Passing structure to unamnaged code in VB.NET

Hello,

     I am working on an application to scan in VB.NET using TWAIN.  but I am having problems with the structures and am getting incorrect data.  I created a class in C++ and found that one of the VB structures (twsIdentity) is 156 bytes using Marshal.SizeOf.  i even tried eliminating the new method and declaring Version as new but it didn't matter.

   <StructLayout(LayoutKind.Sequential, Pack:=2, CharSet:=CharSet.Ansi)> Friend Class twsIdentity '***MAR
        Public Id As IntPtr                     '4
        Public Version As twsVersion            '42
        Public ProtocolMajor As Short           '2
        Public ProtocolMinor As Short           '2
        Public SupportedGroups As Integer       '4
        <MarshalAs(UnmanagedType.ByValTStr, SizeConst:=34)> Public Manufacturer As String
        <MarshalAs(UnmanagedType.ByValTStr, SizeConst:=34)> Public ProductFamily As String
        <MarshalAs(UnmanagedType.ByValTStr, SizeConst:=34)> Public ProductName As String

        Public Sub New()
            MyBase.New()
            Version = New twsVersion
        End Sub
    End Class

    <StructLayout(LayoutKind.Sequential, Pack:=2, CharSet:=CharSet.Ansi)> Friend Class twsVersion '***MAR
        Public MajorNum As Short
        Public MinorNum As Short
        Public Language As Short
        Public Country As Short
        <MarshalAs(UnmanagedType.ByValTStr, SizeConst:=34)> Public Info As String
    End Class

I can't pass the structure between C++ and VB.  I assume because the structure in C is 160 bytes using sizeof.

 typedef unsigned short TW_UINT16;
 typedef unsigned long  TW_UINT32;
 typedef char    TW_STR32[34];

 struct TW_VERSION{
   TW_UINT16  MajorNum;  /* Major revision number of the software. */
   TW_UINT16  MinorNum;  /* Incremental revision number of the software. */
   TW_UINT16  Language;  /* e.g. TWLG_SWISSFRENCH */
   TW_UINT16  Country;   /* e.g. TWCY_SWITZERLAND */
   TW_STR32   Info;      /* e.g. "1.0b3 Beta release" */
 };

 struct TW_IDENTITY{
    TW_UINT32  Id;              /* Unique number.  In Windows, application hWnd      */
    TW_VERSION Version;         /* Identifies the piece of code              */
    TW_UINT16  ProtocolMajor;   /* Application and DS must set to TWON_PROTOCOLMAJOR */
    TW_UINT16  ProtocolMinor;   /* Application and DS must set to TWON_PROTOCOLMINOR */
    TW_UINT32  SupportedGroups; /* Bit field OR combination of DG_ constants */
    TW_STR32   Manufacturer;    /* Manufacturer name, e.g. "Hewlett-Packard" */
    TW_STR32   ProductFamily;   /* Product family name, e.g. "ScanJet"       */
    TW_STR32   ProductName;     /* Product name, e.g. "ScanJet Plus"         */
 };

Although when I use this structure (twsIdentity) in VB (calling twain_32.dll) and pass it to the dll, I get it back OK.  I get problems from another structure () when I try to set the capabilities of the scanner.  I was going to set the capabilities in a C dll to see if that fixes if, but then I can't pass the twsIdentity structure between C and VB.  I could try moving all the functionality to a C DLL, but the boss wants the app done in VB.  I just want to know what I am doing wrong.  Any ideas?

Here is the VB structure that I can't get any good data from:

    <StructLayout(LayoutKind.Sequential, Pack:=2)> Private Structure twsCapability '***MAR
        Public Cap As twCap                   'short
        Public ConType As twContainerTyoe   'short
        Public Handle As IntPtr     'Pointer to a container
    End Structure

copied from this structure:

typedef struct {
   TW_UINT16  Cap; /* id of capability to set or get, e.g. CAP_BRIGHTNESS */
   TW_UINT16  ConType; /* TWON_ONEVALUE, _RANGE, _ENUMERATION or _ARRAY   */
   TW_HANDLE  hContainer; /* Handle to container of type Dat              */
} TW_CAPABILITY, FAR * pTW_CAPABILITY;

And here is the code that tries to get the data from the dll (which allocates the space for it)

           ...other code comes before

            twCapability = New twsCapability
            With twCapability
                .Cap = twCap.IPhysicalWidth
                '.ConType = 7 'TWON_DONTCARE16
                '.Handle = Nothing
            End With
            strWidth = GetOneValue(twCapability)

            ...other code follows



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 help is appreciated, if anyone can direct me to any dull reading that may help me I will read it!!

Thanks for looking

Can someone give me some feedback?

1)   Is my post clear?

2)   Did I post too much code?

3)   Does anyome know anything about marshaling data between managed and unmanaged code?

4)   Any links to any possible help?  I've been searching for days and found nothing.

I was hoping for some tech talk, but so far have gotten zip from this site.  Anybody know anything?

MarDude said:

Can someone give me some feedback?

 

1)   Is my post clear?

2)   Did I post too much code?

3)   Does anyome know anything about marshaling data between managed and unmanaged code?

4)   Any links to any possible help?  I've been searching for days and found nothing.

 

I was hoping for some tech talk, but so far have gotten zip from this site.  Anybody know anything?

There's some stuff that's missing. How is twCap defined? Or twContainerType? Or TW_ONEVALUE?

The first two "structures" in your code aren't structures, they're classes. This may not make a lot of difference in C++, but in VB it does. They have to be structures for this kind of interop to work correctly.

If you don't have to use Twain, the easiest and painfree way is to use WIA. There is a good tutorial here. Also have a look at the coding4fun site because I am sure someone has an implementation that does what you are attempting.

vesuvius said:

If you don't have to use Twain, the easiest and painfree way is to use WIA. There is a good tutorial here. Also have a look at the coding4fun site because I am sure someone has an implementation that does what you are attempting.

WIA is easy, but not supported on all models of scanners.

TWAIN is more difficult to use, but is supported in virtually all types of scanners be produced in last 10+ years.

Unfortunately not much (if exist at all) vendors willing to add WIA support back to older models. And for certain models, if you choose the drivers with WIA support, your scanners will not be fully functional. (This is known for Epson scanners)

Dexter said:

MarDude said:

*snip*

There's some stuff that's missing. How is twCap defined? Or twContainerType? Or TW_ONEVALUE?

Thanks for your reply Dexter.  Here are your answers, twCap and twContainerType are shorts.

    Friend Enum twCap As Short
        XferCount = &H1
        ...more values
        IPhysicalWidth = &H1111                 'Get the max image width
        IPhysicalHeight = &H1112                'Get the max image height
        ...more values
        ICAP_SUPPORTEDEXTIMAGEINFO = &H115E
    End Enum

    Friend Enum twContainerTyoe As Short
        Array = &H3
        [Enum] = &H4
        One = &H5
        Range = &H6
        DontCare = -1
    End Enum

and here is TW_ONEVALUE (I changed Pack:=4 from 2 because the C struc reported 8 bytes and Marshal.Sizeof reported 6 bytes without the change, 8 bytes with):

    <StructLayout(LayoutKind.Sequential, Pack:=4)> Friend Class TW_ONEVALUE
        Public ItemType As Short
        Public Item As Integer
    End Class
 

Sven Groot said:

The first two "structures" in your code aren't structures, they're classes. This may not make a lot of difference in C++, but in VB it does. They have to be structures for this kind of interop to work correctly.

Thanks for your reply Sven.  I called them structures because thats who they are defined in the C dll.  If I change them to structures, then I receive an error from the dll "Attempted to read or write protected memory. This is often an indication that other memory is corrupt."  They work OK as classes and get the Id member filled in.  I was under the inpression that classes and structures are pretty much the same in VB with the exceptions:

1)  A structure gets allocated on the heap if it is declared locally and is not too big.

2)  A structure does not need to be created with the new keywork or a constructor.

Am I incorrect?

MarDude said:

Dexter said:

*snip*

Thanks for your reply Dexter.  Here are your answers, twCap and twContainerType are shorts.

 

    Friend Enum twCap As Short
        XferCount = &H1
        ...more values
        IPhysicalWidth = &H1111                 'Get the max image width
        IPhysicalHeight = &H1112                'Get the max image height
        ...more values
        ICAP_SUPPORTEDEXTIMAGEINFO = &H115E
    End Enum

    Friend Enum twContainerTyoe As Short
        Array = &H3
        [Enum] = &H4
        One = &H5
        Range = &H6
        DontCare = -1
    End Enum

 

and here is TW_ONEVALUE (I changed Pack:=4 from 2 because the C struc reported 8 bytes and Marshal.Sizeof reported 6 bytes without the change, 8 bytes with):

    <StructLayout(LayoutKind.Sequential, Pack:=4)> Friend Class TW_ONEVALUE
        Public ItemType As Short
        Public Item As Integer
    End Class
 

and here is TW_ONEVALUE (I changed Pack:=4 from 2 because the C struc reported 8 bytes and Marshal.Sizeof reported 6 bytes without the change, 8 bytes with):

The twain header file uses packing = 2, the size of that C struct should be 6, not 8. How exactly did you get that 8?

MarDude said:

Sven Groot said:

*snip*

Thanks for your reply Sven.  I called them structures because thats who they are defined in the C dll.  If I change them to structures, then I receive an error from the dll "Attempted to read or write protected memory. This is often an indication that other memory is corrupt."  They work OK as classes and get the Id member filled in.  I was under the inpression that classes and structures are pretty much the same in VB with the exceptions:

1)  A structure gets allocated on the heap if it is declared locally and is not too big.

2)  A structure does not need to be created with the new keywork or a constructor.

 

Am I incorrect?

I tried to call the dll in C, but can't seem to pass the class (defined as a class or structure) twsIdentity.  I get an error "Value of type 'RFQConsole.clsScan.twsIdentity' cannot be converted to 'TW_IDENTITY'."  I have both projects defined in the same solution, and I can step into the C++ code from VB, but this call will not compile:

Twain.Wrapper.GetCap(mtwApp, mtwSource)

Where mtwApp and mtwSource are both twsIdentity.  I changed the Pack attribute from 2 to 4 and now twsIdentity is the same size as TW_IDENTITY.  I don't really want to use C++, but I am trying everything.

I also changed the Pach in the TW_ONEVALUE from 2 to 4 and now it is 8 bytes.  But when I try to get the Physical height or width of the scanner's surface, I get garbage.

Using the call DS_Capability, it returns with a SUCCESS, but these are the 8 bytes that twCapability.Handle points to (should be a TW_ONEVALUE) {16, 212, 34, 0, 48, 1, 214, 3}.  That was the width.  Here's the height: {24, 53, 35, 0, 56, 1, 214, 3}

If the last 4 bytes are the dimensions of the scanner, then it must be pretty big what ever the units!!  If I am getting the data back properly.  I use this to get the structure (or class) filled:

Marshal.PtrToStructure(twCapability.Handle, twOneValue)

and this to inspect the bytes after the pointer:

BitConverter.GetBytes(Marshal.ReadInt64(twCapability.Handle))

Is that the wrong thing to use?

Oh and vesuvius, the boss wants it done with TWAIN, so I am stuck with it.

One more peice of information, I don't know how helpful; I can set twContainerTyoe to 0.  The call sets it to ONEVALUE, which can't be more than 8 bytes.  But those 8 bytes don't look like anything good to me as far as an integer goes where it is supposed to be a dimension of the scanner in some type of human readable units.  8 forums, and no answers.  I'm begining to think it can't be done.

Thanks for anything.

MarDude said:

MarDude said:

*snip*

I tried to call the dll in C, but can't seem to pass the class (defined as a class or structure) twsIdentity.  I get an error "Value of type 'RFQConsole.clsScan.twsIdentity' cannot be converted to 'TW_IDENTITY'."  I have both projects defined in the same solution, and I can step into the C++ code from VB, but this call will not compile:

Twain.Wrapper.GetCap(mtwApp, mtwSource)

Where mtwApp and mtwSource are both twsIdentity.  I changed the Pack attribute from 2 to 4 and now twsIdentity is the same size as TW_IDENTITY.  I don't really want to use C++, but I am trying everything.

 

I also changed the Pach in the TW_ONEVALUE from 2 to 4 and now it is 8 bytes.  But when I try to get the Physical height or width of the scanner's surface, I get garbage.

 

Using the call DS_Capability, it returns with a SUCCESS, but these are the 8 bytes that twCapability.Handle points to (should be a TW_ONEVALUE) {16, 212, 34, 0, 48, 1, 214, 3}.  That was the width.  Here's the height: {24, 53, 35, 0, 56, 1, 214, 3}

 

If the last 4 bytes are the dimensions of the scanner, then it must be pretty big what ever the units!!  If I am getting the data back properly.  I use this to get the structure (or class) filled:

Marshal.PtrToStructure(twCapability.Handle, twOneValue)

and this to inspect the bytes after the pointer:

BitConverter.GetBytes(Marshal.ReadInt64(twCapability.Handle))

 

Is that the wrong thing to use?

 

Oh and vesuvius, the boss wants it done with TWAIN, so I am stuck with it.

 

One more peice of information, I don't know how helpful; I can set twContainerTyoe to 0.  The call sets it to ONEVALUE, which can't be more than 8 bytes.  But those 8 bytes don't look like anything good to me as far as an integer goes where it is supposed to be a dimension of the scanner in some type of human readable units.  8 forums, and no answers.  I'm begining to think it can't be done.

 

Thanks for anything.

 

Hmm, I forgot that twCapability.Handle is not a pointer, you can't use Marshal.PtrToStructure with it. It's a "GlobalAlloc" memory handle, you probably need to call GlobalLock on it to get a pointer:

Dim ptr As IntPtr

ptr = GlobalLock(twCapability.Handle)

Marshal.PtrToStructure(ptr, twOneHandle)

GlobalUnlock(twCapability.Handle)

<DllImport("kernel32.dll", CharSet:=CharSet.Auto, ExactSpelling:=True)> _
Public Shared Function GlobalLock(ByVal handle As IntPtr) As IntPtr
End Function

<DllImport("kernel32.dll", CharSet:=CharSet.Auto, ExactSpelling:=True)> _
Public Shared Function GlobalUnlock(ByVal handle As IntPtr) As Boolean
End Function

PS: sorry if the above doesn't compile, I'm a C++/C# guy

Dexter said:

MarDude said:

*snip*

Hmm, I forgot that twCapability.Handle is not a pointer, you can't use Marshal.PtrToStructure with it. It's a "GlobalAlloc" memory handle, you probably need to call GlobalLock on it to get a pointer:

 

Dim ptr As IntPtr

ptr = GlobalLock(twCapability.Handle)

Marshal.PtrToStructure(ptr, twOneHandle)

GlobalUnlock(twCapability.Handle)

 

<DllImport("kernel32.dll", CharSet:=CharSet.Auto, ExactSpelling:=True)> _
Public Shared Function GlobalLock(ByVal handle As IntPtr) As IntPtr
End Function

<DllImport("kernel32.dll", CharSet:=CharSet.Auto, ExactSpelling:=True)> _
Public Shared Function GlobalUnlock(ByVal handle As IntPtr) As Boolean
End Function

 

PS: sorry if the above doesn't compile, I'm a C++/C# guy

That was it!!  Thanks Dexter.  Being a VB guy memory and pointers are a new world to me.  I thought that a pointer and handle were the same thing, thanks for the info.  I changed my code:

Private Function GetOneValueNew(ByVal twCapability As twsCapability) As String

        Dim rc As twRC
        Dim pOneValue As Integer
        Dim pTemp As Integer
        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
                    pOneValue = GlobalLock(twCapability.Handle)
                    Marshal.PtrToStructure(pOneValue, twOneValue)
                    strTemp = (twOneValue.Item.Whole + (twOneValue.Item.Frac / 65536)).ToString
                Case Else
                    MsgBox("Error, one value expected; " & twCapability.ConType & " returned.")
            End Select
            GlobalFree(twCapability.Handle)
            Return strTemp
        Else
            ErrorStatus()
            Return vbNullString
        End If

    End Function

I also needed to change this structure:

<StructLayout(LayoutKind.Sequential, Pack:=4)> Friend Class TW_ONEVALUE
        Public ItemType As Short
        Public Item As twsFix32
    End Class

now I get 7.5 for the width and 10.6 for the height.  They are reasonable numbers!!