[C#] 某日期為該年的第幾週 -- 使用GregorianCalendar

using System.Globalization;

/// <summary>
/// 取得某一日期在當年的第幾週
/// </summary>
/// <param name="dt">日期</param>
/// <returns>該日期在當年中的週數</returns>
private int GetWeekOfYear(DateTime dt)
{
        GregorianCalendar gc = new GregorianCalendar();
        return gc.GetWeekOfYear(dt, CalendarWeekRule.FirstDay, DayOfWeek.Monday);
}

using System.Globalization;

/// <summary>
/// 取得某一年共有多少週
/// </summary>
/// <param name="year">年份</param>
/// <returns>該年的週數</returns>
private int GetWeekAmount(int year)
{
        DateTime end = new DateTime(year, 12, 31);  //該年最後一天
        System.Globalization.GregorianCalendar gc = new GregorianCalendar();
        return gc.GetWeekOfYear(end, CalendarWeekRule.FirstDay, DayOfWeek.Monday);  //該年的週數
}