如何在 C# 中获取下一个特定日期的日期

Question

How can I get the date of next coming Monday or next coming Friday in C#. lets say today is Wednesday and I want to get date of coming Friday. This is what I've done

DateTime time = DateTime.Now.Date;
DateTime NextFriday;
if (time.DayOfWeek == DayOfWeek.Wednesday)
{
    NextFriday = DateTime.Now.AddDays(2);
}

with this approach I've to initiate 7 variables for each day and 7 conditions for ever day to find the next specific day.

Is there any better and clean code by which I can get the date of any of the next coming day.

Answer 1

Using the following

public int CalculateOffset(DayOfWeek current, DayOfWeek desired) {
    // f( c, d ) = [7 - (c - d)] mod 7
    // f( c, d ) = [7 - c + d] mod 7
    // c is current day of week and 0 <= c < 7
    // d is desired day of the week and 0 <= d < 7
    int c = (int)current;
    int d = (int)desired;
    int offset = (7 - c + d) % 7;
    return offset == 0 ? 7 : offset;
}

You can calculate how far you are from the desired day and then add that to the current date

DateTime now = DateTime.Now.Date;
int offset = CalculateOffset(now.DayOfWeek, DayOfWeek.Friday);
DateTime nextFriday = now.AddDays(offset);

Answer 2

DateTime today = DateTime.Today;
DateTime nextFriday = System.Linq.Enumerable.Range(0, 6)
  .Select(i => today.AddDays(i))
  .Single(day => day.DayOfWeek == DayOfWeek.Friday);

Answer 3

You should probably use a time library that supports this, such as NodaTime.

See date.Next(IsoDayOfWeek.Sunday) on https://nodatime.org/1.3.x/userguide/arithmetic

Here's an alternative solution (please don't use this):

DateTime F(DateTime t, DayOfWeek dayOfWeek) => t.AddDays((7 + (int)dayOfWeek - (int)t.DayOfWeek) % 7);

for (int i = 0; i < 7; i++)
    Console.WriteLine((DayOfWeek)i + " " + F(DateTime.Now, (DayOfWeek)i));

Outputs (on Wednesday 4/25/2018):

Sunday 4/29/2018 12:00:00 AM
Monday 4/30/2018 12:00:00 AM
Tuesday 5/1/2018 12:00:00 AM
Wednesday 4/25/2018 12:00:00 AM
Thursday 4/26/2018 12:00:00 AM
Friday 4/27/2018 12:00:00 AM
Saturday 4/28/2018 12:00:00 AM

Answer 4

DayOfWeek is just an enum between 0 and 6, so with modular arithmetic you can use the difference between your date of interest and target day of week to compute the number of days you must add.

A quick warning, you need to take into account timezone of interest when you ask what is meant by "today". It means a different thing depending on which side of the date line you live.

using System;

public class Program
{

    public static DateTime NextDayForDay(DayOfWeek dayOfWeek, DateTime occurringAfter)
    {
        return occurringAfter.AddDays(((dayOfWeek - occurringAfter.DayOfWeek + 6) % 7)+1); 
    }

    public static void Main()
    {
        for (int i=0; i < 7; i++)
        {
            for (int j=0; j < 7; j++)
            {
                DayOfWeek dayOfWeek = (DayOfWeek)(((int)DayOfWeek.Sunday + j) % 7);

                DateTime test = DateTime.Today.AddDays(i);
                Console.WriteLine($"{test}=>Next {dayOfWeek} is {NextDayForDay(dayOfWeek, test)}");
            }
        }   
    }
}