Made callback a struct.

src/NTwain/Data/TwainTypes.cs

@@ -89,21 +89,19 @@ namespace NTwain.Data
 
     
     [StructLayout(LayoutKind.Sequential, Pack = 2)]
-    partial class TW_CALLBACK
+    partial struct TW_CALLBACK
     {
-        [MarshalAs(UnmanagedType.FunctionPtr)]
+        public TW_MEMREF CallBackProc;
-        CallbackDelegate _callBackProc;
+        TW_UINT32 RefCon;
-        TW_UINT32 _refCon;
+        TW_INT16 Message;
-        TW_INT16 _message;
     }
 
     [StructLayout(LayoutKind.Sequential, Pack = 2)]
-    partial class TW_CALLBACK2
+    partial struct TW_CALLBACK2
     {
-        [MarshalAs(UnmanagedType.FunctionPtr)]
+        public TW_MEMREF CallBackProc;
-        CallbackDelegate _callBackProc;
+        TW_UINTPTR RefCon;
-        TW_UINTPTR _refCon;
+        TW_INT16 Message;
-        TW_INT16 _message;
     }
 
     [StructLayout(LayoutKind.Sequential, Pack = 2)]
@@ -542,6 +540,7 @@ namespace NTwain.Data
         TW_HANDLE _hParent;
     }
 
+    [UnmanagedFunctionPointer(CallingConvention.Cdecl)]
     delegate ReturnCode CallbackDelegate(TW_IDENTITY origin, TW_IDENTITY destination,
             DataGroups dg, DataArgumentType dat, Message msg, TW_MEMREF data);
 

src/NTwain/Data/TwainTypesExtended.cs

@@ -570,71 +570,6 @@ namespace NTwain.Data
         public string Name { get { return _name; } }
     }
 
-
-    /// <summary>
-    /// Used in Callback mechanism for sending messages from the Source to the Application.
-    /// Applications version 2.2 or higher must use <see cref="TW_CALLBACK2"/>.
-    /// </summary>
-    partial class TW_CALLBACK
-    {
-        /// <summary>
-        /// Initializes a new instance of the <see cref="TWCallback"/> class.
-        /// </summary>
-        /// <param name="callback">The callback function’s entry point.</param>
-        public TW_CALLBACK(CallbackDelegate callback)
-        {
-            _callBackProc = callback;
-        }
-
-        ///// <summary>
-        ///// An application defined reference constant.
-        ///// </summary>
-        ///// <value>
-        ///// The reference constant.
-        ///// </value>
-        //public uint RefCon { get { return _refCon; } set { _refCon = value; } }
-
-        ///// <summary>
-        ///// Initialized to any valid DG_CONTROL / DAT_NULL message.
-        ///// </summary>
-        ///// <value>
-        ///// The message.
-        ///// </value>
-        //public short Message { get { return _message; } set { _message = value; } }
-    }
-    /// <summary>
-    /// Used in the Callback mechanism for sending messages from the Source to the Application.
-    /// </summary>
-    partial class TW_CALLBACK2
-    {
-        /// <summary>
-        /// Initializes a new instance of the <see cref="TW_CALLBACK2"/> class.
-        /// </summary>
-        /// <param name="callback">The callback function’s entry point.</param>
-        public TW_CALLBACK2(CallbackDelegate callback)
-        {
-            _callBackProc = callback;
-        }
-
-        ///// <summary>
-        ///// An application defined reference constant. It has a different size on different
-        ///// platforms.
-        ///// </summary>
-        ///// <value>
-        ///// The reference constant.
-        ///// </value>
-        //public UIntPtr RefCon { get { return _refCon; } set { _refCon = value; } }
-
-        ///// <summary>
-        ///// Initialized to any valid DG_CONTROL / DAT_NULL message.
-        ///// </summary>
-        ///// <value>
-        ///// The message.
-        ///// </value>
-        //public short Message { get { return _message; } set { _message = value; } }
-    }
-
-
     //    /// <summary>
     //    /// Used by an application either to get information about, or control the setting of a capability.
     //    /// </summary>

src/NTwain/Triplets/DGControl.Callback.cs

@@ -7,10 +7,10 @@ namespace NTwain.Triplets
 	{
 		internal Callback(TwainSession session) : base(session) { }
 
-		public ReturnCode RegisterCallback(TW_CALLBACK callback)
+		public ReturnCode RegisterCallback(ref TW_CALLBACK callback)
         {
             return NativeMethods.DsmWin32(Session.Config.AppWin32, Session.CurrentSource.Identity,
-                DataGroups.Control, DataArgumentType.Callback2, Message.RegisterCallback, callback);
+                DataGroups.Control, DataArgumentType.Callback, Message.RegisterCallback, ref callback);
         }
 	}
 }

src/NTwain/Triplets/DGControl.Callback2.cs

@@ -7,10 +7,10 @@ namespace NTwain.Triplets
     {
         internal Callback2(TwainSession session) : base(session) { }
 
-        public ReturnCode RegisterCallback(TW_CALLBACK2 callback)
+        public ReturnCode RegisterCallback(ref TW_CALLBACK2 callback)
         {
             return NativeMethods.DsmWin32(Session.Config.AppWin32, Session.CurrentSource.Identity,
-                DataGroups.Control, DataArgumentType.Callback2, Message.RegisterCallback, callback);
+                DataGroups.Control, DataArgumentType.Callback2, Message.RegisterCallback, ref callback);
         }
     }
 }

src/NTwain/Triplets/DGControl.Identity.cs

@@ -16,7 +16,6 @@ namespace NTwain.Triplets
             {
                 Session.State = TwainState.S3;
                 Session.CurrentSource = null;
-                Session.UpdateCallback();
             }
             return rc;
         }
@@ -51,7 +50,7 @@ namespace NTwain.Triplets
             {
                 Session.CurrentSource = Session.GetSourceSingleton(source);
                 Session.State = TwainState.S4;
-                Session.UpdateCallback();
+                Session.RegisterCallback();
             }
             return rc;
         }

src/NTwain/Triplets/DGControl.Parent.cs

@@ -10,7 +10,7 @@ namespace NTwain.Triplets
     {
         internal Parent(TwainSession session) : base(session) { }
 
-        public ReturnCode OpenDSM(ref IntPtr hWnd)
+        public ReturnCode OpenDSM(IntPtr hWnd)
         {
             var rc = NativeMethods.DsmWin32(Session.Config.AppWin32, null, 
                 DataGroups.Control, DataArgumentType.Parent, Message.OpenDSM, ref hWnd);
@@ -29,14 +29,14 @@ namespace NTwain.Triplets
                     }
                     else
                     {
-                        rc = CloseDSM(ref hWnd);
+                        rc = CloseDSM(hWnd);
                     }
                 }
             }
             return rc;
         }
 
-        public ReturnCode CloseDSM(ref IntPtr hWnd)
+        public ReturnCode CloseDSM(IntPtr hWnd)
         {
             var rc = NativeMethods.DsmWin32(Session.Config.AppWin32, null, DataGroups.Control, 
                 DataArgumentType.Parent, Message.CloseDSM, ref hWnd);

src/NTwain/Triplets/NativeMethods.Win32.cs

@@ -70,7 +70,7 @@ namespace NTwain.Triplets
             DataGroups dg,
             DataArgumentType dat,
             Message msg,
-            [In, Out]TW_CALLBACK data);
+            ref TW_CALLBACK data);
 
         [DllImport(WinDsmDll, EntryPoint = EntryName)]
         public static extern ReturnCode DsmWin32(
@@ -79,7 +79,7 @@ namespace NTwain.Triplets
             DataGroups dg,
             DataArgumentType dat,
             Message msg,
-            [In, Out]TW_CALLBACK2 data);
+            ref TW_CALLBACK2 data);
 
         [DllImport(WinDsmDll, EntryPoint = EntryName)]
         public static extern ReturnCode DsmWin32(

src/NTwain/TwainSession.MsgHandling.cs

@@ -0,0 +1,21 @@
+using NTwain.Data;
+using NTwain.Triplets;
+using System;
+using System.Collections.Generic;
+using System.ComponentModel;
+using System.Diagnostics;
+using System.Linq;
+using System.Runtime.InteropServices;
+using System.Text;
+using System.Threading;
+
+namespace NTwain
+{
+    partial class TwainSession
+    {
+        private void HandleSourceMsg(Message msg)
+        {
+            
+        }
+    }
+}

src/NTwain/TwainSession.cs

@@ -3,8 +3,11 @@ using NTwain.Triplets;
 using System;
 using System.Collections.Generic;
 using System.ComponentModel;
+using System.Diagnostics;
 using System.Linq;
+using System.Runtime.InteropServices;
 using System.Text;
+using System.Threading;
 
 namespace NTwain
 {
@@ -14,10 +17,12 @@ namespace NTwain
     public partial class TwainSession
     {
         internal readonly TwainConfig Config;
+
         private IntPtr _hWnd;
-        object _callbackObj; // kept around in this class so it doesn't get gc'ed
         // 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?
+        readonly CallbackDelegate _callbackDelegate;
 
         /// <summary>
         /// Constructs a new <see cref="TwainSession"/>.
@@ -26,6 +31,14 @@ namespace NTwain
         public TwainSession(TwainConfig config)
         {
             Config = config;
+            switch (config.Platform)
+            {
+                case PlatformID.MacOSX:
+                case PlatformID.Unix:
+                default:
+                    _callbackDelegate = new CallbackDelegate(HandleWin32Callback);
+                    break;
+            }
         }
 
         /// <summary>
@@ -36,7 +49,7 @@ namespace NTwain
         public ReturnCode Open(IntPtr hWnd)
         {
             _hWnd = hWnd;
-            return DGControl.Parent.OpenDSM(ref hWnd);
+            return DGControl.Parent.OpenDSM(hWnd);
         }
 
         /// <summary>
@@ -52,7 +65,7 @@ namespace NTwain
                 switch (State)
                 {
                     case TwainState.DsmOpened:
-                        rc = DGControl.Parent.CloseDSM(ref _hWnd);
+                        rc = DGControl.Parent.CloseDSM(_hWnd);
                         if (rc != ReturnCode.Success) return rc;
                         break;
                     case TwainState.SourceOpened:
@@ -64,38 +77,37 @@ namespace NTwain
             return rc;
         }
 
-        internal void UpdateCallback()
+        internal void RegisterCallback()
         {
-            if (State < TwainState.S4)
+            if (State == TwainState.S4)
-            {
-                _callbackObj = null;
-            }
-            else
             {
+                var callbackPtr = Marshal.GetFunctionPointerForDelegate(_callbackDelegate);
+
                 var rc = ReturnCode.Failure;
                 // per the spec (8-10) apps for 2.2 or higher uses callback2 so try this first
                 if (Config.AppWin32.ProtocolMajor > 2 ||
                     (Config.AppWin32.ProtocolMajor >= 2 && Config.AppWin32.ProtocolMinor >= 2))
                 {
-                    var cb = new TW_CALLBACK2(HandleCallback);
+                    var cb = new TW_CALLBACK2 { CallBackProc = callbackPtr };
-                    rc = DGControl.Callback2.RegisterCallback(cb);
+                    rc = DGControl.Callback2.RegisterCallback(ref cb);
-
+                    if (rc == ReturnCode.Success) Debug.WriteLine("Registed Callback2 success.");
-                    if (rc == ReturnCode.Success)
+                    else
                     {
-                        _callbackObj = cb;
+
                     }
                 }
 
                 if (rc != ReturnCode.Success)
                 {
                     // always try old callback
-                    var cb = new TW_CALLBACK(HandleCallback);
+                    var cb = new TW_CALLBACK { CallBackProc = callbackPtr };
 
-                    rc = DGControl.Callback.RegisterCallback(cb);
+                    rc = DGControl.Callback.RegisterCallback(ref cb);
 
-                    if (rc == ReturnCode.Success)
+                    if (rc == ReturnCode.Success) Debug.WriteLine("Registed Callback success.");
+                    else
                     {
-                        _callbackObj = cb;
+
                     }
                 }
             }
@@ -108,8 +120,7 @@ namespace NTwain
         /// <returns></returns>
         public IEnumerable<DataSource> GetSources()
         {
-            TW_IDENTITY srcId;
+            var rc = DGControl.Identity.GetFirst(out TW_IDENTITY srcId);
-            var rc = DGControl.Identity.GetFirst(out srcId);
             while (rc == ReturnCode.Success)
             {
                 yield return GetSourceSingleton(srcId);
@@ -124,8 +135,7 @@ namespace NTwain
         {
             get
             {
-                TW_IDENTITY src;
+                if (DGControl.Identity.GetDefault(out TW_IDENTITY src) == ReturnCode.Success)
-                if (DGControl.Identity.GetDefault(out src) == ReturnCode.Success)
                 {
                     return GetSourceSingleton(src);
                 }
@@ -151,8 +161,7 @@ namespace NTwain
         /// <returns></returns>
         public DataSource ShowSourceSelector()
         {
-            TW_IDENTITY id;
+            if (DGControl.Identity.UserSelect(out TW_IDENTITY id) == ReturnCode.Success)
-            if (DGControl.Identity.UserSelect(out id) == ReturnCode.Success)
             {
                 return GetSourceSingleton(id);
             }
@@ -182,35 +191,12 @@ namespace NTwain
             return source;
         }
 
-        ReturnCode HandleCallback(TW_IDENTITY origin, TW_IDENTITY destination,
+        ReturnCode HandleWin32Callback(TW_IDENTITY origin, TW_IDENTITY destination,
             DataGroups dg, DataArgumentType dat, Message msg, IntPtr data)
         {
-            //if (origin != null && CurrentSource != null && origin.Id == CurrentSource.Identity.Id && State >= S5)
+            Debug.WriteLine($"Thread {Thread.CurrentThread.ManagedThreadId}: {nameof(HandleWin32Callback)}({dg}, {dat}, {msg}, {data})");
-            //{
+            HandleSourceMsg(msg);
-            //    // spec says we must handle this on the thread that enabled the DS.
+            return ReturnCode.Success;
-            //    // by using the internal dispatcher this will be the case.
-
-            //    // In any event the trick to get this thing working is to return from the callback first
-            //    // before trying to process the msg or there will be unpredictable errors.
-
-            //    // changed to sync invoke instead of begininvoke for hp scanjet.
-            //    if (origin.ProductName.IndexOf("scanjet", StringComparison.OrdinalIgnoreCase) > -1)
-            //    {
-            //        _msgLoopHook?.Invoke(() =>
-            //        {
-            //            HandleSourceMsg(msg);
-            //        });
-            //    }
-            //    else
-            //    {
-            //        _msgLoopHook?.BeginInvoke(() =>
-            //        {
-            //            HandleSourceMsg(msg);
-            //        });
-            //    }
-            //    return ReturnCode.Success;
-            //}
-            return ReturnCode.Failure;
         }
     }
 }