protected virtual void OnExternalChange(ExternalChangeEventArgs e)
        {
            AssertValid();
            EventHandler <ExternalChangeEventArgs> h = externalChange;

            if (h != null)
            {
                h(this, e);
            }
        }
        protected virtual void OnExternalChange(ExternalChangeEventArgs e)
        {
            GetCheckedHandle();
            var h = externalChange;

            if (h is not null)
            {
                h(this, e);
            }
        }