c# 获取某日期所在周的第一天和最后一天

using System;  
using System.Collections.Generic;  
using System.Linq;  
using System.Text;  
  
namespace WyfClass  
{  
public class tools  
{  
/// <summary>  
/// 得到本周第一天(以星期天为第一天)  
/// </summary>  
/// <param name="datetime"></param>  
/// <returns></returns>  
public DateTime GetWeekFirstDaySun(DateTime datetime)  
{  
    //星期天为第一天  
    int weeknow = Convert.ToInt32(datetime.DayOfWeek);  
    int daydiff = (-1) * weeknow;  
  
    //本周第一天  
    string FirstDay = datetime.AddDays(daydiff).ToString("yyyy-MM-dd");  
    return Convert.ToDateTime(FirstDay);  
}  
  
/// <summary>  
/// 得到本周第一天(以星期一为第一天)  
/// </summary>  
/// <param name="datetime"></param>  
/// <returns></returns>  
public DateTime GetWeekFirstDayMon(DateTime datetime)  
{  
    //星期一为第一天  
    int weeknow = Convert.ToInt32(datetime.DayOfWeek);  
  
    //因为是以星期一为第一天，所以要判断weeknow等于0时，要向前推6天。  
    weeknow = (weeknow == 0 ? (7 - 1) : (weeknow - 1));  
    int daydiff = (-1) * weeknow;  
  
    //本周第一天  
    string FirstDay = datetime.AddDays(daydiff).ToString("yyyy-MM-dd");  
    return Convert.ToDateTime(FirstDay);  
}  
  
/// <summary>  
/// 得到本周最后一天(以星期六为最后一天)  
/// </summary>  
/// <param name="datetime"></param>  
/// <returns></returns>  
public DateTime GetWeekLastDaySat(DateTime datetime)  
{  
    //星期六为最后一天  
    int weeknow = Convert.ToInt32(datetime.DayOfWeek);  
    int daydiff = (7 - weeknow) - 1;  
  
    //本周最后一天  
    string LastDay = datetime.AddDays(daydiff).ToString("yyyy-MM-dd");  
    return Convert.ToDateTime(LastDay);  
}  
  
/// <summary>  
/// 得到本周最后一天(以星期天为最后一天)  
/// </summary>  
/// <param name="datetime"></param>  
/// <returns></returns>  
public DateTime GetWeekLastDaySun(DateTime datetime)  
{  
    //星期天为最后一天  
    int weeknow = Convert.ToInt32(datetime.DayOfWeek);  
    weeknow = (weeknow == 0 ? 7 : weeknow);  
    int daydiff = (7 - weeknow);  
  
    //本周最后一天  
    string LastDay = datetime.AddDays(daydiff).ToString("yyyy-MM-dd");  
    return Convert.ToDateTime(LastDay);  
}  
}  
}