[vb.net, Microstation V8i, Microstation CE] Activate Transparency of Raster images

Good morning gents,

 I am trying to activate the transparency of a raster image in two different (almost twin) applications: one application uses the MicroStation V8i interoperability, while the other one uses the MicroStation CE interoperability. Here the code that is identical in both the applications:

    Dim RasMan = MstnApp.RasterManager'Attach raster
    Dim mRas = RasMan.Rasters.Attach(rasterFile, CByte(MsdViews.msdViewAll),
                                     MsdAttachMode.msdAttachReference,
                                     MsdFileAccessMode.msdFileAccessModeReadWrite, "", "")

    Dim ri = mRas.RenderingInformation
    ri.Transparency = True
    ri.TransparencyColorLevel = 100
    ri.TransparencyColor = RGB(255, 255, 255)

Both MicroStation V8i and MicroStation CE raise the same exception in correspondence of instruction: TransparencyColorLevel = 100:

“Transparency information is not available”

 I also develop a simple VBA macro both in MicroStation V8i and MicroStation CE with the same scope in order to find a solution. Here the macro source code

Public Sub TransparencyTest()
  Dim k As Integer
  Dim RasMan As RasterManager
  Dim ri  As RenderingInformation
  Set RasMan = Application.RasterManager
  For k = 1 To RasMan.Rasters.Count
    Set ri = RasMan.Rasters.Item(k).RenderingInformation
    ri.Transparency = True
    ri.TransparencyColorLevel = 100
    ri.TransparencyColor = RGB(255, 255, 255)
  Next
End Sub

In MicroStation v8i, macro was completed successfully, while in MicroStation CE, the same exception occurred.

 Any ideas to solve this issue?

thanks