ersten und letzten Tag der Woche berechnen

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public static class Week
{
    /// <summary>
    /// Gets the first day of week.
    /// </summary>
    /// <param name="dateTime">The date time.</param>
    /// <returns>the first day of the week</returns>
    public static DateTime GetFirstDayOfWeek(DateTime dateTime)
    {
        while (dateTime.DayOfWeek != DayOfWeek.Monday)
            dateTime = dateTime.Subtract(new TimeSpan(1, 0, 0, 0));
        return new DateTime(dateTime.Year, dateTime.Month, dateTime.Day);
    }

    /// <summary>
    /// Gets the last day of week.
    /// </summary>
    /// <param name="dateTime">The date time.</param>
    /// <returns>the last day of the week</returns>
    public static DateTime GetLastDayOfWeek(DateTime dateTime)
    {
        while (dateTime.DayOfWeek != DayOfWeek.Sunday)
            dateTime = dateTime.AddDays(1);
        return new DateTime(dateTime.Year, dateTime.Month, dateTime.Day);
    }
}