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Hello,
I am working on using TWAIN from VB.NET. I have gotten to state 4 successfully, but have been unable to set or get capabilities. The project spec requires the source UI to be hidden. But when I hide it, the scanner appears to be waiting for something. I assumed it was some capabilities, but haven’t figured out how to get them, let alone set them. I have an attached scanner that has issues, but I am using it for testing since the scanner that will be used is unavailable to me in debug mode.
I am using Win XP SP3 and VB.NET in Visual Studio 2005. I believe I am making a mistake copying the structure because I get 18874402 for the physical width and height and I get this error when I try to get the units (enumeration) “Attempted to read or write protected memory. This is often an indication that other memory is corrupt.” I tried getting the “OveValue” by changing (hacking) the variable types to different values, the closest I came to reasonalbe values were Width and Hieght = 33, since the scanner is a flatbed scanner (11″ x 8.5″) those values are wrong. I am trying to figure out how to write a class in C++ to see if the returned values are any different. I tried changing twsCapability from a structure to a class, but that fails to return anything.
I read the twain specification and looked at all the examples I can find, but I must be missing something. (Do structure members translate to identicle types between VB and C?) Here is my code:
<[In](), Out()> ByVal pOrigin As twsIdentity, _
<[In]()> ByVal pSource As twsIdentity, _
ByVal dg As twDG, _
ByVal dat As twDAT, _
ByVal msg As twMSG, _
<[In](), Out()> ByRef pData As twsCapability) As twRC
End Function
Public Cap As twCap
Public ConType As twContainerTyoe
Public Handle As IntPtr 'Pointer to a container
End Structure
Public ItemType As Short
Public Item As Integer
End Class
Public Sub NegotiateCapabilities()
...Other Code
twCapability = New twsCapability
With twCapability
.Cap = twCap.IPhysicalWidth
'.ConType = 7 'TWON_DONTCARE16
'.Handle = Nothing
End With
strWidth = GetOneValue(twCapability)
With twCapability
.Cap = twCap.IPhysicalHeight
'.ConType = 7 'TWON_DONTCARE16
'.Handle = Nothing
End With
strHeight = GetOneValue(twCapability)
'Here strWidth and strHeight = 18874402
...Other Code
End Sub
Private Function GetOneValue(ByVal twCapability As twsCapability) As String
Dim rc As twRC
Dim strTemp As String
Dim twOneValue As TW_ONEVALUE
rc = DS_Capability(mtwApp, mtwSource, twDG.Control, twDAT.Capability, twMSG.Get, twCapability)
If rc = twRC.Success Then
strTemp = vbNullString
Select Case twCapability.ConType
Case twContainerTyoe.One
twOneValue = New TW_ONEVALUE
Marshal.PtrToStructure(twCapability.Handle, twOneValue)
strTemp = CType(twOneValue.Item, String)
Case Else
MsgBox("Error, one value expected; " & twCapability.ConType & " returned.")
End Select
Marshal.FreeHGlobal(twCapability.Handle)
Return strTemp
Else
ErrorStatus()
Return vbNullString
End If
End Function
Any assistance is appreciated. I will read anything, no matter how dull, to resolve this, so any links anyone knows of on how to marshall complex structures between VB managed and C unmanaged code will be helpful if no one knows what I am doing wrong. Thanks for looking 🙂 .
Sorry you’ve been having trouble and that it seems as if you’re alone. Have you had a look at codeplex.com/openTwain?
The definitions from that project are correct as are the Imports. If you’re having trouble in vb.net past that let me know..
If you set the guiharness as your startup project, then set a breakpoint in cTwain.vb method UpdateCapabilities() you should see the sample get the values for a couple capabilities with the methods Update_DeviceOnline, Update_EnableDsUiOnly(), Update_AvailableXferMechs(), Update_AvailableImageFileFormats() and Update_CurrentImageFileFormat().
More specifically if you’re looking for the code that gathers a oneValue you’ll have to dig through the twCapability class, but I think you you set a breakpoint at UpdateCapabilities you’ll see how the code determines the values. Lemme know when/if you have more questions.
.