C#返回每月的第一天和最后一天

C#帮助类库：返回每月的第一天和最后一天帮助类库源码

DateFormatHelper源码：

using System;

namespace DateFormatHelper
{
    public class DateFormat
    {
        //返回每月的第一天和最后一天
        public static void ReturnDateFormat(int month, out string firstDay, out string lastDay)
        {
            int year = DateTime.Now.Year + month / 12;
            if (month != 12)
            {
                month = month % 12;
            }
            switch (month)
            {
                case 1:
                    firstDay = DateTime.Now.ToString(year + "-0" + month + "-01");
                    lastDay = DateTime.Now.ToString(year + "-0" + month + "-31");
                    break;
                case 2:
                    firstDay = DateTime.Now.ToString(year + "-0" + month + "-01");
                    if (DateTime.IsLeapYear(DateTime.Now.Year))
                        lastDay = DateTime.Now.ToString(year + "-0" + month + "-29");
                    else
                        lastDay = DateTime.Now.ToString(year + "-0" + month + "-28");
                    break;
                case 3:
                    firstDay = DateTime.Now.ToString(year + "-0" + month + "-01");
                    lastDay = DateTime.Now.ToString("yyyy-0" + month + "-31");
                    break;
                case 4:
                    firstDay = DateTime.Now.ToString(year + "-0" + month + "-01");
                    lastDay = DateTime.Now.ToString(year + "-0" + month + "-30");
                    break;
                case 5:
                    firstDay = DateTime.Now.ToString(year + "-0" + month + "-01");
                    lastDay = DateTime.Now.ToString(year + "-0" + month + "-31");
                    break;
                case 6:
                    firstDay = DateTime.Now.ToString(year + "-0" + month + "-01");
                    lastDay = DateTime.Now.ToString(year + "-0" + month + "-30");
                    break;
                case 7:
                    firstDay = DateTime.Now.ToString(year + "-0" + month + "-01");
                    lastDay = DateTime.Now.ToString(year + "-0" + month + "-31");
                    break;
                case 8:
                    firstDay = DateTime.Now.ToString(year + "-0" + month + "-01");
                    lastDay = DateTime.Now.ToString(year + "-0" + month + "-31");
                    break;
                case 9:
                    firstDay = DateTime.Now.ToString(year + "-0" + month + "-01");
                    lastDay = DateTime.Now.ToString(year + "-0" + month + "-30");
                    break;
                case 10:
                    firstDay = DateTime.Now.ToString(year + "-" + month + "-01");
                    lastDay = DateTime.Now.ToString(year + "-" + month + "-31");
                    break;
                case 11:
                    firstDay = DateTime.Now.ToString(year + "-" + month + "-01");
                    lastDay = DateTime.Now.ToString(year + "-" + month + "-30");
                    break;
                default:
                    firstDay = DateTime.Now.ToString(year + "-" + month + "-01");
                    lastDay = DateTime.Now.ToString(year + "-" + month + "-31");
                    break;
            }
        }
    }
}