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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


About the Author

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Comments and Discussions

Question[Ask] 2 Image in one Click Pin
Reivina15-Mar-09 23:49
memberReivina15-Mar-09 23:49 
AnswerRe: [Ask] 2 Image in one Click Pin
ppmax17-Mar-09 4:57
memberppmax17-Mar-09 4:57 
JokeRe: [Ask] 2 Image in one Click Pin
Wolexie10-Jun-09 2:44
memberWolexie10-Jun-09 2:44 
GeneralRe: [Ask] 2 Image in one Click Pin
realfake116-Sep-09 22:14
memberrealfake116-Sep-09 22:14 
GeneralRe: [Ask] 2 Image in one Click Pin
nakashi27-Nov-11 3:04
membernakashi27-Nov-11 3:04 
GeneralPb with Threading Pin
ppmax9-Mar-09 23:49
memberppmax9-Mar-09 23:49 
Generalscanner & application is gettig hanged Pin
Ashtekar Amit4-Mar-09 20:13
memberAshtekar Amit4-Mar-09 20:13 
General[HELP]No "Select Source..." window appears! Pin
Palermo4ever16-Feb-09 5:32
memberPalermo4ever16-Feb-09 5:32 
Hi all,

I execute the application and i want to select the scanner in order to acquire my images.
But, clicking on the menu "Select source...", no form or dialog window appears!!!
In c:\windows\ and also in c:\windows\system32 i have the file twain_32.dll
How can i solve this problem???
Please, help me!
thanks in advance

GeneralRe: [HELP]No "Select Source..." window appears! Pin
Fiorebat17-Apr-09 3:12
memberFiorebat17-Apr-09 3:12 
QuestionPrinting Profile Pin
kengct5-Feb-09 19:45
memberkengct5-Feb-09 19:45 
GeneralTransferring ONE image a time [modified] Pin
Michele Perniola28-Jan-09 2:54
memberMichele Perniola28-Jan-09 2:54 
GeneralGdipSaveImageToFile function lock file [modified] Pin
lex-luthor27-Jan-09 7:34
memberlex-luthor27-Jan-09 7:34 
GeneralUnable to load DLL 'twain_32.dll': A dynamic link library (DLL) initialization routine failed. (Exception from HRESULT: 0x8007045A) Pin
Member 418037422-Jan-09 0:57
memberMember 418037422-Jan-09 0:57 
GeneralSupportedSizes doesnt work Pin
Member 37116407-Nov-08 4:37
memberMember 37116407-Nov-08 4:37 
GeneralRe: SupportedSizes doesnt work Pin
Member 37116407-Nov-08 4:44
memberMember 37116407-Nov-08 4:44 
GeneralRe: SupportedSizes doesnt work Pin
Ninos Denkha17-Mar-11 13:56
memberNinos Denkha17-Mar-11 13:56 
GeneralNo response after acquire(); Pin
kengct6-Nov-08 19:29
memberkengct6-Nov-08 19:29 
RantRe: No response after acquire(); Pin
kengct13-Nov-08 0:46
memberkengct13-Nov-08 0:46 
GeneralRe: No response after acquire(); Pin
hom.rcp25-Dec-08 5:45
memberhom.rcp25-Dec-08 5:45 
GeneralRe: No response after acquire(); Pin
kengct12-Jan-09 22:10
memberkengct12-Jan-09 22:10 
GeneralRe: No response after acquire(); Pin
Joey Burgett17-Mar-09 17:20
memberJoey Burgett17-Mar-09 17:20 
Questionhow to scan image directly into picturebox instead of painting onto form? Pin
VirtualVoid.NET16-Oct-08 12:13
memberVirtualVoid.NET16-Oct-08 12:13 
AnswerRe: how to scan image directly into picturebox instead of painting onto form? Pin
MacSpudster9-Dec-08 12:16
professionalMacSpudster9-Dec-08 12:16 
GeneralRe: how to scan image directly into picturebox instead of painting onto form? Pin
Burhan ERSOY28-Jul-09 23:52
memberBurhan ERSOY28-Jul-09 23:52 
GeneralRe: how to scan image directly into picturebox instead of painting onto form? Pin
MacSpudster29-Jul-09 8:02
professionalMacSpudster29-Jul-09 8:02 

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Posted 16 Sep 2001


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