领域驱动(DDD)实战---月份类YearMonth

Net中有一个DateTime结构类，涉及时间和日期，这个类大量使用。可是，他的名称已经显著的表明他是表达某个具体的时刻。当我们不需要每天的具体时间时，如：我的程序逻辑仅仅需要年月（发工资的周期？），这个DateTime显得有些累赘，甚至不合用。
一般人们解决的方式，仍然使用DateTime而从数据上，设置hour,mintue等等为0。然而，这与DDD的理念相背，名称有与含义有偏差，另外，数据一致性的维护，散布在各个角落，如，保证日期始终为1，小时，分钟为0。另外，与月份相关的功能，如：得到下一个月份，要么用DateTime本身的功能（AddMonths),要么提炼出一个Utitlies出来。前者，需要开发者时刻重复DateTime到YearMonth的映射逻辑，后者是个反模式。（本文版权属于© 2012 - 2013 予沁安）

这里，我创建出一个基本类型YearMonth，可以作为代码的基本砖块。

[代码] 用Extension的方式，来增强代码流畅性和可读性

public static class YearMonthExtension
    {
        public static YearMonth year(this int year,int month)
        {
            return new YearMonth(year,month);
        }
        public static bool is_later_than(this YearMonth left,YearMonth right) {
            return left.CompareTo(right) > 0;
        }
        public static bool is_ealier_than(this YearMonth left,YearMonth right) {
            return left.CompareTo(right) < 0;
        }
        public static YearMonth get_ealier(this YearMonth left,YearMonth right)
        {
            if (left.is_ealier_than(right))
                return left;
            return right;
        }
        public static YearMonth get_later(this YearMonth left,YearMonth right)
        {
            if (left.is_later_than(right))
                return left;
            return right;
        }
}

[代码] 从测试看功能：公用的测试基类，很简单，就是声明一个YearMonth对象做测试

public class YearMonthSpecs
    {
        protected static YearMonth subject;
 
    }

[代码] 通过构造器，创建YearMonth对象

public class When_init_by_year_month
        :YearMonthSpecs
    {
        private Because of =
            () => subject = new YearMonth(2011,3);
 
        private It year_should_set_properly =
            () => subject.Year.ShouldEqual(2011);
 
         private It month_should_set_properly =
             () => subject.Month.ShouldEqual(3);
 
    }

[代码] 通过流畅接口创建YearMonth: 2012.year(3)。你还可以自己定制为: 2012.年(3)

public class When_create_year_month_through_fluent_interface
    {
        private It should_create_year_month_properly =
            () => 2012.year(3).ShouldEqual(new YearMonth(2012,3));
    }

[代码] 通过字符串创建

public class When_init_by_string : YearMonthSpecs
    {
        private Because of =
            () => subject = new YearMonth("2011年01月");
 
        private It year_should_set_properly =
             () =>
                 {
                     subject.Year.ShouldEqual(2011);
                     subject.Month.ShouldEqual(1);
                 };
    }

[代码] Special Case模式，特别处理：世界末日的下一个月还是世界末日，创世纪的上一个月还是创世纪

private It far_past_last_month_is_still_far_past =
            () => YearMonth.FarPast.get_last().ShouldEqual(YearMonth.FarPast);
        private It far_past_next_month_is_still_far_past =
         () => YearMonth.FarPast.get_next().ShouldEqual(YearMonth.FarPast);
 
        private It far_future_last_month_is_stil_far_future =
          () => YearMonth.FarFuture.get_last().ShouldEqual(YearMonth.FarFuture);
 
        private It far_future_next_month_is_stil_far_future =
            () => YearMonth.FarFuture.get_next().ShouldEqual(YearMonth.FarFuture);

YearMonth结构类型的完整代码

using System;
using Skight.Arch.Domain.Interfaces;
 
namespace Skight.Arch.Domain.Entities
{
    public struct YearMonth : IEquatable<YearMonth>,IComparable<YearMonth>
    {
        private readonly int ticks;
        private readonly int year;
        private readonly int month;
        private const int MONTHS_PER_YEAR=12;
 
        public static YearMonth FarPast = new YearMonth(0,1);
        public static YearMonth FarFuture = new YearMonth(9999,12);
        #region Constructors by ticks,year/month and datetime
        internal YearMonth(int ticks)
        {
            this.ticks = ticks;
            int remain;
            year = Math.DivRem(ticks-1,MONTHS_PER_YEAR,out remain);
            month = remain + 1;
        }
        public YearMonth(int year,int month)
            :this(year*MONTHS_PER_YEAR + month){}
        public YearMonth(DateTime date_time)
            :this(date_time.Year,date_time.Month){}
 
        public YearMonth(string yearMonth):this(int.Parse(yearMonth.Substring(0,4)),int.Parse(yearMonth.Substring(5,2)))
        {}
 
        #endregion 
 
        public int Year { get { return year; } }
        public int Month { get { return month; } }
        public DateTime as_date_Time()
        {
            return new DateTime(Year,Month,1);
        }
 
        public override string ToString()
        {
            return string.Format("{0}年{1}月",year.ToString("0000"),month.ToString("00"));
        }
 
        #region Euqals and Compare
        public bool Equals(YearMonth other)
        {
            return other.ticks == ticks;
        }
 
        public override bool Equals(object obj)
        {
            if (ReferenceEquals(null,obj)) return false;
            if (obj.GetType() != typeof (YearMonth)) return false;
            return Equals((YearMonth) obj);
        }
 
        public override int GetHashCode()
        {
            return ticks;
        }
 
        public int CompareTo(YearMonth other)
        {
            return ticks.CompareTo(other.ticks);
        }
 
        #endregion
 
        #region Discrete interface
        public YearMonth get_last()
        {
            if (Equals(FarPast))
                return FarPast;
 
            if (Equals(FarFuture))
                return FarFuture;
            return new YearMonth(ticks - 1);
        }
        public YearMonth get_last(int Dvalue)
        {
            if (Equals(FarPast))
                return FarPast;
 
            if (Equals(FarFuture))
                return FarFuture;
            return new YearMonth(ticks - Dvalue);
        }
 
        public YearMonth get_next()
        {
            if (Equals(FarPast))
                return FarPast;
 
            if (Equals(FarFuture))
                return FarFuture;
            return new YearMonth(ticks + 1);
        }
 
        public YearMonth get_next(int DValue)
        {
            if (Equals(FarPast))
                return FarPast;
 
            if (Equals(FarFuture))
                return FarFuture;
            return new YearMonth(ticks + DValue);
        }
        #endregion 
 
         public static implicit operator DateTime(YearMonth year_month)
         {
             return year_month.as_date_Time();
         }
 
        public static implicit operator YearMonth(DateTime date_time)
        {
            return new YearMonth(date_time);
        }
    }
}