Expose wndproc hook method and test reset TW_PENDINGXFERS before use.

src/NTwain/Data/TWAINH.cs

@@ -2426,6 +2426,11 @@ namespace NTwain.Data
     [StructLayout(LayoutKind.Sequential, Pack = 2)]
     public struct TW_PENDINGXFERS
     {
+        public TW_PENDINGXFERS()
+        {
+            Count = -1;
+        }
+
         public short Count;
         public uint EOJ;
     }

src/NTwain/TwainAppSession.Windows.cs

@@ -69,7 +69,15 @@ namespace NTwain
             return IntPtr.Zero;
         }
 
-    private bool WndProc(IntPtr hWnd, int msg, IntPtr wParam, IntPtr lParam)
+        /// <summary>
+        /// Method to check window message and handle it if it's for the TWAIN source.'
+        /// </summary>
+        /// <param name="hWnd"></param>
+        /// <param name="msg"></param>
+        /// <param name="wParam"></param>
+        /// <param name="lParam"></param>
+        /// <returns></returns>
+        public 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

@@ -60,7 +60,7 @@ namespace NTwain
             if (xferImage) imgXferMech = Caps.ICAP_XFERMECH.GetCurrent().FirstOrDefault();
             else if (xferAudio) audXferMech = Caps.ACAP_XFERMECH.GetCurrent().FirstOrDefault();
 
-      TW_PENDINGXFERS pending = default;
+            TW_PENDINGXFERS pending = new TW_PENDINGXFERS();
             var sts = WrapInSTS(DGControl.PendingXfers.Get(ref _appIdentity, ref _currentDS, ref pending));
             if (sts.RC == TWRC.SUCCESS)
             {
@@ -76,11 +76,13 @@ namespace NTwain
                     if (readyArgs.Cancel == CancelType.EndNow || _closeDsRequested)
                     {
                         // TODO: need to call EndXfer first?
+                        pending = new TW_PENDINGXFERS();
                         sts = WrapInSTS(DGControl.PendingXfers.Reset(ref _appIdentity, ref _currentDS, ref pending));
                         if (sts.RC == TWRC.SUCCESS && xferImage) State = STATE.S5;
                     }
                     else if (readyArgs.Cancel == CancelType.SkipCurrent)
                     {
+                        pending = new TW_PENDINGXFERS();
                         sts = WrapInSTS(DGControl.PendingXfers.EndXfer(ref _appIdentity, ref _currentDS, ref pending));
                         if (sts.RC == TWRC.SUCCESS)
                         {
@@ -99,6 +101,7 @@ namespace NTwain
                         if (readyArgs.Cancel == CancelType.Graceful)
                         {
                             // ignore rc in this and continue transfer as normal
+                            pending = new TW_PENDINGXFERS();
                             DGControl.PendingXfers.StopFeeder(ref _appIdentity, ref _currentDS, ref pending);
                         }
 
@@ -177,7 +180,9 @@ namespace NTwain
                         TransferCanceled?.Invoke(this, new TransferCanceledEventArgs());
                     }
                     catch { }
+                    pending = new TW_PENDINGXFERS();
                     sts = WrapInSTS(DGControl.PendingXfers.EndXfer(ref _appIdentity, ref _currentDS, ref pending));
+                    pending = new TW_PENDINGXFERS();
                     sts = WrapInSTS(DGControl.PendingXfers.Reset(ref _appIdentity, ref _currentDS, ref pending));
                     if (sts.RC == TWRC.SUCCESS) State = STATE.S5;
                     break;
@@ -192,7 +197,7 @@ namespace NTwain
                         case TWCC.NOMEDIA:
                         case TWCC.PAPERDOUBLEFEED:
                         case TWCC.PAPERJAM:
-              pending = default;
+                            pending = new TW_PENDINGXFERS();
                             sts = WrapInSTS(DGControl.PendingXfers.EndXfer(ref _appIdentity, ref _currentDS, ref pending));
                             break;
                         case TWCC.OPERATIONERROR:
@@ -200,7 +205,9 @@ namespace NTwain
                             if (_userInterface.ShowUI == 0 && indicators == TW_BOOL.False)
                             {
                                 // todo: alert user and drop to S4
+                                pending = new TW_PENDINGXFERS();
                                 sts = WrapInSTS(DGControl.PendingXfers.EndXfer(ref _appIdentity, ref _currentDS, ref pending));
+                                pending = new TW_PENDINGXFERS();
                                 sts = WrapInSTS(DGControl.PendingXfers.Reset(ref _appIdentity, ref _currentDS, ref pending));
                             }
                             break;
@@ -228,7 +235,8 @@ namespace NTwain
                     Transferred?.Invoke(this, args);
                 }
                 catch { }
-;
+
+                pending = new TW_PENDINGXFERS();
                 sts = WrapInSTS(DGControl.PendingXfers.EndXfer(ref _appIdentity, ref _currentDS, ref pending));
                 if (sts.RC == TWRC.SUCCESS)
                 {
@@ -268,6 +276,7 @@ namespace NTwain
                     }
                 }
 
+                pending = new TW_PENDINGXFERS();
                 sts = WrapInSTS(DGControl.PendingXfers.EndXfer(ref _appIdentity, ref _currentDS, ref pending));
                 if (sts.RC == TWRC.SUCCESS)
                 {
@@ -351,6 +360,7 @@ namespace NTwain
                 }
                 catch { }
 
+                pending = new TW_PENDINGXFERS();
                 var sts = WrapInSTS(DGControl.PendingXfers.EndXfer(ref _appIdentity, ref _currentDS, ref pending));
                 if (sts.RC == TWRC.SUCCESS)
                 {
@@ -428,6 +438,7 @@ namespace NTwain
                 }
                 catch { }
 
+                pending = new TW_PENDINGXFERS();
                 var sts = WrapInSTS(DGControl.PendingXfers.EndXfer(ref _appIdentity, ref _currentDS, ref pending));
                 if (sts.RC == TWRC.SUCCESS)
                 {
@@ -456,6 +467,7 @@ namespace NTwain
                 }
                 catch { }
 
+                pending = new TW_PENDINGXFERS();
                 sts = WrapInSTS(DGControl.PendingXfers.EndXfer(ref _appIdentity, ref _currentDS, ref pending));
                 if (sts.RC == TWRC.SUCCESS)
                 {
@@ -509,6 +521,7 @@ namespace NTwain
                     }
 
 
+                    pending = new TW_PENDINGXFERS();
                     sts = WrapInSTS(DGControl.PendingXfers.EndXfer(ref _appIdentity, ref _currentDS, ref pending));
                     if (sts.RC == TWRC.SUCCESS)
                     {