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.NET TWAIN image scanner

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4.91 (224 votes)
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

GeneralRe: Problem of Setting Brightness Pin
Member 260553420-Jan-10 18:38
memberMember 260553420-Jan-10 18:38 
GeneralRe: Problem of Setting Brightness Pin
Member 260553420-Jan-10 19:41
memberMember 260553420-Jan-10 19:41 
AnswerRe: Problem of Setting Brightness Pin
Spike0xFF28-Jan-10 14:56
memberSpike0xFF28-Jan-10 14:56 
GeneralCapability Brightness/Constrast Could not Work on the image Pin
Member 260553418-Jan-10 1:36
memberMember 260553418-Jan-10 1:36 
QuestionAcquire images in picture box from scanner [modified] Pin
manzoor4u18-Jan-10 0:28
membermanzoor4u18-Jan-10 0:28 
QuestionHelp needed getting list of available scanners displayed in my own listbox and NOT via the Twain's GUI... Pin
David Temlak11-Jan-10 3:51
memberDavid Temlak11-Jan-10 3:51 
AnswerRe: Help needed getting list of available scanners displayed in my own listbox and NOT via the Twain's GUI... Pin
Schnapple10-Mar-10 8:56
memberSchnapple10-Mar-10 8:56 
QuestionHelp need to select device from my own dropdownbox error--.Unable to cast object of type 'System.String' to type 'TwainLib.TwIdentity' Pin
jymitra13-Jul-10 20:28
memberjymitra13-Jul-10 20:28 
I have done something for the same issue .i have successfully feel deropdownbox with devices but i m getting problem while i m selecting source device from my own dropdownbox please tell me what kind of changes i have to do so that it can work is my code snippet
..please help me..

these are some methods that i have used and added in Twainlib.cs
public bool SelectSource(int SourceIndex)
      ArrayList _Sources = new ArrayList();
    //  Select correct datasource
    srcds = (TwIdentity)_Sources[SourceIndex];

      //Close Datasource
     // CloseSrc();

      //Invoke set command of datasource
      TwRC rc;
      rc = DSMident(appid, IntPtr.Zero, TwDG.Control, TwDAT.Identity, TwMSG.Set, srcds);
      if (rc != TwRC.Success)
          return false;

      return true;

  //return Arraylist of devices
  public ArrayList GetDataSources()
      //TwRC rc;
      ArrayList ds = new ArrayList();
      ArrayList _Sources = new ArrayList();

      TwIdentity scanner = new TwIdentity();
      TwIdentity defScanner = new TwIdentity();

      //Get the default
      //if (DSMident(appid, IntPtr.Zero, TwDG.Control, TwDAT.Identity, TwMSG.GetDefault, defScanner) != TwRC.Success)
      //    //return _Sources.Count;

          if (DSMident(appid, IntPtr.Zero, TwDG.Control, TwDAT.Identity, TwMSG.GetFirst, scanner) == TwRC.Success)
              //Select default data source
              //if (scanner.Id == defScanner.Id)
                  //  SelectedSource = _Sources.Count;
              //Loop through and find all sources and add to array
              scanner = new TwIdentity();
      while (DSMident(appid, IntPtr.Zero, TwDG.Control, TwDAT.Identity, TwMSG.GetNext, scanner) == TwRC.Success)
         // scanner = new TwIdentity();

      return _Sources;

And in mainform.cs
//For filling up dropdownbox

devices = tw.GetDataSources();
cmbSelectSrc.DropDownStyle = ComboBoxStyle.DropDownList;
cmbSelectSrc.DataSource = devices;

//And by selecting this method is called
private void cmbSelectSrc_SelectedIndexChanged(object sender, EventArgs e)

while i m selecting device it gives

Unable to cast object of type 'System.String' to type 'TwainLib.TwIdentity'
GeneralResolution and color Pin
AnandDegwekar9-Jan-10 3:43
memberAnandDegwekar9-Jan-10 3:43 
GeneralRe: Resolution and color Pin
Member 260553418-Jan-10 0:38
memberMember 260553418-Jan-10 0:38 
GeneralRe: Resolution and color Pin
AnandDegwekar18-Jan-10 5:25
memberAnandDegwekar18-Jan-10 5:25 
QuestionDirect Input of images Pin
AnandDegwekar4-Jan-10 23:14
memberAnandDegwekar4-Jan-10 23:14 
AnswerRe: Direct Input of images Pin
bgmeek7-Jan-10 5:12
memberbgmeek7-Jan-10 5:12 
GeneralRe: Direct Input of images Pin
AnandDegwekar8-Jan-10 7:19
memberAnandDegwekar8-Jan-10 7:19 
GeneralDuplex Mode Pin
morke4-Jan-10 1:49
membermorke4-Jan-10 1:49 
Generaltwain_32.dll question Pin
DroneBG21-Dec-09 3:02
memberDroneBG21-Dec-09 3:02 
GeneralRe: twain_32.dll question Pin
NitinMakwana9-Feb-10 0:21
memberNitinMakwana9-Feb-10 0:21 
Generalthere's nothing un-modern or '16-bit' about TWAIN Pin
Spike0xFF18-Dec-09 14:33
memberSpike0xFF18-Dec-09 14:33 
Questionhow to save TWAIN Scanning image into pdf file Pin
Member 421353221-Nov-09 12:30
memberMember 421353221-Nov-09 12:30 
AnswerRe: how to save TWAIN Scanning image into pdf file Pin
AmitSood198520-Jul-10 23:58
memberAmitSood198520-Jul-10 23:58 
GeneralHello NetMaster Pin
Mike Kostersitz11-Nov-09 1:27
memberMike Kostersitz11-Nov-09 1:27 
Generali need the code with vb Pin
alaa9216111-Nov-09 23:54
memberalaa9216111-Nov-09 23:54 
QuestionHelp on Blank Page Detection Pin
geetesh.12331-Oct-09 2:30
membergeetesh.12331-Oct-09 2:30 
Questionnewbie question: mdi child form problem. Pin
dude25175418-Oct-09 18:49
memberdude25175418-Oct-09 18:49 
QuestionConnect to network scanner Pin
ihsan4u28-Sep-09 0:17
memberihsan4u28-Sep-09 0:17 

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Posted 16 Sep 2001


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