Help request about copymemory

[android]

Hi all.
I should migrate old vb6 code in vb.net but I been clashed with the copymemory api!
Here below is a vb6 method that manages the current sample of a waveform file in a buffer for retrieving the left/right volumePictBox level in order to draw the wavegraph of that file on a picture box:

Declare Sub CopyMemory Lib "kernel32" Alias "RtlMoveMemory" (ByVal pDst As integer, _
ByVal pSrc As Long, _
ByVal ByteLen As Long)

Private Function GetMono16Sample(ByVal Sample As Long) As Double

Dim Ret as Double
Dim Sample16 As Integer
Dim Ptr As Long = Sample * Gformat.nBlockAlign + BufferAlloc

CopyMemory(sample16, Ptr, 2)
Ret = Sample16 / 32768

Return Ret
End Function

So for example in vb6 the routine above works in the following way:
Costant values:
Gformat.NblockAlign = 2
BufferAlloc = 57560544

Input value:
sample = 0
ptr returned value = 57560544

returned value by CopyMemory:
sample16 = 15

or another example:
Input value:
sample = 10
ptr returned value = 57560564

returned value by CopyMemory:
sample16 = 2

in vb.net the function above (with var.type properly updated), returns the following exception: System.NullReferenceException Reference to an object not set on a object istance(????)

its possible to reproduce its vb6 behaviour on vb.net?
Thank you very much for any kind answer.

Best regards
Andrea