C#获取指定月指定周的日期范围 根据当前时间获取本月开始日期和结束日期

/// <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 = startMonth.AddDays(1 - dayOfWeek);  //该月第一周开始日期
//DateTime startDayOfWeeks = startWeek.AddDays((index - 1) * 7);  //index周的起始日期
DateTime startDayOfWeeks = startWeek.AddDays(index * 7);  //index周的起始日期
if ((startDayOfWeeks - startMonth.AddMonths(1)).Days > 0)  //startDayOfWeeks不在该月范围内
{
MessageBox.Show("输入周数大于本月最大周数");
return DateTime.MinValue;
}
return startDayOfWeeks;
}

//当前时间
DateTime dt = DateTime.Now;
//本月第一天时间
DateTime dt_First = dt.AddDays(1 - (dt.Day));
//获得某年某月的天数
int year = dt.Date.Year;
int month = dt.Date.Month;
int dayCount = DateTime.DaysInMonth(year, month);
//本月最后一天时间
DateTime dt_Last = dt_First.AddDays(dayCount - 1);