C # Gets the first and last day of the week on which a date is located

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace WyfClass
{
public class tools
{
/// <summary>
/// get the first day of the week
/// </ summary>
/// <param name = "datetime"> </ param>
/// <returns> </ returns>
public DateTime GetWeekFirstDaySun (DateTime datetime)
{
    // Sunday is the first day
    int weeknow = Convert.ToInt32 (datetime.DayOfWeek);
    int daydiff = (-1) * weeknow;

    // First day of the week
    string FirstDay = datetime.AddDays (daydiff) .ToString ("yyyy-MM-dd");
    return Convert.ToDateTime (FirstDay);
}

/// <summary>
/// Get the first day of the week (take Monday as the first day)
/// </ summary>
/// <param name = "datetime"> </ param>
/// <returns> </ returns>
public DateTime GetWeekFirstDayMon (DateTime datetime)
{
    // Monday is the first day
    int weeknow = Convert.ToInt32 (datetime.DayOfWeek);

    // Because Monday is the first day, if you want to determine that weeknow is equal to 0, push it forward for 6 days.
    weeknow = (weeknow == 0? (7-1): (weeknow-1));
    int daydiff = (-1) * weeknow;

    // First day of the week
    string FirstDay = datetime.AddDays (daydiff) .ToString ("yyyy-MM-dd");
    return Convert.ToDateTime (FirstDay);
}

/// <summary>
/// get the last day of the week (take Saturday as the last day)
/// </ summary>
/// <param name = "datetime"> </ param>
/// <returns> </ returns>
public DateTime GetWeekLastDaySat (DateTime datetime)
{
    // Saturday is the last day
    int weeknow = Convert.ToInt32 (datetime.DayOfWeek);
    int daydiff = (7-weeknow)-1;

    // last day of the week
    string LastDay = datetime.AddDays (daydiff) .ToString ("yyyy-MM-dd");
    return Convert.ToDateTime (LastDay);
}

/// <summary>
/// get the last day of the week (take Sunday as the last day)
/// </ summary>
/// <param name = "datetime"> </ param>
/// <returns> </ returns>
public DateTime GetWeekLastDaySun (DateTime datetime)
{
    // Sunday is the last day
    int weeknow = Convert.ToInt32 (datetime.DayOfWeek);
    weeknow = (weeknow == 0? 7: weeknow);
    int daydiff = (7-weeknow);

    // last day of the week
    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>
/// get the first day of the week (take Sunday as the first day)
/// </ summary>
/// <param name = "datetime"> </ param>
/// <returns> </ returns>
public DateTime GetWeekFirstDaySun (DateTime datetime)
{
    // Sunday is the first day
    int weeknow = Convert.ToInt32 (datetime.DayOfWeek);
    int daydiff = (-1) * weeknow;

    // First day of the week
    string FirstDay = datetime.AddDays (daydiff) .ToString ("yyyy-MM-dd");
    return Convert.ToDateTime (FirstDay);
}

/// <summary>
/// Get the first day of the week (take Monday as the first day)
/// </ summary>
/// <param name = "datetime"> </ param>
/// <returns> </ returns>
public DateTime GetWeekFirstDayMon (DateTime datetime)
{
    // Monday is the first day
    int weeknow = Convert.ToInt32 (datetime.DayOfWeek);

    // Because Monday is the first day, if you want to determine that weeknow is equal to 0, push it forward for 6 days.
    weeknow = (weeknow == 0? (7-1): (weeknow-1));
    int daydiff = (-1) * weeknow;

    // First day of the week
    string FirstDay = datetime.AddDays (daydiff) .ToString ("yyyy-MM-dd");
    return Convert.ToDateTime (FirstDay);
}

/// <summary>
/// get the last day of the week (take Saturday as the last day)
/// </ summary>
/// <param name = "datetime"> </ param>
/// <returns> </ returns>
public DateTime GetWeekLastDaySat (DateTime datetime)
{
    // Saturday is the last day
    int weeknow = Convert.ToInt32 (datetime.DayOfWeek);
    int daydiff = (7-weeknow)-1;

    // last day of the week
    string LastDay = datetime.AddDays (daydiff) .ToString ("yyyy-MM-dd");
    return Convert.ToDateTime (LastDay);
}

/// <summary>
/// get the last day of the week (take Sunday as the last day)
/// </ summary>
/// <param name = "datetime"> </ param>
/// <returns> </ returns>
public DateTime GetWeekLastDaySun (DateTime datetime)
{
    // Sunday is the last day
    int weeknow = Convert.ToInt32 (datetime.DayOfWeek);
    weeknow = (weeknow == 0? 7: weeknow);
    int daydiff = (7-weeknow);

    // last day of the week
    string LastDay = datetime.AddDays (daydiff) .ToString ("yyyy-MM-dd");
    return Convert.ToDateTime (LastDay);
}
}
} 

C # Gets the first and last day of the week on which a date is located