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C++ DLL callback with VB.Net

Hi there,
i have a unmanaged DLL that I use to control a Biometric reader with my app. I'm coding with VB.net. I used a vb6 example to interface with the DLL. Here is the original code to link with the dll function :

Public Declare Function AET60_SetCallback Lib "Librairie externe\AET60.dll" ( _
ByVal CallbackFunction As Long) As Long

Here is the new one in vb.net :

<DllImport("Librairie externe\AET60.dll", EntryPoint:="AET60_SetCallback", _
SetLastError:=True, CallingConvention:=CallingConvention.Cdecl)> _
Public Shared Function AET60_SetCallback(ByVal CallbackFunction As MyCallBack) As Integer
End Function

Here is my delegate :

<UnmanagedFunctionPointer(CallingConvention.Cdecl)> _
Public Delegate Function MyCallBack(ByRef Param As Integer, _
ByVal GuiState As Integer, _
ByVal Response As Byte, _
ByVal Msg As Integer, _
ByVal Progress As Byte, _
ByRef SampleBuffer As FBITMAP) As Integer

And my callback function :

Private Function CALLBACK_FUNCTION(ByRef Param As Integer, _
ByVal GuiState As Integer, _
ByVal Response As Byte, _
ByVal Msg As Integer, _
ByVal Progress As Byte, _
ByRef SampleBuffer As FBITMAP) As Integer

Dim StatusMsg As String
StatusMsg = New String(vbNullChar, 255)

If (GuiState And AET60_MESSAGE_PROVIDED) <> 0 Then
Call AET60_GetMessage(Msg, StatusMsg)
End If
CALLBACK_FUNCTION = 0

End Function

The data structure concerned by the declaration :

'======================================================================
' data structure definition
'======================================================================
Public Structure FDATA
Dim Length As Integer
Dim Data() As Byte
End Structure

Public Structure FBITMAP
Dim Width As Integer
Dim Height As Integer
Dim Bitmap As FDATA
End Structure

And the line where I plug everything :

del = New MyCallBack(AddressOf CALLBACK_FUNCTION)
retcode = AET60_SetCallback(del)

Everything runs smoothly until I call un dll function that should trigger the callback ... i then receive a "Unhandled exception of type 'System.ArgumentNullException' from a unknown module.

If i skip the callback regiostration, everything runs smoothly but i can't have sny status update on the status of my thuimbprint reader ...

Can anyone give me a hand ?

Jason Evans

Hi Moonover.

This might be a long shot, but maybe you should try this:

<DllImport("Librairie externe\AET60.dll", EntryPoint:="AET60_SetCallback", _
SetLastError:=True, CallingConvention:=CallingConvention.Cdecl)> _
Public Shared Function AET60_SetCallback(ByVal CallbackFunction As System.IntPtr) As Integer
End Function

Use the IntPtr in the declaration. Then use the function like so:

del = New MyCallBack(AddressOf CALLBACK_FUNCTION)
retcode = AET60_SetCallback(del) or retcode = AET60_SetCallback(AddressOf del)

If this doesn't work, try using ByRef instead of ByVal in the declaration of AET60_SetCallback.

Hope this helps.
Jas.

David MacDonald

ASKER

Hi, i just tried it ... it tells me that the type MyCalllBack cannot be converted ton SYstem.InPtr ....

Darn ... and to say i knew nothing of delegates last week :)

With "AddressOf del" as param, it tells me that AddressOf needs a method name instead .

Jason Evans

Hi there.

Nope, I just checked and I was wrong. Change it back to this:

<DllImport("Librairie externe\AET60.dll", EntryPoint:="AET60_SetCallback", _
SetLastError:=True, CallingConvention:=CallingConvention.Cdecl)> _
Public Shared Function AET60_SetCallback(ByVal CallbackFunction As MyCallBack) As Integer
End Function

Try this instead:

retcode = AET60_SetCallback(AddressOf CALLBACK_FUNCTION)

You don't need to do 'del = New....', simply pass the address of the CALLBACK_FUNCTION you already have.

Try that.
Cheers.
Jas.

David MacDonald

ASKER

Hi Jas,
still with the null exception ... just for pointers, i tried changing byval for byref on the MyCallBack param ... i get this error :

An unhandled exception of type 'System.AccessViolationException' occurred in AbonnementBiometrie.exe

So i guess Byval is alright ... But i really don't understand why it's not working ...

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Jason Evans

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David MacDonald

ASKER

Hi there,
it seems the problem lies in the structure definition used in the callback, the FBITMAP structure uses the FDATA strucuture wich in turn contains a Byte array ... Is it because my byte array is not initialise by default ... it sure seems like it ...

Public Structure FDATA
        Dim Length As Integer
        Dim Data() As Byte
    End Structure
 
    Public Structure FBITMAP
        Dim Width As Integer
        Dim Height As Integer
        Dim Bitmap As FDATA
    End Structure

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David MacDonald

ASKER

Solved it !

it was the FDATA Byte array that was not initialized that popped the nullException ... Jas, i'm giving you the points anyway because you gave me hope ;-)

Public Structure FDATA
        Dim Length As Integer
        Shared Data(0 To 256) As Byte
    End Structure

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