How can I determine the week number of a certain date?

Question

I\'m trying to make a calendar using wpf. By using itemsPanel and more, I have a grid with 7 columns(sunday-saturday) and 6 rows(week# of month). If i can find the starting

Answer 1

You can use Calendar.GetWeekOfYear from Globalization to do this.

Here's the MSDN docs for it: http://msdn.microsoft.com/en-us/library/system.globalization.calendar.getweekofyear.aspx

You should pass the appropriate culture properties from CultureInfo.CurrentCulture to GetWeekOfYear so that you match the current culture properly.

Example:

int GetWeekOfYear(DateTime date)
{
    return Calendar.GetWeekOfYear(
        date,
        CultureInfo.CurrentCulture.DateTimeFormat.CalendarWeekRule,
        CultureInfo.CurrentCulture.DateTimeFormat.FirstDayOfWeek
    );
}

You could easily modify this into an extension method on DateTime:

static int GetWeekOfYear(this DateTime date)
{
    return Calendar.GetWeekOfYear(
        date,
        CultureInfo.CurrentCulture.DateTimeFormat.CalendarWeekRule,
        CultureInfo.CurrentCulture.DateTimeFormat.FirstDayOfWeek
    );
}

Answer 2

With @Polynomial answer, I have this error:

An object reference is required for the non-static field, method, or property...

If you instanciate GregorianCalendar before then you can call the method GetWeekOfYear !

private static int GetWeekNumber(DateTime time)
{
    GregorianCalendar cal = new GregorianCalendar();
    int week = cal.GetWeekOfYear(time, CalendarWeekRule.FirstFullWeek, DayOfWeek.Monday);
    return week;
}

Answer 3

You must use Calendar.GetDayOfWeek and Calendar.GetWeekOfYear in preference to writing yourself.

You can guarantee that if you write any date / time handling code yourself it will contain faults and won't work in different locales.

public class Row
{
    public string MonthWeek { get; set; }
    public string Year { get; set; }
    public string Month { get; set; }
    public string Day { get; set; }
    public string WeekOfYear { get; set; }
}

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
        var l = new List<Row>();
        DateTime startDate = DateTime.Now;
        DateTime d = new DateTime(startDate.Year, startDate.Month, 1);
        var cal = System.Globalization.DateTimeFormatInfo.CurrentInfo.Calendar;
        var ms = cal.GetWeekOfYear(new DateTime(d.Year, d.Month, 1), System.Globalization.CalendarWeekRule.FirstDay, System.DayOfWeek.Sunday);
        for (var i = 1; d.Month == startDate.Month; d = d.AddDays(1))
        {
            var si = new Row();
            var month_week = (d.Day / 7) + 1;
            si.MonthWeek = month_week.ToString();
            si.Month = d.Year.ToString();
            si.Year = d.Month.ToString();
            si.Day = d.Day.ToString();
            si.WeekOfYear = cal.GetWeekOfYear(d, System.Globalization.CalendarWeekRule.FirstDay, DayOfWeek.Sunday).ToString();
            l.Add(si);
        }
        dataGrid1.ItemsSource = l;
    }
}

together with the obligatory DataGrid in the XAML:

    <DataGrid AutoGenerateColumns="true" Name="dataGrid1" />