vb.net: How to Structure to IntPtr

I have a vb6.0 (basic) project which I want to migrate to the new vs2022. Most of the code just translated "more or less" fine, but there are some open points. I tried checking internet, but I couldnt find solution. So here is my issue:

I have following code:

Public Structure Test
    Dim a as long
    Dim b as long
    ....
End structure

Public Sub xyz()
    'here im filling Test structure locally
    a = test... 
    ..

    A(a)
End sub

external dll function I want to call is A([In] IntPtr Data)

so the calling of A with my Test structure is failing, in vb6.0 it was working fine. I am not sure whats the problem. I tried some things, but I have no clue

I expected that it can just compile like its comiling in vb6.0. I have not touched any of this code part yet.

Another thing is that VarPtr() function is not available anymore. What could be the replacement in vs2022?

When I need to pass a structure to an unmanaged API as a pointer Intptr, I would do the following:

1. Allocate unmanaged memory of the size of the structure. Dim buffer As IntPtr = Marshal.AllocHGlobal(Marshal.SizeOf(GetType(T)))

2. Pass the structure pointer to the unmanaged API.

3. When the unmanaged methods returns, converts the pointer to the structure using: Marshal.PtrToStructure(Of T)(buffer)

4. Do not forget to free the previously allocated unmanaged memory Marshal.FreeHGlobal(buffer)