Updated exception message texts and more cases on stepdown().

src/NTwain/Resources/MsgText.Designer.cs

@@ -68,5 +68,32 @@ namespace NTwain.Resources {
                 return ResourceManager.GetString("MaxStringLengthExceeded", resourceCulture);
             }
         }
+        
+        /// <summary>
+        ///   Looks up a localized string similar to No data type specified..
+        /// </summary>
+        internal static string NoDataTypesSpecified {
+            get {
+                return ResourceManager.GetString("NoDataTypesSpecified", resourceCulture);
+            }
+        }
+        
+        /// <summary>
+        ///   Looks up a localized string similar to This platform ({0}) is not supported..
+        /// </summary>
+        internal static string PlatformNotSupported {
+            get {
+                return ResourceManager.GetString("PlatformNotSupported", resourceCulture);
+            }
+        }
+        
+        /// <summary>
+        ///   Looks up a localized string similar to Source is not from this session..
+        /// </summary>
+        internal static string SourceNotThisSession {
+            get {
+                return ResourceManager.GetString("SourceNotThisSession", resourceCulture);
+            }
+        }
     }
 }

src/NTwain/Resources/MsgText.resx

@@ -120,4 +120,13 @@
   <data name="MaxStringLengthExceeded" xml:space="preserve">
     <value>The string value has exceeded the maximum length of {0}.</value>
   </data>
+  <data name="NoDataTypesSpecified" xml:space="preserve">
+    <value>No data type specified.</value>
+  </data>
+  <data name="PlatformNotSupported" xml:space="preserve">
+    <value>This platform ({0}) is not supported.</value>
+  </data>
+  <data name="SourceNotThisSession" xml:space="preserve">
+    <value>Source is not from this session.</value>
+  </data>
 </root>

src/NTwain/TwainConfigBuilder.cs

@@ -1,5 +1,6 @@
 using NTwain.Data;
 using NTwain.Internals;
+using NTwain.Resources;
 using System;
 using System.Diagnostics;
 using System.Reflection;
@@ -57,7 +58,7 @@ namespace NTwain
             if (image) dg |= DataGroups.Image;
             if (audio) dg |= DataGroups.Audio;
 
-            if (dg == DataGroups.None) throw new InvalidOperationException("No data type specified.");
+            if (dg == DataGroups.None) throw new InvalidOperationException(MsgText.NoDataTypesSpecified);
             _dg = dg;
             return this;
         }
@@ -138,7 +139,7 @@ namespace NTwain
                 case PlatformID.Unix:
                 case PlatformID.MacOSX:
                 default:
-                    throw new PlatformNotSupportedException($"This platform {_platform} is not supported.");
+                    throw new PlatformNotSupportedException(string.Format(MsgText.PlatformNotSupported, _platform));
             }
             return config;
         }

src/NTwain/TwainSession.cs

@@ -1,4 +1,5 @@
 using NTwain.Data;
+using NTwain.Resources;
 using NTwain.Threading;
 using NTwain.Triplets;
 using System;
@@ -24,7 +25,7 @@ namespace NTwain
         readonly Dictionary<string, DataSource> _ownedSources = new Dictionary<string, DataSource>();
         // need to keep delegate around to prevent GC?
         readonly Callback32 _callback32Delegate;
-
+        // for windows only
         readonly WinMsgLoop _winMsgLoop;
 
 
@@ -32,9 +33,10 @@ namespace NTwain
         /// Constructs a new <see cref="TwainSession"/>.
         /// </summary>
         /// <param name="config"></param>
+        /// <exception cref="ArgumentNullException"></exception>
         public TwainSession(TwainConfig config)
         {
-            Config = config;
+            Config = config ?? throw new ArgumentNullException(nameof(config));
             switch (config.Platform)
             {
                 case PlatformID.MacOSX:
@@ -46,15 +48,28 @@ namespace NTwain
             }
         }
 
-
+        /// <summary>
+        /// Synchronously invokes an action on the internal thread if possible.
+        /// </summary>
+        /// <param name="action"></param>
+        /// <exception cref="ArgumentNullException"></exception>
         public void Invoke(Action action)
         {
+            if (action == null) throw new ArgumentNullException(nameof(action));
+
             if (_winMsgLoop != null) _winMsgLoop.Invoke(action);
             else action();
         }
 
+        /// <summary>
+        /// Asynchronously invokes an action on the internal thread if possible.
+        /// </summary>
+        /// <param name="action"></param>
+        /// <exception cref="ArgumentNullException"></exception>
         public void BeginInvoke(Action action)
         {
+            if (action == null) throw new ArgumentNullException(nameof(action));
+
             if (_winMsgLoop != null) _winMsgLoop.BeginInvoke(action);
             else action();
         }
@@ -116,6 +131,14 @@ namespace NTwain
                         rc = DGControl.Identity.CloseDS(CurrentSource.Identity32);
                         if (rc != ReturnCode.Success) return rc;
                         break;
+                    case TwainState.SourceEnabled:
+                        rc = DGControl.UserInterface.DisableDS(ref _lastEnableUI, false);
+                        if (rc != ReturnCode.Success) return rc;
+                        break;
+                    case TwainState.TransferReady:
+                    case TwainState.Transferring:
+                        _disableDSNow = true;
+                        break;
                 }
             }
             return rc;
@@ -210,7 +233,7 @@ namespace NTwain
                 {
                     if (value.Session != this)
                     {
-                        throw new InvalidOperationException("Source is not from this session.");
+                        throw new InvalidOperationException(MsgText.SourceNotThisSession);
                     }
                     var rc = DGControl.Identity.Set(value);
                     RaisePropertyChanged(nameof(DefaultSource));
@@ -274,7 +297,7 @@ namespace NTwain
         /// </returns>
         public override string ToString()
         {
-            return $"Session: {State}";
+            return State.ToString()
         }
     }
 }