C#获取指定月指定周的日期范围

/// <summary>
        /// 获取指定月份指定周数的开始日期
        /// </summary>
        /// <param name="year">年份</param>
        /// <param name="month">月份</param>
        /// <param name="index">周数</param>
        /// <returns></returns>
        private DateTime GetStartDayOfWeeks(int year, int month, int index)
        {
            if (year < 1600 || year > 9999)
            {
                MessageBox.Show("年份超限");
                return DateTime .MinValue ;
            }
            if (month < 0 || month > 12)
            {
                MessageBox.Show("月份错误");
                return DateTime .MinValue ;
            }
            if (index < 1)
            {
                MessageBox.Show("周数错误");
                return DateTime.MinValue;
            }
            DateTime startMonth = new DateTime(year, month, 1);  //该月第一天
            int dayOfWeek = 7;
            if (Convert.ToInt32(startMonth.DayOfWeek.ToString("d")) > 0)
                dayOfWeek = Convert.ToInt32(startMonth.DayOfWeek.ToString("d"));  //该月第一天为星期几
            DateTime startWeek =

补充：软件开发 , C# ,