Properly catch and throw exceptions on message loop .invoke() calls (#79)

samples/Sample.Console/Program.cs

@@ -22,7 +22,14 @@ namespace Sample
             // just an amusing example to do twain in console without UI
             ThreadPool.QueueUserWorkItem(o =>
             {
-                DoTwainWork();
+                try
+                {
+                    DoTwainWork();
+                }
+                catch (Exception ex)
+                {
+                    Console.WriteLine("ERROR: " + ex.ToString());
+                }
             });
             Console.WriteLine("Test started, press Enter to exit.");
             Console.ReadLine();

src/NTwain/Internals/InternalMessageLoopHook.cs

@@ -71,6 +71,8 @@ namespace NTwain.Internals
         {
             if (_dispatcher == null) { throw new InvalidOperationException(Resources.MsgLoopUnavailble); }
 
+            Exception error = null;
+
             if (_dispatcher.CheckAccess())
             {
                 action();
@@ -85,6 +87,7 @@ namespace NTwain.Internals
                         {
                             action();
                         }
+                        catch (Exception ex) { error = ex; }
                         finally
                         {
                             man.Set();
@@ -95,8 +98,17 @@ namespace NTwain.Internals
             }
             else
             {
-                _dispatcher.Invoke(DispatcherPriority.Normal, action);
+                _dispatcher.Invoke(DispatcherPriority.Normal, new Action(() =>
+                {
+                    try
+                    {
+                        action();
+                    }
+                    catch (Exception ex) { error = ex; }
+                }));
             }
+
+            if (error != null) { Rethrow(error); }
         }
     }
 }

src/NTwain/MessageLoopHooks.cs

@@ -1,5 +1,6 @@
 using NTwain.Internals;
 using System;
+using System.Reflection;
 using System.Threading;
 using System.Windows.Interop;
 
@@ -47,13 +48,34 @@ namespace NTwain
             }
             else
             {
+                Exception error = null;
                 SyncContext.Send(o =>
                 {
-                    action();
+                    try
+                    {
+                        action();
+                    }
+                    catch (Exception ex)
+                    {
+                        error = ex;
+                    }
                 }, null);
+
+                if (error != null) { Rethrow(error); }
             }
         }
 
+        /// <summary>
+        /// Rethrows the specified excetion while keeping stack trace.
+        /// </summary>
+        /// <param name="ex">The ex.</param>
+        protected static void Rethrow(Exception ex)
+        {
+            typeof(Exception).GetMethod("PrepForRemoting",
+                BindingFlags.NonPublic | BindingFlags.Instance)?.Invoke(ex, new object[0]);
+            throw ex;
+        }
+
         internal void ThrowInvalidOp()
         {
             throw new InvalidOperationException(InvalidMessage);