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.NET TWAIN image scanner

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12 May 2002Public Domain
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

Questionthunk_16.exe error Pin
mdunlap9913-May-15 11:52
Membermdunlap9913-May-15 11:52 
QuestionTwaingui Pin
mdunlap9913-May-15 11:46
Membermdunlap9913-May-15 11:46 
QuestionHow to show only Scanners which are Twain related? This code shows all WIA as well as Twain related scanners in the list Pin
snprani2-Mar-15 21:27
Membersnprani2-Mar-15 21:27 
QuestionCustomization of Twain Application Pin
K.B.Rajesh24-Feb-15 21:02
MemberK.B.Rajesh24-Feb-15 21:02 
allan-arg20-Jan-15 20:40
Memberallan-arg20-Jan-15 20:40 
QuestionHow to scan a multipage document Pin
WHAK.com30-Dec-14 2:46
professionalWHAK.com30-Dec-14 2:46 
AnswerRe: How to scan a multipage document Pin
Member 85057558-Jun-15 1:34
MemberMember 85057558-Jun-15 1:34 
QuestionExe not compatible Pin
Member 1114919523-Nov-14 23:07
MemberMember 1114919523-Nov-14 23:07 
Questionhow calculate the number image of paper? Pin
nturri13063-Nov-14 6:27
Membernturri13063-Nov-14 6:27 
Member 1119101629-Oct-14 13:01
MemberMember 1119101629-Oct-14 13:01 
QuestionTwainLib PassMessage only returns TwainCommand.Not or TwainCommand.Null Pin
my_gino20-Oct-14 12:12
Membermy_gino20-Oct-14 12:12 
QuestionHow can I set a source in source code? Pin
Member 104012259-Oct-14 1:34
MemberMember 104012259-Oct-14 1:34 
AnswerRe: How can I set a source in source code? Pin
K.B.Rajesh24-Feb-15 21:32
MemberK.B.Rajesh24-Feb-15 21:32 
GeneralRe: How can I set a source in source code? Pin
eqiz17-Jun-15 13:13
Membereqiz17-Jun-15 13:13 
QuestionGreat application Pin
muharremunlu8-Sep-14 22:43
Membermuharremunlu8-Sep-14 22:43 
QuestionError at line 59 Pin
Member 21496282-Sep-14 17:33
MemberMember 21496282-Sep-14 17:33 
AnswerRe: Error at line 59 Pin
MAGSI13-Nov-14 1:44
MemberMAGSI13-Nov-14 1:44 
AnswerRe: Error at line 59 Pin
Member 1136399911-Jul-15 0:11
MemberMember 1136399911-Jul-15 0:11 
QuestionGreat! But id like to Change the filname of Picture Pin
Wolleschorge11-Aug-14 23:38
MemberWolleschorge11-Aug-14 23:38 
AnswerRe: Great! But id like to Change the filname of Picture Pin
K.B.Rajesh24-Feb-15 22:17
MemberK.B.Rajesh24-Feb-15 22:17 
QuestionGreat Pin
Chirantha Jayasinghe16-Jul-14 22:31
MemberChirantha Jayasinghe16-Jul-14 22:31 
Questiongreat thanks! Pin
DmitryDrozd9-Jun-14 3:36
MemberDmitryDrozd9-Jun-14 3:36 
GeneralMy vote of 5 Pin
superduck20-May-14 21:38
Membersuperduck20-May-14 21:38 
GeneralFix for windows 8.0 Pin
prcow12-May-14 12:24
Memberprcow12-May-14 12:24 
GeneralRe: Fix for windows 8.0 Pin
smile869115-Dec-14 6:53
Membersmile869115-Dec-14 6:53 

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Posted 16 Sep 2001


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