Programmatically setting a task to Locked

I would just write a vb script for this within project, but the locked flag is only accessible from PWA.  So we wrote a quick C# application but the DraftTask.IsLockedByManager flag doesn't seem to work.  It almost appears to be a bug.  Can someone point me in the right direction.  We need to lock a task from further input after X happens.  Locked does the trick, but it doesn't seem that you can set it Programmatically.