C# 获取指定年月的第一天和最后一天、获取本月的第一天和最后一天、获取当前日期的星期几等

•获取指定年月的第一天

         public static DateTime GetCurMonthFirstDay(string year,string mon)
         {
             DateTime AssemblDate = Convert.ToDateTime(year + "-" + mon + "-" + "01");  // 组装当前指定月份
             return AssemblDate.AddDays(1 - AssemblDate.Day);  // 返回指定当前月份的第一天
         }

调用

         private void button1_Click(object sender, EventArgs e)
         {
             DateTime FirstDay = GetCurMonthFirstDay("2019","2");  // 2019-02-01 00:00:00
         }

•获取指定年月的最后一天

         public static DateTime GetCurMonthLastDay(string year, string mon)
         {
             DateTime AssemblDate = Convert.ToDateTime(year + "-" + mon + "-" + "01");  // 组装当前指定月份
             return AssemblDate.AddDays(1 - AssemblDate.Day).AddMonths(1).AddDays(-1);  // 返回指定当前月份的最后一天
         }

调用

         private void button1_Click(object sender, EventArgs e)
         {
             DateTime LastDay = GetCurMonthLastDay("2019", "2");  // 2019-02-28 00:00:00
         }

•获取当前月的第一天

         public static DateTime GetCurMonthFirstDay()
         {
             // 第一种写法
             //DateTime CurDate =Convert.ToDateTime(DateTime.Now.ToString());  // 组装当前指定月份
             //return CurDate.AddDays(1 - CurDate.Day);  // 返回指定当前月份的第一天
 
             // 第二种写法
             DateTime nowDate = DateTime.Now;
             return new DateTime(nowDate.Year, nowDate.Month, 1); // 该方法可以指定，年、月、日
         }

调用

         private void button1_Click(object sender, EventArgs e)
         {
             DateTime FirstDay = GetCurMonthFirstDay();  // 2019-02-01 00:00:00
         }

•获取当前月的最后一天

         public static DateTime GetCurMonthLastDay()
         {
             DateTime CurDate = Convert.ToDateTime(DateTime.Now.ToString());  // 组装当前指定月份
             return CurDate.AddDays(1 - CurDate.Day).AddMonths(1).AddDays(-1);  // 返回指定当前月份的最后一天
         }

调用

         private void button1_Click(object sender, EventArgs e)
         {
             DateTime LastDay = GetCurMonthLastDay();  // 2019-02-28 00:00:00
         }

•其他

 private void button1_Click(object sender, EventArgs e)
         {
             string year = DateTime.Now.Year.ToString();  // 获取年份;2019
             string mon = DateTime.Now.Month.ToString();  // 获取月份;2
             string week = DateTime.Now.DayOfWeek.ToString();  // 获取周几;Tuesday
             string days = DateTime.Now.DayOfYear.ToString();  // 获取一年中的第几天；50
         }