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 = (-) * 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 ==  ? ( - ) : (weeknow - ));
    int daydiff = (-) * 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 = ( - weeknow) - ;

    //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 ==  ?  : weeknow);
    int daydiff = ( - weeknow);

    //本周最后一天
    string LastDay = datetime.AddDays(daydiff).ToString("yyyy-MM-dd");
    return Convert.ToDateTime(LastDay);
}
}
}

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 = (-) * 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 ==  ? ( - ) : (weeknow - ));
    int daydiff = (-) * 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 = ( - weeknow) - ;

    //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 ==  ?  : weeknow);
    int daydiff = ( - weeknow);

    //本周最后一天
    string LastDay = datetime.AddDays(daydiff).ToString("yyyy-MM-dd");
    return Convert.ToDateTime(LastDay);
}
}
}