public static ManualResetValueTaskSource <T> Completed <T>(T result, Exception error = null)
        {
            var vts = new ManualResetValueTaskSource <T>();

            if (error != null)
            {
                vts.SetException(error);
            }
            else
            {
                vts.SetResult(result);
            }
            return(vts);
        }
        public static ManualResetValueTaskSource <T> Delay <T>(int delayMs, T result, Exception error = null)
        {
            var vts = new ManualResetValueTaskSource <T>();

            Task.Delay(delayMs).ContinueWith(_ =>
            {
                if (error != null)
                {
                    vts.SetException(error);
                }
                else
                {
                    vts.SetResult(result);
                }
            });
            return(vts);
        }