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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

AnswerRe: How can I check if I scan a blank document? Pin
MacSpudster9-Dec-08 12:50
professionalMacSpudster9-Dec-08 12:50 
QuestionRe: How can I check if I scan a blank document? Pin
NitinMakwana28-May-10 0:27
MemberNitinMakwana28-May-10 0:27 
AnswerChanging brightness Pin
mr_czech24-Aug-08 11:41
Membermr_czech24-Aug-08 11:41 
GeneralRe: Changing brightness Pin
Member 260553418-Jan-10 1:36
MemberMember 260553418-Jan-10 1:36 
Generalbug on cancel Pin
fabi____7-Aug-08 1:06
Memberfabi____7-Aug-08 1:06 
AnswerRe: bug on cancel Pin
Member 350537014-Jan-09 13:05
MemberMember 350537014-Jan-09 13:05 
Questioncould you please give me an simple example for Pin
bkkss20-Jul-08 17:29
Memberbkkss20-Jul-08 17:29 
GeneralVS2008 x64 Vista Problem outside debugger... Pin
Topper Price11-Jul-08 9:49
MemberTopper Price11-Jul-08 9:49 
GeneralRe: VS2008 x64 Vista Problem outside debugger... Pin
Topper Price11-Jul-08 11:46
MemberTopper Price11-Jul-08 11:46 
Questionhow Pin
ibulut3-Jul-08 0:12
Memberibulut3-Jul-08 0:12 
AnswerRe: how Pin
MacSpudster9-Dec-08 12:17
professionalMacSpudster9-Dec-08 12:17 
Generalreturn TwainCommand.Not Pin
Member 462883222-Jun-08 2:36
MemberMember 462883222-Jun-08 2:36 
GeneralRe: return TwainCommand.Not Pin
zedascoves9-Feb-11 4:10
Memberzedascoves9-Feb-11 4:10 
GeneralRe: return TwainCommand.Not Pin
Member 833147720-Oct-11 4:08
MemberMember 833147720-Oct-11 4:08 
GeneralBad value by PixelType Pin
mr_czech6-Jun-08 3:31
Membermr_czech6-Jun-08 3:31 
QuestionVS2008 and Vista 64 bit Pin
nxtwothou5-May-08 8:41
Membernxtwothou5-May-08 8:41 
AnswerRe: VS2008 and Vista 64 bit Pin
nxtwothou5-May-08 10:51
Membernxtwothou5-May-08 10:51 
Questionhow to configure this lib(setting color,size,resolution???) Pin
salihovic22-Apr-08 2:02
Membersalihovic22-Apr-08 2:02 
QuestionHow to set output size using twain capabilities? Pin
kantaiah13-Apr-08 20:14
Memberkantaiah13-Apr-08 20:14 
QuestionThis example doesn't work on VS2008 Pin
Ming_Z26-Feb-08 18:16
MemberMing_Z26-Feb-08 18:16 
AnswerRe: This example doesn't work on VS2008 Pin
Topper Price11-Jul-08 9:57
MemberTopper Price11-Jul-08 9:57 
GeneralRe: This example doesn't work on VS2008 Pin
Ming_Z11-Jul-08 10:48
MemberMing_Z11-Jul-08 10:48 
GeneralScanner problem Pin
piras10-Feb-08 8:35
Memberpiras10-Feb-08 8:35 
QuestionHow do I check if the scanner is plugged in Pin
peter.goessweiner23-Jan-08 11:22
Memberpeter.goessweiner23-Jan-08 11:22 
AnswerRe: How do I check if the scanner is plugged in Pin
zahil5-Jun-08 19:57
Memberzahil5-Jun-08 19:57 

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Posted 16 Sep 2001


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