diff --git a/src/NTwain/Data/TwainTypes.cs b/src/NTwain/Data/TwainTypes.cs
index 2651c2e..f5137a1 100644
--- a/src/NTwain/Data/TwainTypes.cs
+++ b/src/NTwain/Data/TwainTypes.cs
@@ -540,10 +540,6 @@ 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);
-
     [StructLayout(LayoutKind.Sequential, Pack = 2)]
     partial class TW_ENTRYPOINT
     {
diff --git a/src/NTwain/DataSource.Props.cs b/src/NTwain/DataSource.Props.cs
new file mode 100644
index 0000000..4036f80
--- /dev/null
+++ b/src/NTwain/DataSource.Props.cs
@@ -0,0 +1,62 @@
+using System;
+using System.Collections.Generic;
+using System.ComponentModel;
+using System.Linq;
+using System.Text;
+using NTwain.Data;
+
+namespace NTwain
+{
+    partial class DataSource : INotifyPropertyChanged
+    {
+        /// <summary>
+        /// Gets the source name.
+        /// </summary>
+        public string Name => Identity.ProductName;
+
+        /// <summary>
+        /// Gets the source's manufacturer name.
+        /// </summary>
+        public string Manufacturer => Identity.Manufacturer;
+
+        /// <summary>
+        /// Gets the source's product family.
+        /// </summary>
+        public string ProductFamily => Identity.ProductFamily;
+
+        /// <summary>
+        /// Gets the source's version info.
+        /// </summary>
+        public TW_VERSION Version => Identity.Version;
+
+        /// <summary>
+        /// Gets the supported data group.
+        /// </summary>
+        public DataGroups DataGroup => Identity.DataGroup;
+
+        /// <summary>
+        /// Gets the supported TWAIN protocol version.
+        /// </summary>
+        public Version ProtocolVersion { get; }
+
+        /// <summary>
+        /// Gets a value indicating whether this data source is open.
+        /// </summary>
+        public bool IsOpen => Session.State > TwainState.DsmOpened && Session.CurrentSource == this;
+
+        /// <summary>
+        /// Occurs when a property value changes.
+        /// </summary>
+        public event PropertyChangedEventHandler PropertyChanged;
+
+        /// <summary>
+        /// Raises the <see cref="PropertyChanged"/> event.
+        /// </summary>
+        /// <param name="propertyName">Name of the property.</param>
+        internal protected void RaisePropertyChanged(string propertyName)
+        {
+            var handle = PropertyChanged;
+            handle?.Invoke(this, new PropertyChangedEventArgs(propertyName));
+        }
+    }
+}
diff --git a/src/NTwain/DataSource.cs b/src/NTwain/DataSource.cs
index c81b2ba..afd446c 100644
--- a/src/NTwain/DataSource.cs
+++ b/src/NTwain/DataSource.cs
@@ -9,7 +9,7 @@ namespace NTwain
     /// <summary>
     /// A TWAIN data source.
     /// </summary>
-    public class DataSource
+    public partial class DataSource
     {
         internal readonly TwainSession Session;
 
@@ -22,43 +22,6 @@ namespace NTwain
             ProtocolVersion = new Version(src.ProtocolMajor, src.ProtocolMinor);
         }
 
-        /// <summary>
-        /// Gets the source name.
-        /// </summary>
-        public string Name => Identity.ProductName;
-
-        /// <summary>
-        /// Gets the source's manufacturer name.
-        /// </summary>
-        public string Manufacturer => Identity.Manufacturer;
-
-        /// <summary>
-        /// Gets the source's product family.
-        /// </summary>
-        public string ProductFamily => Identity.ProductFamily;
-
-        /// <summary>
-        /// Gets the source's version info.
-        /// </summary>
-        public TW_VERSION Version => Identity.Version;
-
-        /// <summary>
-        /// Gets the supported data group.
-        /// </summary>
-        public DataGroups DataGroup => Identity.DataGroup;
-
-        /// <summary>
-        /// Gets the supported TWAIN protocol version.
-        /// </summary>
-        public Version ProtocolVersion { get; }
-
-        /// <summary>
-        /// Gets a value indicating whether this data source is open.
-        /// </summary>
-        public bool IsOpen => Session.State > TwainState.DsmOpened && Session.CurrentSource == this;
-
-
-
         /// <summary>
         /// Opens the source for capability negotiation.
         /// </summary>
diff --git a/src/NTwain/Internals/LinuxMemoryManager.cs b/src/NTwain/Internals/LinuxMemoryManager.cs
index 4ae8fdb..3254807 100644
--- a/src/NTwain/Internals/LinuxMemoryManager.cs
+++ b/src/NTwain/Internals/LinuxMemoryManager.cs
@@ -10,8 +10,6 @@ namespace NTwain.Internals
 
     class LinuxMemoryManager : IMemoryManager
     {
-        #region IMemoryManager Members
-
         public IntPtr Allocate(uint size)
         {
             return Marshal.AllocHGlobal((int)size);
@@ -32,6 +30,5 @@ namespace NTwain.Internals
             // no op
         }
 
-        #endregion
     }
 }
diff --git a/src/NTwain/Internals/WinMemoryManager.cs b/src/NTwain/Internals/WinMemoryManager.cs
index 7ce356d..a4dd1f5 100644
--- a/src/NTwain/Internals/WinMemoryManager.cs
+++ b/src/NTwain/Internals/WinMemoryManager.cs
@@ -40,22 +40,18 @@ namespace NTwain.Internals
         [SuppressUnmanagedCodeSecurity]
         static class UnsafeNativeMethods
         {
-            #region mem stuff for twain 1.x
-
             [DllImport("kernel32", SetLastError = true, EntryPoint = "GlobalAlloc")]
-            internal static extern IntPtr WinGlobalAlloc(uint uFlags, UIntPtr dwBytes);
+            public static extern IntPtr WinGlobalAlloc(uint uFlags, UIntPtr dwBytes);
 
             [DllImport("kernel32", SetLastError = true, EntryPoint = "GlobalFree")]
-            internal static extern IntPtr WinGlobalFree(IntPtr hMem);
+            public static extern IntPtr WinGlobalFree(IntPtr hMem);
 
             [DllImport("kernel32", SetLastError = true, EntryPoint = "GlobalLock")]
-            internal static extern IntPtr WinGlobalLock(IntPtr handle);
+            public static extern IntPtr WinGlobalLock(IntPtr handle);
 
             [DllImport("kernel32", SetLastError = true, EntryPoint = "GlobalUnlock")]
             [return: MarshalAs(UnmanagedType.Bool)]
-            internal static extern bool WinGlobalUnlock(IntPtr handle);
-
-            #endregion
+            public static extern bool WinGlobalUnlock(IntPtr handle);
         }
     }
 }
diff --git a/src/NTwain/Triplets/DGControl.Identity.cs b/src/NTwain/Triplets/DGControl.Identity.cs
index 958db81..e66b028 100644
--- a/src/NTwain/Triplets/DGControl.Identity.cs
+++ b/src/NTwain/Triplets/DGControl.Identity.cs
@@ -44,6 +44,8 @@ namespace NTwain.Triplets
 
         public ReturnCode OpenDS(TW_IDENTITY source)
         {
+            Session.StepDown(TwainState.DsmOpened);
+
             var rc = NativeMethods.DsmWin32(Session.Config.AppWin32, IntPtr.Zero,
                 DataGroups.Control, DataArgumentType.Identity, Message.OpenDS, source);
             if (rc == ReturnCode.Success)
diff --git a/src/NTwain/TwainSession.cs b/src/NTwain/TwainSession.cs
index f4de1dd..63e36bb 100644
--- a/src/NTwain/TwainSession.cs
+++ b/src/NTwain/TwainSession.cs
@@ -11,6 +11,10 @@ using System.Threading;
 
 namespace NTwain
 {
+    //[UnmanagedFunctionPointer(CallingConvention.Cdecl)]
+    delegate ReturnCode CallbackDelegate(TW_IDENTITY origin, TW_IDENTITY destination,
+            DataGroups dg, DataArgumentType dat, Message msg, IntPtr data);
+
     /// <summary>
     /// Manages a TWAIN session.
     /// </summary>
@@ -24,6 +28,7 @@ namespace NTwain
         // need to keep delegate around to prevent GC?
         readonly CallbackDelegate _callbackDelegate;
 
+
         /// <summary>
         /// Constructs a new <see cref="TwainSession"/>.
         /// </summary>
@@ -79,43 +84,36 @@ namespace NTwain
 
         internal void RegisterCallback()
         {
-            if (State == TwainState.S4)
+            var callbackPtr = Marshal.GetFunctionPointerForDelegate(_callbackDelegate);
+
+            // try new callback first
+            var cb2 = new TW_CALLBACK2 { CallBackProc = callbackPtr };
+            var rc = DGControl.Callback2.RegisterCallback(ref cb2);
+            if (rc == ReturnCode.Success) Debug.WriteLine("Registed Callback2 success.");
+            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))
+            }
+
+
+            if (rc != ReturnCode.Success)
+            {
+                // always register old callback
+                var cb = new TW_CALLBACK { CallBackProc = callbackPtr };
+
+                rc = DGControl.Callback.RegisterCallback(ref cb);
+
+                if (rc == ReturnCode.Success) Debug.WriteLine("Registed Callback success.");
+                else
                 {
-                    var cb = new TW_CALLBACK2 { CallBackProc = callbackPtr };
-                    rc = DGControl.Callback2.RegisterCallback(ref cb);
-                    if (rc == ReturnCode.Success) Debug.WriteLine("Registed Callback2 success.");
-                    else
-                    {
 
-                    }
-                }
-
-                if (rc != ReturnCode.Success)
-                {
-                    // always try old callback
-                    var cb = new TW_CALLBACK { CallBackProc = callbackPtr };
-
-                    rc = DGControl.Callback.RegisterCallback(ref cb);
-
-                    if (rc == ReturnCode.Success) Debug.WriteLine("Registed Callback success.");
-                    else
-                    {
-
-                    }
                 }
             }
         }
 
 
         /// <summary>
-        /// Gets list of sources available on the machine.
+        /// Enumerate list of sources available on the machine.
         /// </summary>
         /// <returns></returns>
         public IEnumerable<DataSource> GetSources()
@@ -168,13 +166,25 @@ namespace NTwain
             return null;
         }
 
+        private DataSource _currentSource;
+
         /// <summary>
         /// Gets the currently open data source.
         /// </summary>
-        /// <value>
-        /// The current source.
-        /// </value>
-        public DataSource CurrentSource { get; internal set; }
+        public DataSource CurrentSource
+        {
+            get { return _currentSource; }
+            internal set
+            {
+                var old = _currentSource;
+                _currentSource = value;
+
+                RaisePropertyChanged(nameof(CurrentSource));
+                old?.RaisePropertyChanged(nameof(DataSource.IsOpen));
+                value?.RaisePropertyChanged(nameof(DataSource.IsOpen));
+            }
+        }
+
 
         internal DataSource GetSourceSingleton(TW_IDENTITY sourceId)
         {
@@ -198,5 +208,16 @@ namespace NTwain
             HandleSourceMsg(msg);
             return ReturnCode.Success;
         }
+
+        /// <summary>
+        /// Returns a <see cref="System.String"/> that represents this instance.
+        /// </summary>
+        /// <returns>
+        /// A <see cref="System.String"/> that represents this instance.
+        /// </returns>
+        public override string ToString()
+        {
+            return $"Session: {State}";
+        }
     }
 }