Converting Array Of GElement Into Block Of Unmanaged Memory?

I have a structure in VB.NET that has a dynamic array of double as a parameter. I am passing an array of structure (GElement) to a C++ DLL. I tried converting my array of GElement into a block of unmanaged memory, it works in VB.NET but crashes in my C++ DLL. I get the same error just trying to simply pass an array of GElement to my DLL instead of a Intptr.

Here is the error
0xC0000005: Access violation reading location 0x00000004

Here is my function and structure in the DLL:
void _stdcall TestGElement(GElement* ge,long& nbge){
nbge=5;
} struct GElement {
double Step2D;
double Step3D ;
[Code] .....

I know it is easier if the array inside the structure as an already defined constant size but that can't be the case here...