Added first struct selection logic by platform & bitness.

src/NTwain/Triplets/BaseTriplet.cs

@@ -20,6 +20,15 @@ namespace NTwain.Triplets
         protected BaseTriplet(TwainSession session)
         {
             this.Session = session ?? throw new ArgumentNullException(nameof(session));
-        }
+
+            // windows can always use 32bit structs even in 64bit app
+            Use32BitData = session.Config.Platform == System.PlatformID.Win32NT ||
+                !session.Config.Is64Bit;
+        }
+
+        /// <summary>
+        /// Whether to use 32bit data structures.
+        /// </summary>
+        protected readonly bool Use32BitData;
     }
 }

src/NTwain/Triplets/Control/Callback.cs

@@ -8,9 +8,13 @@ namespace NTwain.Triplets.Control
 		internal Callback(TwainSession session) : base(session) { }
 
 		public ReturnCode RegisterCallback(ref TW_CALLBACK callback)
+        {
+            if (Use32BitData)
             {
                 return NativeMethods.Dsm32(Session.Config.App32, Session.CurrentSource.Identity,
                     DataGroups.Control, DataArgumentType.Callback, Message.RegisterCallback, ref callback);
             }
+            return ReturnCode.Failure;
+        }
 	}
 }

src/NTwain/Triplets/Control/Callback2.cs

@@ -8,9 +8,13 @@ namespace NTwain.Triplets.Control
         internal Callback2(TwainSession session) : base(session) { }
 
         public ReturnCode RegisterCallback(ref TW_CALLBACK2 callback)
+        {
+            if (Use32BitData)
             {
                 return NativeMethods.Dsm32(Session.Config.App32, Session.CurrentSource.Identity,
                 DataGroups.Control, DataArgumentType.Callback2, Message.RegisterCallback, ref callback);
             }
+            return ReturnCode.Failure;
+        }
     }
 }

src/NTwain/Triplets/Control/DeviceEvent.cs

@@ -8,9 +8,13 @@ namespace NTwain.Triplets.Control
 		internal DeviceEvent(TwainSession session) : base(session) { }
 
         public ReturnCode Get(ref TW_DEVICEEVENT sourceDeviceEvent)
+        {
+            if (Use32BitData)
             {
                 return NativeMethods.Dsm32(Session.Config.App32, Session.CurrentSource.Identity,
                 DataGroups.Control, DataArgumentType.DeviceEvent, Message.Get, ref sourceDeviceEvent);
             }
+            return ReturnCode.Failure;
+        }
 	}
 }

src/NTwain/Triplets/Control/EntryPoint.cs

@@ -13,8 +13,12 @@ namespace NTwain.Triplets.Control
         public ReturnCode Get(out TW_ENTRYPOINT entryPoint)
         {
             entryPoint = new TW_ENTRYPOINT();
+            if (Use32BitData)
+            {
                 return NativeMethods.Dsm32(Session.Config.App32, null,
                 DataGroups.Control, DataArgumentType.EntryPoint, Message.Get, entryPoint);
             }
+            return ReturnCode.Failure;
+        }
     }
 }

src/NTwain/Triplets/Control/Identity.cs

@@ -10,8 +10,14 @@ namespace NTwain.Triplets.Control
 
         public ReturnCode CloseDS(TW_IDENTITY source)
         {
-            var rc = NativeMethods.Dsm32(Session.Config.App32, IntPtr.Zero,
+            var rc = ReturnCode.Failure;
+
+            if (Use32BitData)
+            {
+                rc = NativeMethods.Dsm32(Session.Config.App32, IntPtr.Zero,
                 DataGroups.Control, DataArgumentType.Identity, Message.CloseDS, source);
+            }
+
             if (rc == ReturnCode.Success)
             {
                 Session.State = TwainState.S3;
@@ -23,17 +29,25 @@ namespace NTwain.Triplets.Control
         public ReturnCode GetDefault(out TW_IDENTITY source)
         {
             source = new TW_IDENTITY();
+            if (Use32BitData)
+            {
                 return NativeMethods.Dsm32(Session.Config.App32, IntPtr.Zero,
                 DataGroups.Control, DataArgumentType.Identity, Message.GetDefault, source);
             }
+            return ReturnCode.Failure;
+        }
 
 
         public ReturnCode GetFirst(out TW_IDENTITY source)
         {
             source = new TW_IDENTITY();
+            if (Use32BitData)
+            {
                 return NativeMethods.Dsm32(Session.Config.App32, IntPtr.Zero,
                 DataGroups.Control, DataArgumentType.Identity, Message.GetFirst, source);
             }
+            return ReturnCode.Failure;
+        }
 
         public ReturnCode GetNext(out TW_IDENTITY source)
         {
@@ -45,9 +59,14 @@ namespace NTwain.Triplets.Control
         public ReturnCode OpenDS(TW_IDENTITY source)
         {
             Session.StepDown(TwainState.DsmOpened);
+            var rc = ReturnCode.Failure;
 
-            var rc = NativeMethods.Dsm32(Session.Config.App32, IntPtr.Zero,
+            if (Use32BitData)
+            {
+                rc = NativeMethods.Dsm32(Session.Config.App32, IntPtr.Zero,
                 DataGroups.Control, DataArgumentType.Identity, Message.OpenDS, source);
+            }
+
             if (rc == ReturnCode.Success)
             {
                 Session.CurrentSource = Session.GetSourceSingleton(source);
@@ -58,16 +77,24 @@ namespace NTwain.Triplets.Control
         }
 
         public ReturnCode Set(DataSource source)
+        {
+            if (Use32BitData)
             {
                 return NativeMethods.Dsm32(Session.Config.App32, IntPtr.Zero,
                 DataGroups.Control, DataArgumentType.Identity, Message.Set, source?.Identity);
             }
+            return ReturnCode.Failure;
+        }
 
         public ReturnCode UserSelect(out TW_IDENTITY source)
         {
             source = new TW_IDENTITY();
+            if (Use32BitData)
+            {
                 return NativeMethods.Dsm32(Session.Config.App32, IntPtr.Zero,
                 DataGroups.Control, DataArgumentType.Identity, Message.UserSelect, source);
             }
+            return ReturnCode.Failure;
+        }
     }
 }

src/NTwain/Triplets/Control/Parent.cs

@@ -12,8 +12,14 @@ namespace NTwain.Triplets.Control
 
         public ReturnCode OpenDSM(IntPtr hWnd)
         {
-            var rc = NativeMethods.Dsm32(Session.Config.App32, null, 
+            var rc = ReturnCode.Failure;
+
+            if (Use32BitData)
+            {
+                rc = NativeMethods.Dsm32(Session.Config.App32, null,
                 DataGroups.Control, DataArgumentType.Parent, Message.OpenDSM, ref hWnd);
+            }
+
             if (rc == ReturnCode.Success)
             {
                 Session.State = TwainState.DsmOpened;
@@ -37,9 +43,18 @@ namespace NTwain.Triplets.Control
 
         public ReturnCode CloseDSM(IntPtr hWnd)
         {
-            var rc = NativeMethods.Dsm32(Session.Config.App32, null, DataGroups.Control, 
+            var rc = ReturnCode.Failure;
+
+            if (Use32BitData)
+            {
+                rc = NativeMethods.Dsm32(Session.Config.App32, null, DataGroups.Control,
                 DataArgumentType.Parent, Message.CloseDSM, ref hWnd);
-            if (rc == ReturnCode.Success) Session.State = TwainState.DsmLoaded;
+            }
+
+            if (rc == ReturnCode.Success)
+            {
+                Session.State = TwainState.DsmLoaded;
+            }
             return rc;
         }
     }