Attempt to resurrect my old triplet ideas with new data types and native methods.

src/NTwain/TWAINWorkingGroup/TWAINH_EXTRAS.cs

@@ -14,7 +14,7 @@ namespace TWAINWorkingGroup
   {
     static TwainPlatform()
     {
-      Is64bit = IntPtr.Size == 8;
+      Is32bit = IntPtr.Size == 4;
 
 #if NETFRAMEWORK
       if (Environment.OSVersion.Platform == PlatformID.Win32NT)
@@ -64,7 +64,14 @@ namespace TWAINWorkingGroup
     /// <summary>
     /// Whether the code is running in 64bit or 32bit.
     /// </summary>
-    public static bool Is64bit { get; }
+    public static bool Is32bit { get; }
+
+    /// <summary>
+    /// Whether to use the older DSM lib on Windows and Mac. 
+    /// On Windows it only takes effect when running in 32bit. 
+    /// Defaults to false.
+    /// </summary>
+    public static bool PreferLegacyDSM { get; set; }
   }
 
   /// <summary>

src/NTwain/Triplets/DATParent.cs

@@ -0,0 +1,85 @@
+using System;
+using TWAINWorkingGroup;
+
+namespace NTwain.Triplets
+{
+  /// <summary>
+  /// Contains calls used with <see cref="DG.CONTROL"/> and <see cref="DAT.PARENT"/>.
+  /// </summary>
+  public class DATParent : TripletBase
+  {
+    public DATParent(TwainSession session) : base(session)
+    {
+    }
+
+    /// <summary>
+    /// Loads and opens the DSM.
+    /// </summary>
+    /// <param name="hwnd">Required if on Windows.</param>
+    /// <returns></returns>
+    public STS OpenDSM(ref IntPtr hwnd)
+    {
+      STS rc;
+      if ((rc = DoIt(MSG.OPENDSM, ref hwnd)) == STS.SUCCESS)
+      {
+        Session.State = STATE.S3;
+      }
+      return rc;
+    }
+
+    /// <summary>
+    /// Closes the DSM.
+    /// </summary>
+    /// <param name="hwnd">Required if on Windows.</param>
+    /// <returns></returns>
+    public STS CloseDSM(ref IntPtr hwnd)
+    {
+      STS rc;
+      if ((rc = DoIt(MSG.CLOSEDSM, ref hwnd)) == STS.SUCCESS)
+      {
+        Session.State = STATE.S2;
+      }
+      return rc;
+    }
+
+
+    STS DoIt(MSG msg, ref IntPtr hwnd)
+    {
+      var rc = STS.FAILURE;
+      if (TwainPlatform.IsWindows)
+      {
+        var app = Session._appIdentityLegacy;
+        if (TwainPlatform.Is32bit && TwainPlatform.PreferLegacyDSM)
+        {
+          rc = (STS)NativeMethods.WindowsTwain32DsmEntryParent(ref app, IntPtr.Zero, DG.CONTROL, DAT.PARENT, msg, ref hwnd);
+        }
+        else
+        {
+          rc = (STS)NativeMethods.WindowsTwaindsmDsmEntryParent(ref app, IntPtr.Zero, DG.CONTROL, DAT.PARENT, msg, ref hwnd);
+        }
+        if (rc == STS.SUCCESS) Session._appIdentity = app;
+      }
+      else if (TwainPlatform.IsLinux)
+      {
+        var app = Session._appIdentityLegacy;
+
+        rc = (STS)NativeMethods.LinuxDsmEntryParent(ref app, IntPtr.Zero, DG.CONTROL, DAT.PARENT, msg, ref hwnd);
+        if (rc == STS.SUCCESS) Session._appIdentity = app;
+      }
+      else if (TwainPlatform.IsMacOSX)
+      {
+        var app = Session._appIdentityOSX;
+        if (TwainPlatform.PreferLegacyDSM)
+        {
+          rc = (STS)NativeMethods.MacosxTwainDsmEntryParent(ref app, IntPtr.Zero, DG.CONTROL, DAT.PARENT, msg, ref hwnd);
+        }
+        else
+        {
+          rc = (STS)NativeMethods.MacosxTwaindsmDsmEntryParent(ref app, IntPtr.Zero, DG.CONTROL, DAT.PARENT, msg, ref hwnd);
+        }
+        if (rc == STS.SUCCESS) Session._appIdentityOSX = app;
+      }
+      return rc;
+    }
+  }
+}

src/NTwain/Triplets/DGAudio.cs

@@ -0,0 +1,14 @@
+using TWAINWorkingGroup;
+
+namespace NTwain.Triplets
+{
+  /// <summary>
+  /// Contains calls used with <see cref="DG.AUDIO"/>.
+  /// </summary>
+  public class DGAudio : TripletBase
+  {
+    public DGAudio(TwainSession session) : base(session)
+    {
+    }
+  }
+}

src/NTwain/Triplets/DGControl.cs

@@ -0,0 +1,16 @@
+namespace NTwain.Triplets
+{
+  /// <summary>
+  /// Contains calls used with <see cref="DG.CONTROL"/>.
+  /// </summary>
+  public class DGControl : TripletBase
+  {
+    public DGControl(TwainSession session) : base(session)
+    {
+    }
+
+    private DATParent? _parent;
+    public DATParent Parent => _parent ??= new DATParent(Session);
+
+  }
+}

src/NTwain/Triplets/DGImage.cs

@@ -0,0 +1,14 @@
+using TWAINWorkingGroup;
+
+namespace NTwain.Triplets
+{
+  /// <summary>
+  /// Contains calls used with <see cref="DG.IMAGE"/>.
+  /// </summary>
+  public class DGImage : TripletBase
+  {
+    public DGImage(TwainSession session) : base(session)
+    {
+    }
+  }
+}

src/NTwain/Triplets/Readme.md

@@ -0,0 +1,22 @@
+All TWAIN operations are done through the a combination of 
+Data Group (DG), Data Argument Type (DAT), and Message (MSG) 
+triplets. Rather than letting consumers of this lib deal 
+with all the combinations themselves and risk passing the 
+wrong thing, all valid triplet combinations are simply 
+made available under this namespace.
+
+Example:
+To get the status of the DS, just use the 
+"Get" method (represents MSG), in the
+"Status" property (represnts DAT), in the
+"DGControl" class (represents DG).
+
+or better explained in code:
+
+DGControl.Status.Get(...)
+
+Only triplets usable by the
+application-side are defined here.
+
+Due to the number of things involved, the properties are 
+lazy-loaded until used.

src/NTwain/Triplets/TripletBase.cs

@@ -0,0 +1,25 @@
+using System;
+
+namespace NTwain.Triplets
+{
+  /// <summary>
+  /// Base class for grouping triplet operations messages.
+  /// </summary>
+  public abstract class TripletBase
+  {
+    /// <summary>
+    /// Initializes a new instance of the <see cref="TripletBase" /> class.
+    /// </summary>
+    /// <param name="session">The session.</param>
+    /// <exception cref="System.ArgumentNullException"></exception>
+    public TripletBase(TwainSession session)
+    {
+      Session = session ?? throw new ArgumentNullException("session");
+    }
+
+    /// <summary>
+    /// Gets the twain session.
+    /// </summary>
+    public TwainSession Session { get; }
+  }
+}

src/NTwain/TwainSession.PropEvents.cs

@@ -1,4 +1,5 @@
-using System;
+using NTwain.Triplets;
+using System;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Linq;
@@ -42,7 +43,7 @@ namespace NTwain
     public STATE State
     {
       get => _state;
-      private set
+      internal set
       {
         if (_state != value)
         {
@@ -53,8 +54,21 @@ namespace NTwain
     }
 
     /// <summary>
-    /// Fired when state changes.
+    /// Fired when <see cref="State"/> changes.
     /// </summary>
     public event Action<TwainSession, STATE>? StateChanged;
+
+    /// <summary>
+    /// TWAIN triplet API calls with <see cref="DG.CONTROL"/>.
+    /// </summary>
+    public DGControl DGControl { get; }
+    /// <summary>
+    /// TWAIN triplet API calls with <see cref="DG.IMAGE"/>.
+    /// </summary>
+    public DGImage DGImage { get; }
+    /// <summary>
+    /// TWAIN triplet API calls with <see cref="DG.AUDIO"/>.
+    /// </summary>
+    public DGAudio DGAudio { get; }
   }
 }

src/NTwain/TwainSession.cs

@@ -1,4 +1,5 @@
-using System;
+using NTwain.Triplets;
+using System;
 using System.Diagnostics;
 using System.Text;
 using TWAINWorkingGroup;
@@ -62,7 +63,7 @@ namespace NTwain
         ProductName = productName,
         ProtocolMajor = (ushort)TWON_PROTOCOL.MAJOR,
         ProtocolMinor = (ushort)TWON_PROTOCOL.MINOR,
-        SupportedGroups = (uint)(supportedTypes | DG.APP2),
+        SupportedGroups = (uint)(supportedTypes | DG.CONTROL | DG.APP2),
         Version = new TW_VERSION
         {
           Country = appCountry,
@@ -74,6 +75,10 @@ namespace NTwain
       };
       if (TwainPlatform.IsLinux) _appIdentity = _appIdentityLegacy;
       else if (TwainPlatform.IsMacOSX) _appIdentityOSX = _appIdentityLegacy;
+
+      DGControl = new DGControl(this);
+      DGImage = new DGImage(this);
+      DGAudio = new DGAudio(this);
     }
   }
 }