Marshaling data between managed VB.NET and an unmanaged C Dll

**tlhintoq** · Feb 18 '10, 07:35 PM

Another post here on Bytes for nearly the same thing.

http://bytes.com/topic/net/answers/881730-using-c-libs-headers-through-dll-c

Hint: There is a search box in the upper right of the Bytes webpage.

**MarDude** · Feb 19 '10, 02:55 PM

Thanks for the reply. For the record I searched through quite a few pages on this site which took a couple of hours and found NOTHING helpful. Perhaps no one used the same keywords I used, although it did pick up several quotes that contained the keywords which the article had no relation.
And that link you provided doesn't provide anything useful, besides it discusses C#, NOT VB. since the problem I am having involves passing data from one language to another, including a third unnecessarily complicates things. Perhaps I need to give more detailed information, here goes:

I am writing a VB.NET application that uses TWAIN (dumb dll). When I call the function and get data back it is incorrect. What I don't know is what I am doing wrong. Here are the C structures:

Code:

typedef unsigned short TW_UINT16, FAR *pTW_UINT16;
typedef unsigned long  TW_UINT32, FAR *pTW_UINT32;
typedef HANDLE         TW_HANDLE;

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;

typedef struct {
   TW_UINT16  ItemType;
   TW_UINT32  Item;
} TW_ONEVALUE, FAR * pTW_ONEVALUE;

Which have been converted to VB.NET structure/class:

Code:

    <StructLayout(LayoutKind.Sequential, Pack:=4)> Private Structure twsCapability
        Public Cap As twCap         'short
        Public ConType As twContainerTyoe   'short
        Public Handle As IntPtr     'Pointer to a container
    End Structure

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

Then I run this routine and get back 64356640 for the physical height and width:

Code:

'Call into the C library
    <DllImport("twain_32.dll", EntryPoint:="#1")> Private Shared Function DS_Capability( _
        <[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

'Start here:
            ...previous code
            twCapability = New twsCapability
            With twCapability
                .Cap = twCap.IPhysicalWidth
                .ConType = twContainerTyoe.One   '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)
            ...more code

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

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)
                    pTemp = VarPtr(twOneValue)
                    CopyMemory(pTemp, pOneValue, Marshal.SizeOf(twOneValue))
                    GlobalUnlock(twCapability.Handle)
                    strTemp = CType(twOneValue.Item, String)
                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 included the function GetOneValueNew( because I found that method somewhere on the net, but it was written in VB6. Unfortunately I get 0 back from that.

I tried searching the net but didn't find my answer. I used "VB.NET marshal structure C dll". I included this incase someone may have a better search string.

I even tried writing the code to access the dll in C++. But I get an error in the VB routine that calls it "Attempted to read or write protected memory. This is often an indication that other memory is corrupt." I presume that is also a problem form the unmanaged C dll trying to write to memory allocated in managed C++.

Thanks for looking!

**MarDude** · Feb 23 '10, 06:22 PM

I found out that a pointer to a structure is not the same as a pointer to the memory that the structure occupies. Changed my code to copy the members explicitly.

Code:

        Dim rc As twRC
        Dim pOneValue As IntPtr
        Dim pPosition As Int32 = 0
        Dim strValue As String
        Dim twFix32 As twsFix32
        Dim twOneValue As TW_ONEVALUE


        rc = DS_Capability(mtwApp, mtwSource, twDG.Control, twDAT.Capability, twMSG.Get, twCapability)
        If rc = twRC.Success Then

            twOneValue = New TW_ONEVALUE
            pOneValue = GlobalLock(twCapability.Handle)
            twOneValue.ItemType = CType(Runtime.InteropServices.Marshal.ReadInt16(pOneValue, pPosition), ItemType)
            pPosition += 2

            Select Case twOneValue.ItemType

                Case ItemType.Fix32
                    Dim origFrac As Int32
                    twFix32 = New twsFix32

                    twOneValue.Item = Marshal.ReadInt16(pOneValue, pPosition)
                    twFix32.Whole = twOneValue.Item
                    pPosition += 2


                    origFrac = Marshal.ReadInt16(pOneValue, pPosition)
                    If origFrac < 0 Then
                        origFrac += 65536
                    End If
                    pPosition += 2

                    'twFix32.Frac = CUShort(origFrac)
                    strValue = (twFix32.Whole + (origFrac / 65536)).ToString

                Case Else
                    MsgBox("Unexpected One Value.  Ignored")

            End Select

            GlobalUnlock(twCapability.Handle)

            Return strValue

        End If

Marshaling data between managed VB.NET and an unmanaged C Dll

Marshaling data between managed VB.NET and an unmanaged C Dll

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