在C＃中获取星期的第一个日期和当前月份的最后一个日期

Question

Need help how to get first date and last date of a week in current month.

Below what I have done so far, but still fall for certain case especially where the start date and end date is sunday:

    var givenDate = new DateTime(2018,4,1);
    var intervalToStart = givenDate.DayOfWeek - DayOfWeek.Monday;
    var startDate = givenDate.AddDays(-intervalToStart);

    DateTime endDate;
    int intervalToEnd = 0; 

    if (startDate.Month != givenDate.Month)
        startDate = new DateTime(givenDate.Year, givenDate.Month, 1);

    var dayOfWeekStartDate = startDate.DayOfWeek.ToString().ToLower();

    switch (dayOfWeekStartDate)
        {
            case "sunday": intervalToEnd = 0;
                break;
            case "monday": intervalToEnd = 6;
                break;
            case "tuesday": intervalToEnd = 5;
                break;
            case "wednesday": intervalToEnd = 4;
                break;
            case "thursday": intervalToEnd = 3;
                break;
            case "friday": intervalToEnd = 2;
                break;
            case "saturday": intervalToEnd = 1;
                break;
        }

    endDate = startDate.AddDays(intervalToEnd);


    Console.WriteLine(startDate);
    Console.WriteLine(endDate);

Case 1:

• Given date: 05 May 2018 (saturday)

• First date of week: 01 May 2018 (tuesday)

• Last date of week: 06 May 208 (sunday)

Case 2:

• Given date: 09 May 2018 (wednesday)

• First date of week: 07 May 2018 (monday)

• Last date of week: 13 May 2018 (sunday)

Case 3:

• Given date: 01 Apr 2018 (sunday)

• First date of week: 01 Apr 2018 (sunday)

• Last date of week: 01 Apr 2018 (sunday)

case 4:

• Given date: 02 June 2018 (saturday)

• First date of week: 01 June 2018 (friday)

• Last date of week: 03 June 2018 (sunday)

Concept or answer is highly appreciated.

Thank you in advance.

Answer 1

You want to return the first day of the month if the monday would fall into the previous month?

public static Tuple<DateTime, DateTime> GetFirstAndLastWeekDate(DateTime dt, DayOfWeek firstDayOfWeek = DayOfWeek.Monday)
{
    int diff = (7 + (dt.DayOfWeek - firstDayOfWeek)) % 7;
    DateTime firstDay = dt.AddDays(-1 * diff).Date;
    DateTime lastDay = firstDay.AddDays(6);
    if (dt.Month != firstDay.Month)
    {
        firstDay = new DateTime(dt.Year, dt.Month, 1);
    }

    return Tuple.Create(firstDay, lastDay);
}

Answer 2

Use DateTime.DayOfWeek

and the code would be like this

var givenDate = new DateTime(2018, 4, 1);
int addMax = DateTime.DaysInMonth(givenDate.Year, givenDate.Month) - givenDate.Day;
int add = Math.Min(addMax, ((DayOfWeek.Saturday - givenDate.DayOfWeek) + 1) % 7);

int subMax = givenDate.Day - 1;
int sub = Math.Min(subMax, (givenDate.DayOfWeek - DayOfWeek.Monday + 7) % 7);

var firstDate = givenDate.AddDays(-sub);
var lastDate = givenDate.AddDays(add);
Console.Write("Give: {0}({1})\nFirst: {2}({3})\nLast: {4}({5})", 
    givenDate, givenDate.DayOfWeek, 
    firstDate, firstDate.DayOfWeek, 
    lastDate, lastDate.DayOfWeek);