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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


About the Author

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Switzerland Switzerland
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Comments and Discussions

Questionthanks Pin
Member 786391322-May-13 13:15
memberMember 786391322-May-13 13:15 
QuestionHow can i scan multi pages Pin
Member 397326719-May-13 18:33
memberMember 397326719-May-13 18:33 
QuestionNeed help to fix Issue in Twain .net Library Pin
vinaykn.rao22-Apr-13 4:07
membervinaykn.rao22-Apr-13 4:07 
QuestionException throws when I use in Win7 64 bit Pin
anisfathima18-Apr-13 23:11
memberanisfathima18-Apr-13 23:11 
AnswerRe: Exception throws when I use in Win7 64 bit Pin
Jnnxp5-Aug-13 17:11
memberJnnxp5-Aug-13 17:11 
Questionhow set Image info Pin
Nabawoka15-Apr-13 7:59
memberNabawoka15-Apr-13 7:59 
QuestionScanner Error Pin
asheykh14-Apr-13 23:05
memberasheykh14-Apr-13 23:05 
QuestionTwainCommand.TransferReady: hbitmap to gdi+ bitmap Pin
liuxu093826-Mar-13 22:21
memberliuxu093826-Mar-13 22:21 
bool IMessageFilter.PreFilterMessage( ref Message m )
TwainCommand cmd = tw.PassMessage( ref m );
if( cmd == TwainCommand.Not )
return false;

switch( cmd )
case TwainCommand.CloseRequest:
case TwainCommand.CloseOk:
case TwainCommand.DeviceEvent:
case TwainCommand.TransferReady:
System.Collections.ArrayList pics = tw.TransferPictures();

for (int i = 0; i < pics.Count; i++)
IntPtr img = (IntPtr)pics[i];
Bitmap bmp = Bitmap.FromHbitmap(img);
pictureBox1.Image = bmp;
// PicForm newpic = new PicForm( img );
// newpic.MdiParent = this;
// int picnum = i + 1;
// newpic.Text = "ScanPass" + picnumber.ToString() + "_Pic" + picnum.ToString();
// newpic.Show();

return true;

The program runs at Bitmap bmp = Bitmap.FromHbitmap(img); Error: GDI + generic error occurred.
QuestionThe "" is damaged? Pin
liuxu093826-Mar-13 1:16
memberliuxu093826-Mar-13 1:16 
QuestionScanning on Win7 64Bit Pin
khalilsa27-Feb-13 5:19
memberkhalilsa27-Feb-13 5:19 
AnswerRe: Scanning on Win7 64Bit Pin
ricardofrias20-Mar-13 8:48
memberricardofrias20-Mar-13 8:48 
AnswerRe: Scanning on Win7 64Bit Pin
Jnnxp5-Aug-13 17:12
memberJnnxp5-Aug-13 17:12 
QuestionWIA driver is not working with multiple pages Pin
anisfathima9-Jan-13 23:59
memberanisfathima9-Jan-13 23:59 
AnswerRe: WIA driver is not working with multiple pages Pin
smile869114-Feb-13 9:51
membersmile869114-Feb-13 9:51 
nturri13065-Jan-13 6:07
membernturri13065-Jan-13 6:07 
QuestionNeed help Pin
Member 806548423-Nov-12 18:54
memberMember 806548423-Nov-12 18:54 
AnswerRe: Need help Pin
Abdullah Çetinkaya23-Jan-13 0:11
memberAbdullah Çetinkaya23-Jan-13 0:11 
Questionget error in multipage scanning Pin
yuee17-Nov-12 3:08
memberyuee17-Nov-12 3:08 
AnswerRe: get error in multipage scanning Pin
smile86917-Nov-12 3:09
membersmile86917-Nov-12 3:09 
Questionhelp Pin
bijarcity24-Oct-12 7:09
memberbijarcity24-Oct-12 7:09 
AnswerRe: help Pin
smile86911-Nov-12 6:34
membersmile86911-Nov-12 6:34 
Questionhelp me Pin
bijarcity24-Oct-12 6:24
memberbijarcity24-Oct-12 6:24 
Questionvery helpful and tested with some scanners Pin
_Nizar21-Oct-12 10:09
member_Nizar21-Oct-12 10:09 
QuestionNeed support, Pin
mastermindinfo25-Sep-12 5:25
membermastermindinfo25-Sep-12 5:25 
AnswerNeed support, till pending Pin
mastermindinfo3-Oct-12 0:47
membermastermindinfo3-Oct-12 0:47 

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