Made TwainSession utilize both internal and external thread contexts.

src/NTwain/Threading/IThreadContext.cs

@@ -7,6 +7,16 @@ namespace NTwain.Threading
     /// </summary>
     interface IThreadContext
     {
+        /// <summary>
+        /// Starts the context if supported.
+        /// </summary>
+        void Start();
+        /// <summary>
+        /// Stops the context if supported.
+        /// </summary>
+        void Stop();
+
+
         /// <summary>
         /// Runs the action synchronously on the associated thread.
         /// </summary>

src/NTwain/Threading/UIThreadContext.cs

@@ -16,15 +16,6 @@ namespace NTwain.Threading
     {
         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"/>.
@@ -32,8 +23,7 @@ namespace NTwain.Threading
         /// <param name="context"></param>
         public UIThreadContext(SynchronizationContext context)
         {
-            this.context = context ?? throw new ArgumentNullException(nameof(context));
+            this.context = context;
-            ;
         }
 
         /// <summary>
@@ -91,5 +81,13 @@ namespace NTwain.Threading
                 BindingFlags.NonPublic | BindingFlags.Instance)?.Invoke(ex, new object[0]);
             throw ex;
         }
+
+        void IThreadContext.Start()
+        {
+        }
+
+        void IThreadContext.Stop()
+        {
+        }
     }
 }

src/NTwain/TwainSession.Disposable.cs

@@ -1,52 +0,0 @@
-//using NTwain.Data;
-//using NTwain.Triplets;
-//using System;
-//using System.Collections.Generic;
-//using System.ComponentModel;
-//using System.Linq;
-//using System.Text;
-
-//namespace NTwain
-//{
-//    partial class TwainSession : IDisposable
-//    {
-//        private bool disposedValue = false; // To detect redundant calls
-
-//        /// <summary>
-//        /// Handles actual disposal logic.
-//        /// </summary>
-//        /// <param name="disposing"></param>
-//        protected virtual void Dispose(bool disposing)
-//        {
-//            if (!disposedValue)
-//            {
-//                if (disposing)
-//                {
-//                    StepDown(TwainState.DsmLoaded);
-//                    // TODO: dispose managed state (managed objects).
-//                }
-
-//                // TODO: free unmanaged resources (unmanaged objects) and override a finalizer below.
-//                // TODO: set large fields to null.
-
-//                disposedValue = true;
-//            }
-//        }
-
-//        // TODO: override a finalizer only if Dispose(bool disposing) above has code to free unmanaged resources.
-//        // ~TwainSession() {
-//        //   // Do not change this code. Put cleanup code in Dispose(bool disposing) above.
-//        //   Dispose(false);
-//        // }
-
-//        /// <summary>
-//        /// Closes any open TWAIN objects.
-//        /// </summary>
-//        public void Dispose()
-//        {
-//            Dispose(true);
-//            // TODO: uncomment the following line if the finalizer is overridden above.
-//            // GC.SuppressFinalize(this);
-//        }
-//    }
-//}

src/NTwain/TwainSession.MsgHandling.cs

@@ -22,6 +22,8 @@ namespace NTwain
         ReturnCode Handle32BitCallback(TW_IDENTITY origin, TW_IDENTITY destination,
             DataGroups dg, DataArgumentType dat, Message msg, IntPtr data)
         {
+            // from the docs this needs to return Success 
+            // before handling the msg thus BeginInvoke is used.
             Debug.WriteLine($"Thread {Thread.CurrentThread.ManagedThreadId}: {nameof(Handle32BitCallback)}({dg}, {dat}, {msg}, {data})");
             InternalBeginInvoke(() =>
             {

src/NTwain/TwainSession.Props.cs

@@ -54,8 +54,11 @@ namespace NTwain
         /// <param name="e"></param>
         internal protected virtual void OnSourceDisabled(SourceDisabledEventArgs e)
         {
-            var handler = SourceDisabled;
+            ExternalInvoke(() =>
-            handler?.Invoke(this, e);
+            {
+                var handler = SourceDisabled;
+                handler?.Invoke(this, e);
+            });
         }
 
         /// <summary>
@@ -69,8 +72,11 @@ namespace NTwain
         /// <param name="e"></param>
         protected virtual void OnDeviceEventReceived(DeviceEventArgs e)
         {
-            var handler = DeviceEventReceived;
+            ExternalInvoke(() =>
-            handler?.Invoke(this, e);
+            {
+                var handler = DeviceEventReceived;
+                handler?.Invoke(this, e);
+            });
         }
 
         /// <summary>
@@ -84,8 +90,11 @@ namespace NTwain
         /// <param name="e"></param>
         protected virtual void OnTransferReady(TransferReadyEventArgs e)
         {
-            var handler = TransferReady;
+            ExternalInvoke(() =>
-            handler?.Invoke(this, e);
+            {
+                var handler = TransferReady;
+                handler?.Invoke(this, e);
+            });
         }
 
 
@@ -93,6 +102,21 @@ namespace NTwain
         ///// Occurs when data has been transferred.
         ///// </summary>
         //public event EventHandler<DataTransferredEventArgs> DataTransferred;
+
+        ///// <summary>
+        ///// Raises the <see cref="DataTransferred"/> event.
+        ///// </summary>
+        ///// <param name="e"></param>
+        //protected virtual void OnDataTransferred(DataTransferredEventArgs e)
+        //{
+        //    ExternalInvoke(() =>
+        //    {
+        //        var handler = DataTransferred;
+        //        handler?.Invoke(this, e);
+        //    });
+        //}
+
+
         /// <summary>
         /// Occurs when an error has been encountered during transfer.
         /// </summary>
@@ -104,8 +128,11 @@ namespace NTwain
         /// <param name="e"></param>
         protected virtual void OnTransferError(TransferErrorEventArgs e)
         {
-            var handler = TransferError;
+            ExternalInvoke(() =>
-            handler?.Invoke(this, e);
+            {
+                var handler = TransferError;
+                handler?.Invoke(this, e);
+            });
         }
 
 
@@ -121,8 +148,11 @@ namespace NTwain
         /// <param name="propertyName">Name of the property.</param>
         protected void RaisePropertyChanged(string propertyName)
         {
-            var handler = PropertyChanged;
+            ExternalInvoke(() =>
-            handler?.Invoke(this, new PropertyChangedEventArgs(propertyName));
+            {
+                var handler = PropertyChanged;
+                handler?.Invoke(this, new PropertyChangedEventArgs(propertyName));
+            });
         }
     }
 }

src/NTwain/TwainSession.cs

@@ -25,10 +25,10 @@ namespace NTwain
         // need to keep delegate around to prevent GC
         readonly Callback32 _callback32Delegate;
         // for windows only
-        readonly WinMsgLoop _winMsgLoop;
+        readonly IThreadContext _internalContext;
 
         private IntPtr _hWnd;
-        private IThreadContext _extSyncContext;
+        private IThreadContext _externalContext;
 
 
         /// <summary>
@@ -39,12 +39,13 @@ namespace NTwain
         public TwainSession(TwainConfig config)
         {
             Config = config ?? throw new ArgumentNullException(nameof(config));
+            SetSynchronizationContext(SynchronizationContext.Current);
             switch (config.Platform)
             {
                 case PlatformID.MacOSX:
                 case PlatformID.Unix:
                 default:
-                    _winMsgLoop = new WinMsgLoop(this);
+                    _internalContext = new WinMsgLoop(this);
                     _callback32Delegate = new Callback32(Handle32BitCallback);
                     break;
             }
@@ -59,7 +60,8 @@ namespace NTwain
         /// <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);
+            if (context == null) _externalContext = null;
+            else _externalContext = new UIThreadContext(context);
         }
 
         /// <summary>
@@ -68,7 +70,7 @@ namespace NTwain
         /// <param name="action"></param>
         void ExternalInvoke(Action action)
         {
-            if (_extSyncContext != null) _extSyncContext.Invoke(action);
+            if (_externalContext != null) _externalContext.Invoke(action);
             action();
         }
 
@@ -78,7 +80,7 @@ namespace NTwain
         /// <param name="action"></param>
         void ExternalBeginInvoke(Action action)
         {
-            if (_extSyncContext != null) _extSyncContext.BeginInvoke(action);
+            if (_externalContext != null) _externalContext.BeginInvoke(action);
             action();
         }
 
@@ -88,7 +90,7 @@ namespace NTwain
         /// <param name="action"></param>
         internal void InternalInvoke(Action action)
         {
-            if (_winMsgLoop != null) _winMsgLoop.Invoke(action);
+            if (_internalContext != null) _internalContext.Invoke(action);
             else action();
         }
 
@@ -98,7 +100,7 @@ namespace NTwain
         /// <param name="action"></param>
         void InternalBeginInvoke(Action action)
         {
-            if (_winMsgLoop != null) _winMsgLoop.BeginInvoke(action);
+            if (_internalContext != null) _internalContext.BeginInvoke(action);
             else action();
         }
 
@@ -113,7 +115,7 @@ namespace NTwain
             var rc = DGControl.Parent.OpenDSM(hWnd);
             if (rc == ReturnCode.Success)
             {
-                _winMsgLoop?.Start();
+                _internalContext?.Start();
             }
             return rc;
         }
@@ -127,7 +129,7 @@ namespace NTwain
             var rc = DGControl.Parent.CloseDSM(_hWnd);
             if (rc == ReturnCode.Success)
             {
-                _winMsgLoop?.Stop();
+                _internalContext?.Stop();
             }
             return rc;
         }