Passing a 2d array from vb.net to unmanaged C++ dll

Hi,

I'm trying to pass a bi-dimensional array of integers from vb.net to a c++
dll (in fact - a matrix that has to be processed in the dll) and then I need
the results back in the vb.net application.
I tried using the same approach as when passing a simple array (see example
below) but didn't succeed.

C++ function to process the array:
int Cmfcw001App::TestArr(int *ar, int size) {

//read the first element for test

int k;

k = ar[5];

return k;

}

......

The function TestArr has been defined in the .def file.

The declaration in vb.net module

Public Declare Function TestArr Lib "<path>\mfcw001.dll" (ByRef ar As
Integer, ByVal size As Integer) As Integer

//****** Usage in vb.net code:

Dim ar(9) As Integer

For i = 0 To 9

ar(i) = -1 * i

Next

s = TestArr(ar(0), 10)

MsgBox(s)

On this pattern I tried replacing
int* ar with int** ar ,
ar(9) with ar(3, 3)
the call is now TestArr(ar(0, 0), 10)
in the dll the function tries to read k = ar[0][0]

The error message I get in vb.net is "Object reference not set to an
instance of an object" when reaching the call to TestArr.

Can anybody help, please ?

Thank you,

Andrei.