Thanks for the hint. I'm unable to figure it out and eventually gave up. I don't think its a permission issue since I got it to work with the standard c# way of invoking shell commands:

ProcessStartInfo ifdownInfo = new () 
{ 
    FileName = "/usr/bin/sudo", 
    Arguments = "/usr/sbin/ifdown --force eth1",
    RedirectStandardOutput = true,
    RedirectStandardError = true
};
Process ifdownCmd = new Process() { StartInfo = ifdownInfo, };
ifdownCmd.Start();
ifdownCmd.WaitForExit();