Changed to use SynchronizationContext instead of the DIY threadmarshaller.

samples/WinForm32/Form1.cs

@@ -11,6 +11,7 @@ using System.IO;
 using System.Linq;
 using System.Reflection;
 using System.Runtime.Versioning;
+using System.Threading;
 using System.Windows.Forms;
 
 namespace WinFormSample
@@ -28,7 +29,7 @@ namespace WinFormSample
 
       TWPlatform.PreferLegacyDSM = false;
 
-      twain = new TwainAppSession(new WinformMarshaller(this), Assembly.GetExecutingAssembly().Location);
+      twain = new TwainAppSession(SynchronizationContext.Current!, Assembly.GetExecutingAssembly().Location);
       twain.StateChanged += Twain_StateChanged;
       twain.DefaultSourceChanged += Twain_DefaultSourceChanged;
       twain.CurrentSourceChanged += Twain_CurrentSourceChanged;

src/NTwain/ThreadMarshaller.Windows.cs

@@ -1,58 +0,0 @@
-#if WINDOWS || NETFRAMEWORK
-using System;
-using System.Windows.Forms;
-using System.Windows.Threading;
-
-namespace NTwain
-{
-  /// <summary>
-  /// An <see cref="IThreadMarshaller"/> that can be used
-  /// to integrate <see cref="TwainAppSession"/> with
-  /// an existing Winforms app.
-  /// </summary>
-  public class WinformMarshaller : IThreadMarshaller
-  {
-    private readonly Control control;
-
-    public WinformMarshaller(System.Windows.Forms.Control control)
-    {
-      this.control = control;
-    }
-
-    public void BeginInvoke(Delegate work, params object[] args)
-    {
-      control.BeginInvoke(work, args);
-    }
-
-    public object? Invoke(Delegate work, params object[] args)
-    {
-      return control.Invoke(work, args);
-    }
-  }
-
-  /// <summary>
-  /// An <see cref="IThreadMarshaller"/> that can be used
-  /// to integrate <see cref="TwainAppSession"/> with
-  /// an existing WPF app.
-  /// </summary>
-  public class WpfMarshaller : IThreadMarshaller
-  {
-    private readonly Dispatcher dispatcher;
-
-    public WpfMarshaller(Dispatcher dispatcher)
-    {
-      this.dispatcher = dispatcher;
-    }
-
-    public void BeginInvoke(Delegate work, params object[] args)
-    {
-      dispatcher.BeginInvoke(work, args);
-    }
-
-    public object? Invoke(Delegate work, params object[] args)
-    {
-      return dispatcher.Invoke(work, args);
-    }
-  }
-}
-#endif

src/NTwain/ThreadMarshaller.cs

@@ -1,46 +0,0 @@
-using System;
-
-namespace NTwain
-{
-  /// <summary>
-  /// Allows work to be marshalled to a different (usually UI) thread as necessary.
-  /// </summary>
-  public interface IThreadMarshaller
-  {
-    /// <summary>
-    /// Starts work asynchronously and returns immediately.
-    /// </summary>
-    /// <param name="work"></param>
-    /// <param name="args"></param>
-    void BeginInvoke(Delegate work, params object[] args);
-
-    /// <summary>
-    /// Starts work synchronously until it returns.
-    /// </summary>
-    /// <param name="work"></param>
-    /// <param name="args"></param>
-    /// <returns></returns>
-    object? Invoke(Delegate work, params object[] args);
-  }
-
-  /// <summary>
-  /// No marshalling occurs. Invokes happen right in place synchronously.
-  /// </summary>
-  public class InPlaceMarshaller : IThreadMarshaller
-  {
-    /// <summary>
-    /// No async invocation here.
-    /// </summary>
-    /// <param name="work"></param>
-    /// <param name="args"></param>
-    public void BeginInvoke(Delegate work, params object[] args)
-    {
-      work.DynamicInvoke(args);
-    }
-
-    public object? Invoke(Delegate work, params object[] args)
-    {
-      return work.DynamicInvoke(args);
-    }
-  }
-}

src/NTwain/TwainAppSession.Callbacks.cs

@@ -112,23 +112,24 @@ namespace NTwain
           if (!_inTransfer)
           {
             // this should be done on ui thread (or same one that enabled the ds)
-            _uiThreadMarshaller.BeginInvoke(() =>
+            _uiThreadMarshaller.Post(obj =>
             {
-              DisableSource();
+              ((TwainAppSession)obj!).DisableSource();
-            });
+            }, this);
           }
           break;
         case MSG.DEVICEEVENT:
           if (DeviceEvent != null && DGControl.DeviceEvent.Get(ref _appIdentity, ref _currentDS, out TW_DEVICEEVENT de) == TWRC.SUCCESS)
           {
-            _uiThreadMarshaller.BeginInvoke(() =>
+            _uiThreadMarshaller.Post(obj =>
             {
               try
               {
-                DeviceEvent.Invoke(this, de);
+                var twain = (TwainAppSession)obj!;
+                twain.DeviceEvent!.Invoke(twain, de);
               }
               catch { }
-            });
+            }, this);
           }
           break;
       }

src/NTwain/TwainAppSession.PropEvents.cs

@@ -57,17 +57,14 @@ namespace NTwain
         if (_state != value)
         {
           _state = value;
-          if (StateChanged != null)
+          _uiThreadMarshaller.Send(obj =>
           {
-            _uiThreadMarshaller.Invoke(() =>
+            try
             {
-              try
+              ((TwainAppSession)obj!).StateChanged?.Invoke(this, value);
-              {
+            }
-                StateChanged.Invoke(this, value);
+            catch { }
-              }
+          }, this);
-              catch { }
-            });
-          }
         }
       }
     }

src/NTwain/TwainAppSession.Windows.cs

@@ -60,16 +60,16 @@ namespace NTwain
 
     bool IMessageFilter.PreFilterMessage(ref Message m)
     {
-      return CheckIfTwainMessage(m.HWnd, m.Msg, m.WParam, m.LParam);
+      return WndProc(m.HWnd, m.Msg, m.WParam, m.LParam);
     }
 
     IntPtr WpfHook(IntPtr hwnd, int msg, IntPtr wParam, IntPtr lParam, ref bool handled)
     {
-      handled = CheckIfTwainMessage(hwnd, msg, wParam, lParam);
+      handled = WndProc(hwnd, msg, wParam, lParam);
       return IntPtr.Zero;
     }
 
-    private bool CheckIfTwainMessage(IntPtr hWnd, int msg, IntPtr wParam, IntPtr lParam)
+    private bool WndProc(IntPtr hWnd, int msg, IntPtr wParam, IntPtr lParam)
     {
       // this handles the message from a typical WndProc message loop and checks if it's for the TWAIN source.
       bool handled = false;

src/NTwain/TwainAppSession.Xfers.cs

@@ -74,14 +74,15 @@ namespace NTwain
           var readyArgs = new TransferReadyEventArgs(pending.Count, (TWEJ)pending.EOJ);
           if (TransferReady != null)
           {
-            _uiThreadMarshaller.Invoke(() =>
+            _uiThreadMarshaller.Send(_ =>
             {
               try
               {
+
                 TransferReady.Invoke(this, readyArgs);
               }
               catch { } // don't let consumer kill the loop if they have exception
-            });
+            }, null);
           }
 
           if (readyArgs.Cancel == CancelType.EndNow || _closeDsRequested)
@@ -154,17 +155,15 @@ namespace NTwain
             }
             catch (Exception ex)
             {
-              if (TransferError != null)
+              _uiThreadMarshaller.Send(obj =>
               {
-                _uiThreadMarshaller.Invoke(() =>
+                try
                 {
-                  try
+                  var twain = ((TwainAppSession)obj!);
-                  {
+                  twain.TransferError?.Invoke(twain, new TransferErrorEventArgs(ex));
-                    TransferError?.Invoke(this, new TransferErrorEventArgs(ex));
+                }
-                  }
+                catch { }
-                  catch { }
+              }, this);
-                });
-              }
             }
           }
         } while (sts.RC == TWRC.SUCCESS && pending.Count != 0);
@@ -173,10 +172,10 @@ namespace NTwain
       HandleXferCode(sts);
       if (State >= STATE.S5)
       {
-        _uiThreadMarshaller.Invoke(() =>
+        _uiThreadMarshaller.Send(obj =>
         {
-          DisableSource();
+          ((TwainAppSession)obj!).DisableSource();
-        });
+        }, this);
       }
       _inTransfer = false;
     }
@@ -191,13 +190,11 @@ namespace NTwain
           break;
         case TWRC.CANCEL:
           // might eventually have option to cancel this or all like transfer ready
-          if (TransferCanceled != null)
+          _uiThreadMarshaller.Send(obj =>
           {
-            _uiThreadMarshaller.Invoke(() =>
+            var twain = ((TwainAppSession)obj!);
-            {
+            twain.TransferCanceled?.Invoke(twain, new TransferCanceledEventArgs());
-              TransferCanceled.Invoke(this, new TransferCanceledEventArgs());
+          }, this);
-            });
-          };
           TW_PENDINGXFERS pending = default;
           DGControl.PendingXfers.EndXfer(ref _appIdentity, ref _currentDS, ref pending);
           // todo: also reset?

src/NTwain/TwainAppSession.cs

@@ -23,7 +23,7 @@ namespace NTwain
     /// <param name="exeFilePath"></param>
     /// <param name="appLanguage"></param>
     /// <param name="appCountry"></param>
-    public TwainAppSession(IThreadMarshaller uiThreadMarshaller,
+    public TwainAppSession(SynchronizationContext uiThreadMarshaller,
       string exeFilePath,
       TWLG appLanguage = TWLG.ENGLISH_USA, TWCY appCountry = TWCY.USA) :
       this(uiThreadMarshaller, FileVersionInfo.GetVersionInfo(exeFilePath), appLanguage, appCountry)
@@ -35,7 +35,7 @@ namespace NTwain
     /// <param name="appInfo"></param>
     /// <param name="appLanguage"></param>
     /// <param name="appCountry"></param>
-    public TwainAppSession(IThreadMarshaller uiThreadMarshaller,
+    public TwainAppSession(SynchronizationContext uiThreadMarshaller,
         FileVersionInfo appInfo,
         TWLG appLanguage = TWLG.ENGLISH_USA, TWCY appCountry = TWCY.USA) :
         this(uiThreadMarshaller,
@@ -57,7 +57,7 @@ namespace NTwain
     /// <param name="appLanguage"></param>
     /// <param name="appCountry"></param>
     /// <param name="supportedTypes"></param>
-    public TwainAppSession(IThreadMarshaller uiThreadMarshaller,
+    public TwainAppSession(SynchronizationContext uiThreadMarshaller,
         string companyName, string productFamily, string productName,
         Version productVersion, string productDescription = "",
         TWLG appLanguage = TWLG.ENGLISH_USA, TWCY appCountry = TWCY.USA,
@@ -104,7 +104,7 @@ namespace NTwain
 #endif
     // test threads a bit
     //readonly BlockingCollection<MSG> _bgPendingMsgs = new();
-    private readonly IThreadMarshaller _uiThreadMarshaller;
+    private readonly SynchronizationContext _uiThreadMarshaller;
     bool _closeDsRequested;
     bool _inTransfer;
     readonly AutoResetEvent _xferReady = new(false);