Find active unc path for a DFS target with NetDfsGetInfo

Question

I'm using the NetDfsGetInfo example from pinvoke.net! to find the active unc path for a DFS target. If I call the sub DFSInfo with a DFS path which has two targets I get back that both targets are active. I have searched the internet and found a suggestion to replace the vbNull for calling the ServerName and the ShareName with vbNullString but this makes no difference.

Can someone explain me what I'm doing wrong? Here is the code I'm using:

Option Explicit On
'Option Strict On

Imports System
Imports System.ComponentModel
Imports System.Runtime.InteropServices


Module modDFS
    Declare Unicode Function NetDfsGetInfo Lib "NETAPI32.DLL" ( _
        ByVal DfsEntryPath As String, _
        ByVal ServerName As String, _
        ByVal ShareName As String, _
        ByVal Level As Integer, _
        ByRef Buffer As IntPtr) As Integer

    Declare Unicode Function NetApiBufferFree Lib "netapi32.dll" _
    (ByVal buffer As IntPtr) As Long

    <StructLayout(LayoutKind.Sequential, CharSet:=CharSet.Auto)> _
    Public Structure _DFS_INFO_3
        <MarshalAs(UnmanagedType.LPWStr)> Public EntryPath As String
        <MarshalAs(UnmanagedType.LPWStr)> Public Comment As String
        Public State As Int32
        Public NumberOfStorages As Int32
        Public storage As Int32
    End Structure

    <StructLayout(LayoutKind.Sequential, CharSet:=CharSet.Auto)> _
    Public Structure _DFS_STORAGE_INFO
        Public State As Integer
        <MarshalAs(UnmanagedType.LPWStr)> Public ServerName As String
        <MarshalAs(UnmanagedType.LPWStr)> Public ShareName As String
    End Structure

    Public Sub DFSInfo(ByVal DfsEntryPath)
        Dim DfsInfo3 As New _DFS_INFO_3
        Dim DfsStorageInfo As New _DFS_STORAGE_INFO
        Dim err_code As Integer
        Dim bufPtf As IntPtr
        Dim bufPtf2 As IntPtr

        err_code = NetDfsGetInfo(DfsEntryPath, vbNullString, vbNullString, 3, bufPtf)
        DfsInfo3 = CType(Marshal.PtrToStructure(bufPtf, GetType(_DFS_INFO_3)), _DFS_INFO_3)
        bufPtf2 = DfsInfo3.storage

        For i As Integer = 0 To DfsInfo3.NumberOfStorages - 1
            'Increment the buffer and re-cast for each server in the list
            DfsStorageInfo = CType(Marshal.PtrToStructure(bufPtf2, GetType(_DFS_STORAGE_INFO)), _DFS_STORAGE_INFO)
            DfsInfo3.storage += Marshal.SizeOf(DfsStorageInfo)
            bufPtf2 = DfsInfo3.storage
            Debug.Print("\\" & DfsStorageInfo.ServerName & "\" & DfsStorageInfo.ShareName & " : " & DfsStorageInfo.State)
        Next

        NetApiBufferFree(bufPtf)

    End Sub
End Module