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Posted 16 Sep 2001


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.NET TWAIN image scanner

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4.91/5 (226 votes)
12 May 2002Public Domain2 min read
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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Comments and Discussions Framework 4 windows form hits this line and stops debugger with no error message Pin
Member 1011884610-Jul-20 13:22
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QuestionHandling System.AccessViolationException potential solution? Pin
James McCullough5-Sep-19 6:57
professionalJames McCullough5-Sep-19 6:57 
AnswerRe: Handling System.AccessViolationException potential solution? PinPopular
Brent Lamborn9-Sep-19 9:31
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Questionis ther any way to get this for VB.Net? Pin
friedmad6-Jun-19 9:53
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QuestionPRESCAN Pin
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QuestionGreat starter program Pin
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QuestionIs it possible to scan all the pages of the document at once and then retrieve the data from the scanner in one call ? Pin
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QuestionSetting custom capabilities Pin
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QuestionHow i can convert IntPtr to Image? Pin
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QuestionError in windows 10 Pin
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Questionset setting inside the code Pin
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QuestionChoose the profile Pin
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Questionsave images directly to disc Pin
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QuestionUpdated continuation of this code on Github Pin
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QuestionScan again after error Feed Pin
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AnswerRe: Scan again after error Feed Pin
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Questionacquire more than one paper Pin
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AnswerRe: BadImageFormatException Pin
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QuestionHow to set duplex scanning option as true Pin
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AnswerRe: How to set duplex scanning option as true Pin
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QuestionHow I can select the active scanner Pin
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