Marshal Structure containing arrays to function in DLL

[David Fort]

Hi,
I'm upgrading a VB6 app to VB.net and I'm having a problem with a call to a
function provided in a DLL.

The function takes the address of a structure which it will fill in with
values.

I get an error:
----------------
An unhandled exception of type 'System.NullRef erenceException ' occured in
Project1.exe

Additional Information: Object reference not set to an instance of an object.
----------------

Based on the UPGRADE_TODOs etc that the upgrade wizard put in my code, I
suspect the problem is with marshaling the VB struct to the unmanaged
function.

Below is a (rather long) description of how the VB looked in VB6, what the
wizard turned it into, the C signature of the function, and some of the
things I've tried.

I would be very grateful for suggestions on how to pass the structure
correctly or where to find a good explanation of the rules. Thanks!
in VB6, it was like this
' the structure that the function takes as a parameter
Public Type NifInterfaceInf o ' Interface Infor structure
interfaceName(N IF_NAME_LEN) As Byte
DeviceID(DEV_ID _SIZE) As Byte
End Type

' the declaration of the function that takes the NifInterfaceInf o struct
Declare Function nifGetInterface List Lib "nifbstd" (ByVal session As Long,
ByRef numIntf As Integer, ByRef info As NifInterfaceInf o) As Long

' an example of a subroutine which invokes the function
Private Sub OpenSess()
' Open a fieldbus Session
Ret (nifOpenSession (vbNull, SessionDesc))

' Pass the max interface number to function "nifGetInterfac eList"
NoOfInterfaces = MAX_INTERFACES

' Get all the interface information
Ret (nifGetInterfac eList(SessionDe sc, NoOfInterfaces, InterfaceInfo(0 )))

For i = 0 To NoOfInterfaces - 1
' The interfaceName and DeviceID are array of byte, need to be
converted by function "getString"
ListInterface.A ddItem getString(Inter faceInfo(i).int erfaceName)
Next

End Sub

The upgrade wizard did this:
Public Structure NifInterfaceInf o ' Interface Infor structure
<VBFixedArray(N IF_NAME_LEN)> Dim interfaceName() As Byte
<VBFixedArray(D EV_ID_SIZE)> Dim DeviceID() As Byte

'UPGRADE_TODO: "Initialize " must be called to initialize instances
of this structure. Click for more:
'ms-help://MS.VSCC.2003/commoner/redir/redirect.htm?ke yword="vbup1026 "'
Public Sub Initialize()
ReDim interfaceName(N IF_NAME_LEN)
ReDim DeviceID(DEV_ID _SIZE)
End Sub
End Structure

'UPGRADE_WARNIN G: Structure NifInterfaceInf o may require marshalling
attributes to be passed as an argument in this Declare statement. Click for
more: 'ms-help://MS.VSCC.2003/commoner/redir/redirect.htm?ke yword="vbup1050 "'
Declare Function nifGetInterface List Lib "nifbstd" (ByVal session As
Integer, ByRef numIntf As Short, ByRef info As NifInterfaceInf o) As Integer
Private Sub OpenSess()
' Open a fieldbus Session
Ret((nifOpenSes sion(VariantTyp e.Null, SessionDesc)))

' Pass the max interface number to function "nifGetInterfac eList"
NoOfInterfaces = MAX_INTERFACES

' Get all the interface information
Ret((nifGetInte rfaceList(Sessi onDesc, NoOfInterfaces,
InterfaceInfo(0 ))))

For i = 0 To NoOfInterfaces - 1
' The interfaceName and DeviceID are array of byte, need to be
converted by function "getString"
ListInterface.I tems.Add(getStr ing(InterfaceIn fo(i).interface Name))
Next

End Sub

--------------

The underlying C declarations are like this:
typedef struct nifInterfaceInf o_t {
char interfaceName[NIF_NAME_LEN];
char deviceID[DEV_ID_SIZE + 1];
} nifInterfaceInf o_t;

extern nifError_t nifGetInterface List(nifDesc_t ud, int16 *numInterfaces,
nifInterfaceInf o_t *info);
--------------

As suggested by the UPGRADE_TODO vbup1026, I added a call to Initialize()
like this:
Private Sub OpenSess()
' Open a fieldbus Session
Ret((nifOpenSes sion(VariantTyp e.Null, SessionDesc)))

' Pass the max interface number to function "nifGetInterfac eList"
NoOfInterfaces = MAX_INTERFACES

InterfaceInfo.I nitialize()
For i = InterfaceInfo.G etLowerBound(0) To
InterfaceInfo.G etUpperBound(0)
InterfaceInfo(i ).Initialize()
Next

' Get all the interface information
Ret((nifGetInte rfaceList(Sessi onDesc, NoOfInterfaces,
InterfaceInfo(0 ))))

For i = 0 To NoOfInterfaces - 1
' The interfaceName and DeviceID are array of byte, need to be
converted by function "getString"
ListInterface.I tems.Add(getStr ing(InterfaceIn fo(i).interface Name))
Next

End Sub

I examined InterfaceInfo in the debugger, and it looks ok following the
call(s) to Initialize().

The call to nifGetInterface List fails with an error
----------------
An unhandled exception of type 'System.NullRef erenceException ' occured in
Project1.exe

Additional Information: Object reference not set to an instance of an object.
----------------

if I add this
<StructLayout ( LayoutKind.Sequ ential, CharSet:=CharSe t.ANSI)>

to the structure declaration, I get the same error.
If I add MarshalAs attributes (as suggested by UPGRADE_WARNING : Structure
NifInterfaceInf o may require marshalling attributes to be passed as an
argument in this Declare statement. Click for more:
'ms-help://MS.VSCC.2003/commoner/redir/redirect.htm?ke yword="vbup1050 "):

<StructLayout(L ayoutKind.Seque ntial, CharSet:=CharSe t.Ansi)> _
Public Structure NifInterfaceInf o ' Interface Infor structure
<VBFixedArray(N IF_NAME_LEN), MarshalAs(Unman agedType.ByValT Str,
SizeConst:=NIF_ NAME_LEN)> Dim interfaceName() As Byte
<VBFixedArray(D EV_ID_SIZE), MarshalAs(Unman agedType.ByValT Str,
SizeConst:=DEV_ ID_SIZE)> Dim DeviceID() As Byte

'UPGRADE_TODO: "Initialize " must be called to initialize instances
of this structure. Click for more:
'ms-help://MS.VSCC.2003/commoner/redir/redirect.htm?ke yword="vbup1026 "'
Public Sub Initialize()
ReDim interfaceName(N IF_NAME_LEN)
ReDim DeviceID(DEV_ID _SIZE)
End Sub
End Structure

then I get error:
----------------
An unhandled exception of type 'System.TypeLoa dException' occured in
Project1.exe

Additional Information: Can not marshal field interfaceName of type
NifInterfaceInf o: This type can not be marshaled as a structure field
----------------


Anyone have suggestions for the right way to pass a VB.net struct containing
fixed length arrays to a DLL?

[Tom Shelton]

David Fort (donotspam) wrote:

Here is the magic syntax:

<StructLayout(L ayoutKind.Seque ntial)> _
Public Structure NifInterfaceInf o
<MarshalAs(Unma nagedType.ByVal Array, SizeConst:=NIF_ NAME_LEN + 1)> _
Public interfaceName As Byte()
<MarshalAs(Unma nagedType.ByVal Array, SizeConst:=DEV_ ID_SIZE + 1)> _
Public DeviceID As Byte()
Public Sub Initialize()
ReDim interfaceName(N IF_NAME_LEN)
ReDim DeviceID(DEV_ID _SIZE)
End Sub
End Structure
Declare Function nifGetInterface List Lib "nifbstd" (ByVal session As
Integer, ByRef numIntf As Short, <Out()> ByVal info As NifInterfaceInf o()) As
Integer

Public InterfaceInfo(M AX_INTERFACES) As NifInterfaceInf o
and call it like this:
nifGetInterface List(SessionDes c, NoOfInterfaces, InterfaceInfo)
So, the only change from Tom's last suggestion was the addition of <Out()>
in the function declaration on the third argument.

David - I'm glad you got this figured out. I've been busy the last
couple of days, and wasn't able to respond.

--
Tom Shelton [MVP]