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.NET TWAIN image scanner

, 12 May 2002 Public Domain
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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


About the Author

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Comments and Discussions

GeneralRe: activatign a scanner through a button.... Pin
fifthwheel11-Feb-04 14:26
memberfifthwheel11-Feb-04 14:26 
GeneralRe: activatign a scanner through a button.... Pin
mlkeS22-Feb-04 20:08
membermlkeS22-Feb-04 20:08 
GeneralRe: activatign a scanner through a button.... Pin
jbuxtonNOSPAM@appintell.nopleasespam.com16-Apr-04 6:35
sussjbuxtonNOSPAM@appintell.nopleasespam.com16-Apr-04 6:35 
GeneralRe: activatign a scanner through a button.... Pin
mlkeS17-Apr-04 0:25
membermlkeS17-Apr-04 0:25 
GeneralRe: activatign a scanner through a button.... Pin
cougarlvr19-Aug-08 11:54
membercougarlvr19-Aug-08 11:54 
GeneralHelp Needed Urgently Pin
the_mumford@yahoo.com17-Jan-04 18:34
memberthe_mumford@yahoo.com17-Jan-04 18:34 
GeneralSet PixelType Capability Pin
Byron Vinzant15-Jan-04 12:52
memberByron Vinzant15-Jan-04 12:52 
AnswerRe: Set PixelType Capability Pin
AlexKak21-Dec-05 3:44
memberAlexKak21-Dec-05 3:44 

try to change TwType.Int16 to TwType.Fix32 or TwType.UInt16

public TwCapability( TwCap cap, short sval)

Cap = (short) cap;
ConType = (short) TwOn.One;
Handle = Twain.GlobalAlloc( 0x42, 6 );
IntPtr pv = Twain.GlobalLock( Handle );

Marshal.WriteInt16( pv, 0, (short) TwType.Fix32 );

Marshal.WriteInt32( pv, 2, (int) sval );
Twain.GlobalUnlock( Handle );

Generaljumbled up image Pin
cablewu8-Jan-04 1:07
membercablewu8-Jan-04 1:07 
GeneralRe: jumbled up image Pin
fifthwheel15-Jan-04 11:10
memberfifthwheel15-Jan-04 11:10 
QuestionLaunching this TWAIN scanner from a Web Application? Pin
gin_blossoms30-Dec-03 5:12
membergin_blossoms30-Dec-03 5:12 
GeneralAutomatic Image Scanner Pin
kobyb26-Nov-03 10:01
memberkobyb26-Nov-03 10:01 
GeneralSize and orientation of page Pin
filippos7013-Nov-03 4:47
memberfilippos7013-Nov-03 4:47 
GeneralHangs when no TWAIN device installed Pin
Carl Mercier24-Oct-03 5:23
memberCarl Mercier24-Oct-03 5:23 
GeneralRe: Hangs when no TWAIN device installed Pin
musicstar9-Nov-03 19:58
membermusicstar9-Nov-03 19:58 
GeneralRe: Hangs when no TWAIN device installed Pin
Parag Kavi21-Nov-03 2:28
sussParag Kavi21-Nov-03 2:28 
QuestionHow to set Duplex capability Pin
keyser-soze22-Oct-03 22:56
susskeyser-soze22-Oct-03 22:56 
AnswerRe: How to set Duplex capability Pin
MuzzyB20-Jul-16 22:34
memberMuzzyB20-Jul-16 22:34 
GeneralRe: How to set Duplex capability Pin
MuzzyB21-Jul-16 1:20
memberMuzzyB21-Jul-16 1:20 
Questionhow set up file transfer mode? Pin
Niklaus1-Oct-03 22:08
memberNiklaus1-Oct-03 22:08 
AnswerRe: how set up file transfer mode? Pin
sdelbos11-Dec-03 23:29
membersdelbos11-Dec-03 23:29 
QuestionAny way to perform a fast convert to System.Drawing.Bitmap? Pin
killy_cn24-Sep-03 23:28
memberkilly_cn24-Sep-03 23:28 
GeneralImage size Pin
len00x19-Sep-03 5:04
memberlen00x19-Sep-03 5:04 
GeneralRe: Image size Pin
NETMaster19-Sep-03 8:30
memberNETMaster19-Sep-03 8:30 
Questionhow to convert a 24bit bpm file into a monochrome (2 bit) bmp Pin
Rai Yawar Ijaz12-Sep-03 10:51
memberRai Yawar Ijaz12-Sep-03 10:51 

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