LINQ-分组后使用聚合函数

1.实现代码

        /// <summary>
        ///  获取指定课程编号的总时长
        /// </summary>
        /// <param name="courseId">课程编号</param>
        /// <returns></returns>
        public async Task<decimal> GetTotalDurationByCourseId(int courseId)
        {
            decimal totalDuration = 0;

            var result = _sectionInfoRepository.GetAll().AsNoTracking().Where(x => x.CourseId == courseId)
                .GroupBy(x => new { x.CourseId })
                .Select(g => new
                {
                    TotalDuration = g.Sum(x => x.ResourceDuration)
                }).FirstOrDefault();

            var r = result.GetType().GetProperty("TotalDuration");
            if (r != null)
            {
                totalDuration = Convert.ToDecimal(r.GetValue(result,null));
            }

            return await Task.Fromresult(totalDuration);
        }