Updated with suggestion from #63.

src/NTwain/ITwainSession.cs

@@ -94,6 +94,15 @@ namespace NTwain
         /// </value>
         bool StopOnTransferError { get; set; }
 
+        /// <summary>
+        /// Gets the reason a source was disabled (dropped from state 5) if it's due to user action.
+        /// Mostly only <see cref="Message.CloseDSOK"/> or <see cref="Message.CloseDSReq"/>.
+        /// </summary>
+        /// <value>
+        /// The dialog result.
+        /// </value>
+        Message DisableReason { get; }
+
         /// <summary>
         /// Try to show the built-in source selector dialog and return the selected source.
         /// This is not recommended and is only included for completeness.

src/NTwain/Properties/VersionInfo.cs

@@ -23,7 +23,7 @@ namespace NTwain
         /// <summary>
         /// The build release version number.
         /// </summary>
-        public const string Build = "3.3.9.4"; // change this for each nuget release
+        public const string Build = "3.3.9.5"; // change this for each nuget release
 
 
     }

src/NTwain/TwainSession.cs

@@ -211,6 +211,15 @@ namespace NTwain
         /// </value>
         public bool StopOnTransferError { get; set; }
 
+        /// <summary>
+        /// Gets the reason a source was disabled (dropped from state 5) if it's due to user action.
+        /// Mostly only <see cref="Message.CloseDSOK" /> or <see cref="Message.CloseDSReq" />.
+        /// </summary>
+        /// <value>
+        /// The dialog result.
+        /// </value>
+        public Message DisableReason { get; private set; }
+
         /// <summary>
         /// Opens the data source manager. This must be the first method used
         /// before using other TWAIN functions. Calls to this must be followed by

src/NTwain/TwainSessionInternal.cs

@@ -159,6 +159,7 @@ namespace NTwain
         ReturnCode ITwainSessionInternal.EnableSource(SourceEnableMode mode, bool modal, IntPtr windowHandle)
         {
             _closeRequested = false;
+            DisableReason = Message.Null;
             var rc = ReturnCode.Failure;
 
             _msgLoopHook.Invoke(() =>
@@ -306,6 +307,7 @@ namespace NTwain
                     break;
                 case Message.CloseDSReq:
                 case Message.CloseDSOK:
+                    DisableReason = msg;
                     // even though it says closeDS it's really disable.
                     // dsok is sent if source is enabled with uionly