wpd c# enumerating devices content when more then one device is present

I developed an application that enumerates and merges the content maps of all drives ever connected to a system. Doing the same for devices with the WPD API Interop.PortableDeviceApiLib.dll, there is a marshal limit for managing contemporary devices.

I managed to follow the trick cited in https://blogs.msdn.microsoft.com/dimeby8/2006/12/05/enumerating-wpd-devices-in-c/
to dis-assemble and re-assemble Interop.PortableDeviceApiLib.dll in order to manage more then one device at a time, replacing

instance void GetDevices([in][out] string& marshal( lpwstr) pPnPDeviceIDs

with

instance void GetDevices([in][out] string[] marshal( lpwstr[]) pPnPDeviceIDs

BUT, with two connected devices, the second item of the returned array is ALWAYS null !!!

This is the code snippet:

    public string[] Refresh() {
        deviceManager.RefreshDeviceList();
        string[] nullArray = { null };
        uint count = 1;
        try { deviceManager.GetDevices(null, ref count); }    // <-- I tried also with nullArray instead of null
        catch (Exception ex) { Console.WriteLine(ex.Message); count = 0; }
        if (count == 0)
            return null;
        var deviceIds = new string[count];
        deviceManager.GetDevices(deviceIds, ref count);
        foreach (var deviceId in deviceIds)
        {
            Add(new PortableDevice(deviceId));
        }
        return deviceIds;
    }

Note that there are two already connected devices (on each I responded to Allow connection: YES), so the var count receives the value 2 and deviceIds[0] is OK, but deviceIds[1] is always null ! (even after swapping the devices on the USB slots).

I use Windows Studio Professional 2017 on Windows 10 The two devices are a Honor9 and a USB disk or an iPad. In the project.csproj file I inserted: ...

<Reference Include="Interop.PortableDeviceApiLib">
  <HintPath>.\Interop.PortableDeviceApiLib.dll</HintPath>
</Reference>
<Reference Include="Interop.PortableDeviceTypesLib">
  <HintPath>.\Interop.PortableDeviceTypesLib.dll</HintPath>
</Reference>

... instead to reference the COM files:

    <COMReference Include="PortableDeviceApiLib">
  <Guid>{1F001332-1A57-4934-BE31-AFFC99F4EE0A}</Guid>
  <VersionMajor>1</VersionMajor>
  <VersionMinor>0</VersionMinor>
  <Lcid>0</Lcid>
  <WrapperTool>tlbimp</WrapperTool>
  <Isolated>False</Isolated>
  <EmbedInteropTypes>False</EmbedInteropTypes>
</COMReference>
<COMReference Include="PortableDeviceTypesLib">
  <Guid>{2B00BA2F-E750-4BEB-9235-97142EDE1D3E}</Guid>
  <VersionMajor>1</VersionMajor>
  <VersionMinor>0</VersionMinor>
  <Lcid>0</Lcid>
  <WrapperTool>tlbimp</WrapperTool>
  <Isolated>False</Isolated>
  <EmbedInteropTypes>False</EmbedInteropTypes>
</COMReference>

because previously it gave me two problems: 1) every compilation overwrote the new bin/Debug/Interop.PortableDeviceApiLib.dll; 2) It gave me the exception: Impossible to find void PortableDeviceApiLib.IPortableDeviceManager.GetDevices(System.String ByRef, UInt32 ByRef)

Is there anybody to solve the null value always returned for the non-first devices?