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Posted 16 Sep 2001
Licenced Public Domain

.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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QuestionRe: Out of Memory Exception Pin
nilesh kakade26-Feb-13 19:17
membernilesh kakade26-Feb-13 19:17 
AnswerRe: Out of Memory Exception Pin
Jaydeep Jadav1-Mar-13 23:18
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QuestionLoader Lock Error Problem Pin
leenak8-Jun-12 2:11
memberleenak8-Jun-12 2:11 
GeneralRe: Loader Lock Error Problem Pin
smile869110-Jul-12 11:59
membersmile869110-Jul-12 11:59 
QuestionDuplex Scanning Pin
Isuru Alahakoon8-Jun-12 1:14
memberIsuru Alahakoon8-Jun-12 1:14 
Questionhide progress bar window. Pin
Member 46759534-Jun-12 21:32
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QuestionError on 64 bits machine Pin
david bong1-Jun-12 6:09
memberdavid bong1-Jun-12 6:09 
AnswerRe: Error on 64 bits machine Pin
boblogan13-Jun-12 6:00
memberboblogan13-Jun-12 6:00 
You might check your project output - and make sure it is specify 32 bit code only...
GeneralRe: Error on 64 bits machine Pin
Abdullah Çetinkaya22-Jan-13 23:33
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Questionset Flatbed or Feader Pin
Andrew8231-May-12 21:22
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QuestionCan use project as web Pin
ramysamir29-May-12 0:40
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QuestionDo you have code for automatically control scanner Pin
golanw12-May-12 22:52
membergolanw12-May-12 22:52 
QuestionDLL Pin
Member 888329025-Apr-12 9:57
memberMember 888329025-Apr-12 9:57 
QuestionHow Can I change scaning variables with c# code? Pin
Radu23717-Apr-12 20:46
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AnswerRe: How Can I change scaning variables with c# code? Pin
Member 100259915-May-13 16:36
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QuestionHow to check scanner is connected befor Acquire Pin
swapnil7434513-Apr-12 21:02
memberswapnil7434513-Apr-12 21:02 
Questionconverting? Pin
darkalastair19-Mar-12 22:15
memberdarkalastair19-Mar-12 22:15 
AnswerRe: converting? Pin
_Nizar21-Oct-12 8:58
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QuestionTwainLib Visual Studio 2010 Windows 7 64 bit Pin
sanvi615-Mar-12 3:10
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AnswerRe: TwainLib Visual Studio 2010 Windows 7 64 bit Pin
UstesGreenridge26-Mar-12 3:55
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AnswerRe: TwainLib Visual Studio 2010 Windows 7 64 bit Pin
DanielMarzan14-May-12 8:19
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GeneralRe: TwainLib Visual Studio 2010 Windows 7 64 bit Pin
smile869110-Jul-12 12:02
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GeneralMy vote of 5 Pin
Manoj Kumar Choubey26-Feb-12 18:23
professionalManoj Kumar Choubey26-Feb-12 18:23 
Questionproblem ..!!!!!!!!!! Pin
A7mad_22-Feb-12 7:43
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QuestionEpson V30 scan problem with 16 bit grayscale tiff Pin
smarino6021-Feb-12 7:52
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