/// <summary>
/// 公历转为农历的函数
/// </summary>
/// <param name="solarDateTime">公历日期</param>
/// <returns>农历的日期</returns>
static string SolarToChineseLunisolarDate(DateTime solarDateTime)
{
    System.Globalization.ChineseLunisolarCalendar cal = new System.Globalization.ChineseLunisolarCalendar();

    int year = cal.GetYear(solarDateTime);
    int month = cal.GetMonth(solarDateTime);
    int day = cal.GetDayOfMonth(solarDateTime);
    int leapMonth = cal.GetLeapMonth(year);
    return string.Format("农历{0}{1}（{2}）年{3}{4}月{5}{6}"
                        , "甲乙丙丁戊己庚辛壬癸"[(year - 4) % 10]
                        , "子丑寅卯辰巳午未申酉戌亥"[(year - 4) % 12]
                        , "鼠牛虎兔龙蛇马羊猴鸡狗猪"[(year - 4) % 12]
                        , month == leapMonth ? "闰" : ""
                        , "无正二三四五六七八九十冬腊"[leapMonth > 0 && leapMonth <= month ? month - 1 : month]
                        , "初十廿三"[day / 10]
                        , "日一二三四五六七八九"[day % 10]
                        );
}
使用的方法非常简单：

string 农历 = SolarToChineseLunisolarDate(DateTime.Today);