Added xfer error event instead of throwing exceptions during xfers.

NTwain/Data/TypesExtended.cs

@@ -2189,12 +2189,12 @@ namespace NTwain.Data
         /// <summary>
         /// Condition Code describing the status.
         /// </summary>
-        public ConditionCode ConditionCode { get { return (ConditionCode)_conditionCode; } set { _conditionCode = (ushort)value; } }
+        public ConditionCode ConditionCode { get { return (ConditionCode)_conditionCode; } internal set { _conditionCode = (ushort)value; } }
         /// <summary>
         /// Valid for TWAIN 2.1 and later. This field contains additional
         /// scanner-specific data. If there is no data, then this value must be zero.
         /// </summary>
-        public ushort Data { get { return _data; } set { _data = Data; } }
+        public ushort Data { get { return _data; } internal set { _data = Data; } }
     }
 
     /// <summary>

NTwain/NTwain.csproj

@@ -72,6 +72,7 @@
     </Compile>
     <Compile Include="Properties\VersionInfo.cs" />
     <Compile Include="TentativeStateCommitable.cs" />
+    <Compile Include="TransferErrorEventArgs.cs" />
     <Compile Include="TransferReadyEventArgs.cs" />
     <Compile Include="Triplets\DGControl\DGControl.Callback2.cs" />
     <Compile Include="Triplets\DGImage\DGImage.Filter.cs" />

NTwain/TwainSession.cs

@@ -455,6 +455,11 @@ namespace NTwain
         /// </summary>
         public event EventHandler<DataTransferredEventArgs> DataTransferred;
 
+        /// <summary>
+        /// Occurs when an error has been encountered during transfer.
+        /// </summary>
+        public event EventHandler<TransferErrorEventArgs> TransferError;
+
         /// <summary>
         /// Called when <see cref="State"/> changed
         /// and raises the <see cref="StateChanged" /> event.
@@ -557,6 +562,22 @@ namespace NTwain
             }
         }
 
+        /// <summary>
+        /// Raises the <see cref="E:TransferError" /> event.
+        /// </summary>
+        /// <param name="e">The <see cref="TransferErrorEventArgs"/> instance containing the event data.</param>
+        protected virtual void OnTransferError(TransferErrorEventArgs e)
+        {
+            var hand = TransferError;
+            if (hand != null)
+            {
+                try
+                {
+                    hand(this, e);
+                }
+                catch { }
+            }
+        }
         #endregion
 
         #region TWAIN logic during xfer work
@@ -780,6 +801,14 @@ namespace NTwain
                     }
                     OnDataTransferred(new DataTransferredEventArgs { NativeData = lockedPtr });
                 }
+                else
+                {
+                    OnTransferError(new TransferErrorEventArgs { ReturnCode = xrc, SourceStatus = this.GetSourceStatus() });
+                }
+            }
+            catch (Exception ex)
+            {
+                OnTransferError(new TransferErrorEventArgs { Exception = ex });
             }
             finally
             {
@@ -812,6 +841,10 @@ namespace NTwain
             {
                 OnDataTransferred(new DataTransferredEventArgs { FileDataPath = filePath });
             }
+            else
+            {
+                OnTransferError(new TransferErrorEventArgs { ReturnCode = xrc, SourceStatus = this.GetSourceStatus() });
+            }
         }
 
         #endregion
@@ -839,6 +872,14 @@ namespace NTwain
                     }
                     OnDataTransferred(new DataTransferredEventArgs { NativeData = lockedPtr, ImageInfo = imgInfo });
                 }
+                else
+                {
+                    OnTransferError(new TransferErrorEventArgs { ReturnCode = xrc, SourceStatus = this.GetSourceStatus() });
+                }
+            }
+            catch (Exception ex)
+            {
+                OnTransferError(new TransferErrorEventArgs { Exception = ex });
             }
             finally
             {
@@ -876,6 +917,10 @@ namespace NTwain
                 }
                 OnDataTransferred(new DataTransferredEventArgs { FileDataPath = filePath, ImageInfo = imgInfo });
             }
+            else
+            {
+                OnTransferError(new TransferErrorEventArgs { ReturnCode = xrc, SourceStatus = this.GetSourceStatus() });
+            }
         }
 
         private void DoImageMemoryXfer()
@@ -953,11 +998,14 @@ namespace NTwain
                         }
                         else
                         {
-                            // todo: provide better mechanism for failed xfer, like event handler
+                            OnTransferError(new TransferErrorEventArgs { ReturnCode = xrc, SourceStatus = this.GetSourceStatus() });
-                            throw new TwainException("Failed to transfer data.");
                         }
                     }
                 }
+                catch (Exception ex)
+                {
+                    OnTransferError(new TransferErrorEventArgs { Exception = ex });
+                }
                 finally
                 {
                     State = 6;
@@ -1074,6 +1122,14 @@ namespace NTwain
                         }
                         File.Move(tempFile, finalFile);
                     }
+                    else
+                    {
+                        OnTransferError(new TransferErrorEventArgs { ReturnCode = xrc, SourceStatus = this.GetSourceStatus() });
+                    }
+                }
+                catch (Exception ex)
+                {
+                    OnTransferError(new TransferErrorEventArgs { Exception = ex });
                 }
                 finally
                 {