If it's for non commercial then MBROLA has Aribic support, I think its a command line application, so while it does not support .NET as such it should be easy enough to incorporate it into a .NET application by just launching the MBROLA binary from within your .NET application.
Dim WithEvents Player As WMPLib.WindowsMediaPlayer
Private Sub PlayFile(ByVal url As String)
Player = New WMPLib.WindowsMediaPlayer
Player.URL = url
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
Dim readtext As String
readtext = "SOMETHING ARABIC"
PlayFile("http://translate.google.com/translate_tts?ie=UTF-8&q=" & readtext & "&tl=ar")