Update to allow preferring old dsm for #43

README.md

@@ -154,13 +154,24 @@ application due to their use of modal dialogs, so if you find yourself in that p
 you'll have to find another way to synchronize data to UI threads. 
 
 
-64-bit OS
+Using the new twaindsm.dll
 --------------------------------------
-If the application process is going to be running in 64-bit then you will need to have the 
+By default NTwain will use the newer [data source manager](http://sourceforge.net/projects/twain-dsm/files/TWAIN%20DSM%202%20Win/)
-newer data source manager (twaindsm.dll) from below installed. 
+(twaindsm.dll) if available. To override this behavior
+set the PlatformInfo's PreferNewDSM flag to false. Some older sources does not work with the newer dsm so it's
+necessary to set it.
 
-[DSM from TWAIN.org](http://sourceforge.net/projects/twain-dsm/files/TWAIN%20DSM%202%20Win/)
+```
+#!c#
+// go back to using twain_32.dll under windows,
+// do this once at app startup.
+NTwain.PlatformInfo.Current.PreferNewDSM = false;
 
-In fact, installing the new DSM is recommended whether you're running in 64-bit or not.
+```
+
+
+If the application process is going to be running in 64-bit then this flag will have no effect and you will 
+always need to have the newer dsm installed. 
+
+If the scanner's TWAIN driver is still 32-bit then you'll have need to compile the application exe in x86 or you won't see the driver.
 
-If the scanner's TWAIN driver is still 32-bit then you'll have no choice but to compile the application exe in x86 or you won't see the driver.

samples/Sample.WPF/Launcher.xaml

@@ -1,10 +1,16 @@
 <Window x:Class="Sample.WPF.Launcher"
         xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+        xmlns:twain="clr-namespace:NTwain;assembly=NTwain"
         Title="Scan Launcher" Height="200" Width="250"
         ResizeMode="NoResize" Style="{StaticResource AppWindow}">
     <Grid>
         <StackPanel VerticalAlignment="Center">
+            <CheckBox DataContext="{x:Static twain:PlatformInfo.Current}"
+                      IsChecked="{Binding PreferNewDSM}"
+                      IsEnabled="{Binding IsWindows, Mode=OneWay}"
+                      Content="Prefer new DSM"
+                      HorizontalAlignment="Center"></CheckBox>
             <Button Content="Open scan window" Click="Button_Click" Margin="4 0"></Button>
             <TextBlock Text="This is to test opening/closing multiple twain sessions in an app" Margin="8"
                        TextWrapping="Wrap"></TextBlock>

src/NTwain/IPlatformInfo.cs

@@ -1,4 +1,5 @@
 using System;
+using System.IO;
 namespace NTwain
 {
     /// <summary>
@@ -46,6 +47,14 @@ namespace NTwain
         /// </value>
         bool UseNewWinDSM { get; }
 
+        /// <summary>
+        /// Gets or sets a value indicating whether to prefer using the new DSM on Windows over old twain_32 dsm if applicable.
+        /// </summary>
+        /// <value>
+        ///   <c>true</c> to prefer new DSM; otherwise, <c>false</c>.
+        /// </value>
+        bool PreferNewDSM { get; set; }
+
         /// <summary>
         /// Gets a value indicating whether the current runtime is mono.
         /// </summary>

src/NTwain/PlatformInfo.cs

@@ -2,6 +2,7 @@
 using NTwain.Triplets;
 using System;
 using System.Collections.Generic;
+using System.Diagnostics;
 using System.IO;
 using System.Linq;
 using System.Text;
@@ -36,32 +37,13 @@ namespace NTwain
             {
                 _defaultMemManager = new WinMemoryManager();
 
-                var newDsmPath = Path.Combine(Environment.SystemDirectory, Dsm.WIN_NEW_DSM_NAME);
+                newDsmPath = Path.Combine(Environment.SystemDirectory, Dsm.WIN_NEW_DSM_NAME);
 #if NET35
-                var oldDsmPath = Path.Combine(Environment.GetEnvironmentVariable("windir"), Dsm.WIN_OLD_DSM_NAME);
+                oldDsmPath = Path.Combine(Environment.GetEnvironmentVariable("windir"), Dsm.WIN_OLD_DSM_NAME);
 #else
-                var oldDsmPath = Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.Windows), Dsm.WIN_OLD_DSM_NAME);
+                oldDsmPath = Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.Windows), Dsm.WIN_OLD_DSM_NAME);
 #endif
-
+                PreferNewDSM = true;
-                if (IsApp64Bit)
-                {
-                    ExpectedDsmPath = newDsmPath;
-                    IsSupported = DsmExists = File.Exists(ExpectedDsmPath);
-                    UseNewWinDSM = true;
-                }
-                else
-                {
-                    if (File.Exists(newDsmPath))
-                    {
-                        ExpectedDsmPath = newDsmPath;
-                        UseNewWinDSM = IsSupported = DsmExists = true;
-                    }
-                    else
-                    {
-                        ExpectedDsmPath = oldDsmPath;
-                        IsSupported = DsmExists = File.Exists(ExpectedDsmPath);
-                    }
-                }
             }
             else if (IsLinux)
             {
@@ -77,6 +59,54 @@ namespace NTwain
             }
         }
 
+        string oldDsmPath;
+        string newDsmPath;
+
+        private bool _preferNewDSM;
+
+        /// <summary>
+        /// Gets a value indicating whether to prefer using the new DSM on Windows over old twain_32 dsm if applicable.
+        /// </summary>
+        /// <value>
+        ///   <c>true</c> to prefer new DSM; otherwise, <c>false</c>.
+        /// </value>
+        public bool PreferNewDSM
+        {
+            get { return _preferNewDSM; }
+            set
+            {
+                if (IsWindows)
+                {
+                    _preferNewDSM = value;
+
+                    if (IsApp64Bit)
+                    {
+                        ExpectedDsmPath = newDsmPath;
+                        IsSupported = DsmExists = File.Exists(ExpectedDsmPath);
+                        UseNewWinDSM = true;
+                        Debug.WriteLine("Using new dsm in windows.");
+                    }
+                    else
+                    {
+                        if (_preferNewDSM && File.Exists(newDsmPath))
+                        {
+                            ExpectedDsmPath = newDsmPath;
+                            UseNewWinDSM = IsSupported = DsmExists = true;
+                            Debug.WriteLine("Using new dsm in windows.");
+                        }
+                        else
+                        {
+                            ExpectedDsmPath = oldDsmPath;
+                            IsSupported = DsmExists = File.Exists(ExpectedDsmPath);
+                            UseNewWinDSM = false;
+                            Debug.WriteLine("Using old dsm in windows.");
+                        }
+                    }
+                }
+            }
+        }
+
+
         /// <summary>
         /// Gets a value indicating whether the lib is expecting to use new DSM.
         /// </summary>