Attempt on enable source that doesn't work yet.

samples/WinForm32/Form1.Designer.cs

@@ -42,6 +42,7 @@
       label3 = new System.Windows.Forms.Label();
       btnOpenDef = new System.Windows.Forms.Button();
       btnClose = new System.Windows.Forms.Button();
+      btnShowSettings = new System.Windows.Forms.Button();
       ((System.ComponentModel.ISupportInitialize)splitContainer1).BeginInit();
       splitContainer1.Panel1.SuspendLayout();
       splitContainer1.Panel2.SuspendLayout();
@@ -158,6 +159,7 @@
       // 
       // splitContainer1.Panel2
       // 
+      splitContainer1.Panel2.Controls.Add(btnShowSettings);
       splitContainer1.Panel2.Controls.Add(btnClose);
       splitContainer1.Panel2.Controls.Add(label2);
       splitContainer1.Panel2.Controls.Add(lblCurrent);
@@ -203,6 +205,16 @@
       btnClose.UseVisualStyleBackColor = true;
       btnClose.Click += btnClose_Click;
       // 
+      // btnShowSettings
+      // 
+      btnShowSettings.Location = new System.Drawing.Point(161, 32);
+      btnShowSettings.Name = "btnShowSettings";
+      btnShowSettings.Size = new System.Drawing.Size(132, 23);
+      btnShowSettings.TabIndex = 5;
+      btnShowSettings.Text = "Show settings only";
+      btnShowSettings.UseVisualStyleBackColor = true;
+      btnShowSettings.Click += btnShowSettings_Click;
+      // 
       // Form1
       // 
       AutoScaleDimensions = new System.Drawing.SizeF(7F, 15F);
@@ -236,5 +248,6 @@
     private System.Windows.Forms.Button btnOpenDef;
     private System.Windows.Forms.Label lblState;
     private System.Windows.Forms.Label label3;
+    private System.Windows.Forms.Button btnShowSettings;
   }
 }

samples/WinForm32/Form1.cs

@@ -100,5 +100,10 @@ namespace WinForm32
 
       twain.DGControl.Identity.OpenDS(twain.DefaultSource);
     }
+
+    private void btnShowSettings_Click(object sender, EventArgs e)
+    {
+      twain.EnableSource(true, true);
+    }
   }
 }

src/NTwain/DSM/OSXLegacyDSM.cs

@@ -81,5 +81,12 @@ namespace NTwain.DSM
         ref TW_IDENTITY_MACOSX origin, ref TW_IDENTITY_MACOSX dest,
         DG dg, DAT dat, MSG msg, ref DG xfergroup
     );
+
+    [DllImport(DsmName, CharSet = CharSet.Ansi)]
+    internal static extern ushort DSM_Entry
+    (
+        ref TW_IDENTITY_MACOSX origin, ref TW_IDENTITY_MACOSX dest,
+        DG dg, DAT dat, MSG msg, ref TW_USERINTERFACE userinterface
+    );
   }
 }

src/NTwain/DSM/OSXNewDSM.cs

@@ -81,5 +81,12 @@ namespace NTwain.DSM
         ref TW_IDENTITY_MACOSX origin, ref TW_IDENTITY_MACOSX dest,
         DG dg, DAT dat, MSG msg, ref DG xfergroup
     );
+
+    [DllImport(DsmName, CharSet = CharSet.Ansi)]
+    internal static extern ushort DSM_Entry
+    (
+        ref TW_IDENTITY_MACOSX origin, ref TW_IDENTITY_MACOSX dest,
+        DG dg, DAT dat, MSG msg, ref TW_USERINTERFACE userinterface
+    );
   }
 }

src/NTwain/DSM/WinLegacyDSM.cs

@@ -81,5 +81,12 @@ namespace NTwain.DSM
         ref TW_IDENTITY_LEGACY origin, ref TW_IDENTITY_LEGACY dest,
         DG dg, DAT dat, MSG msg, ref DG xfergroup
     );
+
+    [DllImport(DsmName, CharSet = CharSet.Ansi)]
+    internal static extern ushort DSM_Entry
+    (
+        ref TW_IDENTITY_LEGACY origin, ref TW_IDENTITY_LEGACY dest,
+        DG dg, DAT dat, MSG msg, ref TW_USERINTERFACE userinterface
+    );
   }
 }

src/NTwain/DSM/WinNewDSM.cs

@@ -81,5 +81,12 @@ namespace NTwain.DSM
         ref TW_IDENTITY_LEGACY origin, ref TW_IDENTITY_LEGACY dest,
         DG dg, DAT dat, MSG msg, ref DG xfergroup
     );
+
+    [DllImport(DsmName, CharSet = CharSet.Ansi)]
+    internal static extern ushort DSM_Entry
+    (
+        ref TW_IDENTITY_LEGACY origin, ref TW_IDENTITY_LEGACY dest,
+        DG dg, DAT dat, MSG msg, ref TW_USERINTERFACE userinterface
+    );
   }
 }

src/NTwain/Triplets/ControlDATs/UserInterface.cs

@@ -0,0 +1,92 @@
+using NTwain.DSM;
+using TWAINWorkingGroup;
+
+namespace NTwain.Triplets.ControlDATs
+{
+  /// <summary>
+  /// Contains calls used with <see cref="DG.CONTROL"/> and <see cref="DAT.USERINTERFACE"/>.
+  /// </summary>
+  public class UserInterface : TripletBase
+  {
+    public UserInterface(TwainSession session) : base(session)
+    {
+    }
+
+    /// <summary>
+    /// Disables source to bring state down to <see cref="STATE.S4"/>.
+    /// </summary>
+    /// <param name="data"></param>
+    /// <returns></returns>
+    public STS DisableDS(ref TW_USERINTERFACE data)
+    {
+      var rc = DoIt(MSG.DISABLEDS, ref data);
+      if (rc == STS.SUCCESS)
+      {
+        Session.State = STATE.S4;
+      }
+      return rc;
+    }
+
+    /// <summary>
+    /// Enables source to bring state up to <see cref="STATE.S5"/>.
+    /// </summary>
+    /// <param name="data"></param>
+    /// <returns></returns>
+    public STS EnableDS(ref TW_USERINTERFACE data)
+    {
+      var rc = DoIt(MSG.ENABLEDS, ref data);
+      if (rc == STS.SUCCESS || (data.ShowUI == 0 && rc == STS.CHECKSTATUS))
+      {
+        Session.State = STATE.S5;
+      }
+      return rc;
+    }
+
+    /// <summary>
+    /// Enables source to bring state up to <see cref="STATE.S5"/>.
+    /// </summary>
+    /// <param name="data"></param>
+    /// <returns></returns>
+    public STS EnableDSUIOnly(ref TW_USERINTERFACE data)
+    {
+      var rc = DoIt(MSG.ENABLEDSUIONLY, ref data);
+      if (rc == STS.SUCCESS)
+      {
+        Session.State = STATE.S5;
+      }
+      return rc;
+    }
+
+    STS DoIt(MSG msg, ref TW_USERINTERFACE data)
+    {
+      var rc = STS.FAILURE;
+      if (TwainPlatform.IsWindows)
+      {
+        var app = Session.AppIdentity;
+        var ds = Session.CurrentSource;
+        if (TwainPlatform.Is32bit && TwainPlatform.PreferLegacyDSM)
+        {
+          rc = (STS)WinLegacyDSM.DSM_Entry(ref app, ref ds, DG.CONTROL, DAT.USERINTERFACE, msg, ref data);
+        }
+        else
+        {
+          rc = (STS)WinNewDSM.DSM_Entry(ref app, ref ds, DG.CONTROL, DAT.USERINTERFACE, msg, ref data);
+        }
+      }
+      else if (TwainPlatform.IsMacOSX)
+      {
+        TW_IDENTITY_MACOSX app = Session.AppIdentity;
+        TW_IDENTITY_MACOSX ds = Session.CurrentSource;
+        if (TwainPlatform.PreferLegacyDSM)
+        {
+          rc = (STS)OSXLegacyDSM.DSM_Entry(ref app, ref ds, DG.CONTROL, DAT.USERINTERFACE, msg, ref data);
+        }
+        else
+        {
+          rc = (STS)OSXNewDSM.DSM_Entry(ref app, ref ds, DG.CONTROL, DAT.USERINTERFACE, msg, ref data);
+        }
+      }
+      return rc;
+    }
+  }
+}

src/NTwain/Triplets/DGControl.cs

@@ -41,5 +41,8 @@ namespace NTwain.Triplets
     private XferGroup? _xferGroup;
     public XferGroup XferGroup => _xferGroup ??= new XferGroup(Session);
 
+    private UserInterface? _userInterface;
+    public UserInterface UserInterface => _userInterface ??= new UserInterface(Session);
+
   }
 }

src/NTwain/Triplets/TripletBase.cs

@@ -14,7 +14,7 @@ namespace NTwain.Triplets
     /// <exception cref="System.ArgumentNullException"></exception>
     public TripletBase(TwainSession session)
     {
-      Session = session ?? throw new ArgumentNullException("session");
+      Session = session ?? throw new ArgumentNullException(nameof(session));
     }
 
     /// <summary>

src/NTwain/TwainSession.Callbacks.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Diagnostics;
 using System.Runtime.InteropServices;
 using TWAINWorkingGroup;
 
@@ -55,6 +56,7 @@ namespace NTwain
         IntPtr twnull
     )
     {
+      Debug.WriteLine($"Legacy callback got {msg}");
       HandleSourceMsg(msg);
       return (ushort)STS.SUCCESS;
     }
@@ -69,6 +71,7 @@ namespace NTwain
         IntPtr twnull
     )
     {
+      Debug.WriteLine($"OSX callback got {msg}");
       HandleSourceMsg(msg);
       return (ushort)STS.SUCCESS;
     }
@@ -82,10 +85,17 @@ namespace NTwain
         case MSG.DEVICEEVENT:
           break;
         case MSG.CLOSEDSOK:
+          DisableSource();
           break;
         case MSG.CLOSEDSREQ:
+          DisableSource();
           break;
       }
     }
+
+    public void HandleWin32Message(uint msg)
+    {
+
+    }
   }
 }

src/NTwain/TwainSession.Sources.cs

@@ -21,5 +21,37 @@ namespace NTwain
         rc = DGControl.Identity.GetNext(out ds);
       }
     }
+
+    /// <summary>
+    /// Enables the currently open source.
+    /// </summary>
+    /// <param name="showUI">Whether to show driver interface.</param>
+    /// <param name="uiOnly">If true try to display only driver dialog (no capture). 
+    /// Otherwise capturing will begin after this.</param>
+    /// <returns></returns>
+    public STS EnableSource(bool showUI, bool uiOnly)
+    {
+      var ui = new TW_USERINTERFACE
+      {
+        ShowUI = (ushort)((showUI || uiOnly) ? 1 : 0),
+        hParent = _hwnd,
+      };
+      var rc = uiOnly ? DGControl.UserInterface.EnableDSUIOnly(ref ui) : DGControl.UserInterface.EnableDS(ref ui);
+      if (rc == STS.SUCCESS || (!uiOnly && !showUI && rc == STS.CHECKSTATUS))
+      {
+        // keep it around for disable use
+        _userInterface = ui;
+      };
+      return rc;
+    }
+
+    /// <summary>
+    /// Disables the currently enabled source.
+    /// </summary>
+    /// <returns></returns>
+    public STS DisableSource()
+    {
+      return DGControl.UserInterface.DisableDS(ref _userInterface);
+    }
   }
 }

src/NTwain/TwainSession.cs

@@ -87,6 +87,7 @@ namespace NTwain
     }
 
     internal IntPtr _hwnd;
+    internal TW_USERINTERFACE _userInterface;
 
     /// <summary>
     /// Tries to bring the TWAIN session down to some state.
@@ -102,6 +103,9 @@ namespace NTwain
 
         switch (State)
         {
+          case STATE.S5:
+            DGControl.UserInterface.DisableDS(ref _userInterface);
+            break;
           case STATE.S4:
             DGControl.Identity.CloseDS();
             break;