Well, those Transparency radio buttons will be activated once you have selected Illustrator 8.0 above as Version.
In case you select Preserve Appearance and Overprints, that option will work well, that is manually. from within the UI, but in case of Scripting, for example in Visual Basic (which I have been testing too in combination with the latest Creative Cloud application) that option Preserve Appearance and Overprints does not work for me, as if instead Preserve Paths is always being selected.
My test in Visual Basic does follow exactly the Visual Basic Scripting specs (does follow the exact same lines of Visual Basic source code) as had been formulated with the not yet released newest version of Creative Cloud. It is supposed to work, but it does not work for me anymore since the release of CS6. Why?
Title of the original post refers explicitly to C# .NET, but a similar test has also been done with Visual Basic .NET in Visual Studio 2012 which has been communicating with Adobe Illustrator 2017 CC.
Visual Basic source code actually used by me and to be tried out by others can be found here below:
Public Class Form1
'Visual Basic .NET Script for saving Adobe Illustrator 2017 CC content to AI version 8.0 file format.
Private Sub Form1_Load(sender As Object, e As EventArgs) _
'Request path which allows storage of AI output file
Dim LocalAppData As String = Environment.GetFolderPath(Environment.SpecialFolder.LocalApplicationData)
' Set name of file to be saved in textbox.
TextBox1.Text = LocalAppData + "\\Temp\\Test.ai"
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
'Defined reference: Adobe Illustrator CC 2017 Type Library (selected SriptingSupport.aip)
'When build with setting Any CPU then this Visual Basic .NET utility will only work with 32-Bit
'Easiest approach is first to startup Illustrator CC 2017 manually,
'then create required content (something with transparency)
'and then run this VB .NET program to let the application save its content to AI 8.0 format
'after clicking the push button (transparency will be removed instead of being flattened!).
Dim appRef As New Illustrator.Application
Dim SaveOptions As New Illustrator.IllustratorSaveOptions
SaveOptions.Compatibility = 8 'aiIllustrator8
SaveOptions.FlattenOutput = 1 'aiPreserveAppearance
'File must first be deleted in case file already exists.
If System.IO.File.Exists(TextBox1.Text) = True Then
Call appRef.Documents(1).SaveAs(TextBox1.Text, SaveOptions)
Private Sub TextBox1_TextChanged(sender As Object, e As EventArgs) Handles TextBox1.TextChanged