public App()
        {
            WindowsIdentity principal = WindowsIdentity.GetCurrent();
            var user = this.GetUserNamefromSID( principal.User );
        }

        /// <summary>
        /// Liefert den UserAccount vom SID in Form von Domain\UserName.
        /// </summary>
        /// <param name="SID">Der SecurityIdentifier</param>
        public string GetUserNamefromSID(SecurityIdentifier SID)
        {
            NTAccount NTUser = (NTAccount)SID.Translate(typeof(NTAccount));
            return NTUser.ToString();
        }