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Posted 16 Sep 2001
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.NET TWAIN image scanner

, 12 May 2002
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Using TWAIN API to scan images

Sample Screenshot


In Windows imaging applications, the most used API for scanning is TWAIN Unfortunately, the new .NET Framework has no built-in support for TWAIN. So we have to work with the interop methods of .NET to access this API. This article doesn't explain this interop techniques, and good knowledge of the TWAIN 1.9 specifications is assumed! The sample code included doesn't present a finished library, only some essential steps for a minimal TWAIN adaption to .NET applications.


First step was to port the most important parts of TWAIN.H, these are found in TwainDefs.cs. The real logic for calling TWAIN is coded in the class Twain, in file TwainLib.cs.. As the TWAIN API is exposed by the Windows DLL, twain_32.dll, we have to use the .NET DllImport mechanism for interop with legacy code. This DLL has the central DSM_Entry(), ordinal #1 function exported as the entry point to TWAIN. This call has numerous parameters, and the last one is of variable type! It was found to be best if we declare multiple variants of the call like:

[DllImport("twain_32.dll", EntryPoint="#1")]
private static extern TwRC DSMparent(
    [In, Out] TwIdentity origin,
    IntPtr zeroptr,
    TwDG dg, TwDAT dat, TwMSG msg,
    ref IntPtr refptr );

The Twain class has a simple 5-step interface:

class Twain

For some sort of 'callbacks', TWAIN uses special Windows messages, and these must be caught from the application-message-loop. In .NET, the only way found was IMessageFilter.PreFilterMessage(), and this filter has to be activated with a call like Application.AddMessageFilter(). Within the filter method, we have to forward each message to Twain.PassMessage(), and we get a hint (enum TwainCommand) back for how we have to react.

Sample App

The sample is a Windows Forms MDI-style application. It has the two TWAIN-related menu items Select Source... and Acquire... Once an image is scanned in, we can save it to a file in any of the GDI+ supported file formats (BMP, GIF, TIFF, JPEG...)


All code was only tested on Windows 2000SP2, with an Epson Perfection USB scanner and an Olympus digital photo camera. The scanned picture is (by TWAIN spec) a Windows DIB, and the sample code has VERY little checking against error return codes and bitmap formats. Unfortunately, no direct method is available in .NET to convert a DIB to the managed Bitmap class... Some known problems may show up with color palettes and menus.

Note, TWAIN has it's root in 16-Bit Windows! For a more modern API supported on Windows ME/XP, have a look at Windows Image Acquisition (WIA).


This article, along with any associated source code and files, is licensed under A Public Domain dedication


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Question.NET TWAIN image scanner Convert to for VB 2010 Pin
Dibakar Mishra30-Oct-13 4:55
memberDibakar Mishra30-Oct-13 4:55 
QuestionGdipSaveImageToStream brings back a empty stream Pin
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GeneralMy vote of 5 Pin
Lucas Lima Araujo27-Sep-13 7:37
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QuestionWindows 8 Fails Pin
Lucas Lima Araujo27-Sep-13 5:39
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AnswerRe: Windows 8 Fails Pin
blastnsmash10-Dec-13 7:59
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GeneralRe: Windows 8 Fails Pin
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AnswerRe: Windows 8 Fails Pin
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Questionsave in Db Pin
partia3-Sep-13 4:16
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AnswerMessage Closed Pin
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GeneralRe: save in Db Pin
partia20-Sep-13 4:31
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GeneralRe: save in Db Pin
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QuestionPage rotation Pin
Ricardo Valero29-Aug-13 3:11
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QuestionTWAIN Pin
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AnswerRe: TWAIN Pin
p312ry3-Dec-13 16:41
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Questionusing TWAIN Kyocera FS-1035 Pin
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QuestionPass parameters to without using ui Pin
saijagadeesh30-Jul-13 4:13
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GeneralMy vote of 5 Pin
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Questionc# n vb Pin
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QuestionError in the Code Pin
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AnswerRe: Error in the Code Pin
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GeneralRe: Error in the Code Pin
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GeneralRe: Error in the Code Pin
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Questionthanks Pin
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QuestionHow can i scan multi pages Pin
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QuestionNeed help to fix Issue in Twain .net Library Pin
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