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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

QuestionHow to used scanner in vs2008 aspx Pin
Member 426204223-Feb-10 23:56
memberMember 426204223-Feb-10 23:56 
GeneralComponentize Pin
Member 383947323-Feb-10 22:15
memberMember 383947323-Feb-10 22:15 
GeneralSave Image To folder Without Opening Save dialogbox Pin
NitinMakwana14-Feb-10 19:18
memberNitinMakwana14-Feb-10 19:18 
GeneralRe: Save Image To folder Without Opening Save dialogbox Pin
NitinMakwana17-Feb-10 2:40
memberNitinMakwana17-Feb-10 2:40 
GeneralRe: Save Image To folder Without Opening Save dialogbox Pin
Alessandro Test3-Mar-10 3:50
memberAlessandro Test3-Mar-10 3:50 
GeneralRe: Save Image To folder Without Opening Save dialogbox Pin
NitinMakwana4-Mar-10 20:30
memberNitinMakwana4-Mar-10 20:30 
GeneralRe: Save Image To folder Without Opening Save dialogbox Pin
Abdullah Çetinkaya23-Jan-13 1:15
memberAbdullah Çetinkaya23-Jan-13 1:15 
GeneralExtended Image Info. Pin
MonkFox8-Feb-10 11:18
memberMonkFox8-Feb-10 11:18 
Ok I found the re-written version of this project in and so the syntax is different, but same concept. I've added the TW_EXTIMAGEINFO struct and TW_INFO struct to the project. Also added the TWEI enum and another dll function called 'DSeiinf(...)'.

Here is the code:

just the declaration in TwainLib.vb:
<br />
        <DllImport("twain_32.dll", EntryPoint:="#1")> Private Shared Function DSeiinf(<[In](), Out()> ByVal origin As TwIdentity, <[In]()> ByVal dest As TwIdentity, ByVal dg As TwDG, ByVal dat As TwDAT, ByVal msg As TwMSG, <[In](), Out()> ByVal extimginf As TW_EXTIMAGEINFO) As TwRC<br />
        End Function<br />

Here is the code added in the TwainDefs.vb that I added:

just declaring the TWEI constants (only one i'm interested in)
Friend Enum TwEi As Short
End Enum

definitions of the two structs needed for the DC_IMAGE/DAT_EXTIMAGEINFO/MSG_GET combo
<StructLayout(LayoutKind.Sequential, Pack:=2)> Friend Class TW_EXTIMAGEINFO
    Public NumInfos As Integer = 1
    Public Info(0) As TW_INFO
End Class

<StructLayout(LayoutKind.Sequential, Pack:=2)> Friend Class TW_INFO
    Public InfoID As Short
    Public ItemType As Short
    Public NumItems As Short
    Public ReturnCode As Short
    Public Item As IntPtr
End Class

Code added in a sub to get the data after scanning has been done in TwainLib.vb.

Public Sub getPictureInfo()
    Dim extimginf As New TW_EXTIMAGEINFO
    Dim inf1 As New TW_INFO
        inf1.InfoID = CType(TwEi.TWEI_BARCODETEXT, Short)
        inf1.ItemType = 0
        inf1.NumItems = 0
        inf1.ReturnCode = 0
        inf1.Item = 0
        extimginf.Info(0) = inf1

        Dim rc As TwRC = DSeiinf(appid, srcds, TwDG.Image, TwDAT.ExtImageInfo, TwMSG.Get, extimginf)
        If (rc <> TwRC.Success) Then
        End If
    Catch ex As Exception
    End Try
End Sub

This sub is called on a menuItem_click event and I've been calling it after the image(s) are scanned. But rc is always 'failure'. I've read about you can only get the extimageinfo at state 7 of the scan process, but I have no clue how to do that. Any help would be awesome.


GeneralRe: Extended Image Info. Pin
MonkFox8-Feb-10 11:28
memberMonkFox8-Feb-10 11:28 
GeneralProblem of Setting Brightness Pin
Member 260553419-Jan-10 0:49
memberMember 260553419-Jan-10 0:49 
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 
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 

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Posted 16 Sep 2001


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