if (_extSyncContext != null) _extSyncContext.BeginInvoke(action);
+            action();
+        }
+
+        /// <summary>
+        /// Synchronously invokes an action on the internal TWAIN thread if possible.
+        /// </summary>
+        /// <param name="action"></param>
+        internal void InternalInvoke(Action action)
+        {
             if (_winMsgLoop != null) _winMsgLoop.Invoke(action);
             else action();
         }
 
         /// <summary>
-        /// Asynchronously invokes an action on the internal thread if possible.
+        /// Asynchronously invokes an action on the internal TWAIN thread if possible.
         /// </summary>
         /// <param name="action"></param>
-        /// <exception cref="ArgumentNullException"></exception>
+        void InternalBeginInvoke(Action action)
-        public void BeginInvoke(Action action)
         {
-            if (action == null) throw new ArgumentNullException(nameof(action));
-
             if (_winMsgLoop != null) _winMsgLoop.BeginInvoke(action);
             else action();
         }