C# 一行代碼獲取當月首尾日期

本月第一天

DateTime firstDay = DateTime.Today.AddDays(1 - DateTime.Today.Day);

本月最後一天

DateTime lastDay = DateTime.Today.AddDays(1 - DateTime.Today.Day).AddMonths(1).AddDays(-1);

原理解析

• DateTime.Today.Day 獲取當前日 (1-31)
• AddDays(1 - Day) 回退到 1 號 00:00:00
• AddMonths(1) 進入下月 1 號
• AddDays(-1) 退回上月最後一天 23:59:59.999

擴展：獲取任意月首尾

public (DateTime First, DateTime Last) GetMonthRange(int year, int month) {
    DateTime first = new DateTime(year, month, 1);
    DateTime last = first.AddMonths(1).AddDays(-1);
    return (first, last);
}