HOW TO: Use Twain in a Background Thread

Attached is a simple C# 2005 project that uses LEADTOOLS .NET 16.5 to show how to do Twain scanning in a background thread.  As pages are scanned it will populate a RasterImageList and let you select and view images while additional pages are being scanned.

One should note that there can be some limitations based on the scanner driver.  Most scanners' dialogs are modeless, meaning you can still use your application even though the scanner's dialog is active and on the screen.  However, most scanner manufacturers make their dialogs the top most window (just like Task Manager) so even though you can click and move your application's window around, the scanner's dialog will still be drawn over it.  This is true for Fujitsu, Panasonic, Canon, and the sample Twain Source.  AFAIK, the only manufacturer who doesn't make their dialog the top most window is Kodak.  With Kodak's dialog present you can click on your application and your application will be painted over Kodak's dialog.

According to the Twain specifications, CAP_INDICATORS will enable/disable the progress dialog.  You can only suppress the progress indicator when you choose NOT to display the UI.  Therefore the only way to truly use Twain in a background thread and have full access to your main form's GUI without interruption on the vast majority of scanner models is to set CAP_INDICATORS to false and not show the UI when acquiring.

The attached project has 2 changes:
1. It is updated to use LEADTOOLS version 17.
2. The project now does all the Twain functions inside one thread.

The reason for the second change is that some scanners showed problems when the Twain session was created in one thread and used in a different thread.

The attached project is updated to use LEADTOOLS v19.