给C#新增一个时间类型: YearMonth

在.Net Framework中，我们常用的时间类型是DateTime。直到.Net6微软加入了两个新的时间类型：DateOnly和TimeOnly，才弥补了之前的不足。

DateOnly：表示仅日期。比如：某人的生日，我只关心日期，就适合用DateOnly。

TimeOnly：表示仅时间。比如：每天定时执行某个任务，我只关心时间，就适合用TimeOnly。

由此可见，DateOnly和TimeOnly都有相应的应用场景。可笔者在实际项目中遇到了这样的业务场景：需要每月给客户生成月账单。这里我所关心的是某个月份，于是我首先想到用DateOnly表示(不考虑字符串)。

var date = new DateOnly(2023, 2, 1);    // 代表2023年2月1日 

虽然DateOnly可用，但从字面理解和表现形式上还是略显尴尬。 DateOnly真正表达的是某一天并不是某个月， 在代码层面也容易混淆，所以并不符合博主的心理期望。经过一番纠结和思考，博主决定自己动手创建一个表示年/月的时间类型：YearMonth。

var ym = new YearMonth(2023, 2);  // 代表2023年2月

 YearMonth的源码如下：

 /// <summary>
 /// 表示年/月的时间类型
 /// </summary>
 [JsonConverter(typeof(YearMonthJsonConverter))]
 public readonly struct YearMonth
 {
     public int Year { get; }

     public int Month { get; }

     public YearMonth(int year, int month)
     {
         Year = year;
         Month = month;
     }
        
     public YearMonth AddMonths(int value)
     {
         var date = new DateOnly(Year, Month, 1);
         return FromDateOnly(date.AddMonths(value));
     }

     public YearMonth AddYears(int value)
     {
         var date = new DateOnly(Year, Month, 1);
         return FromDateOnly(date.AddYears(value));
     }

     public DateOnly FirstDay()
     {
         return new DateOnly(Year, Month, 1);
     }

     public DateOnly LastDay()
     {
         var nextMonth = AddMonths(1);
         var date = new DateOnly(nextMonth.Year, nextMonth.Month, 1);
         return date.AddDays(-1);
     }

     public int DaysInMonth()
     {
         return DateTime.DaysInMonth(Year, Month);
     }

     public static YearMonth Current 
     {
         get { return FromDateTime(DateTime.Now); }
     }

     public static YearMonth UtcCurrent
     {
         get { return FromDateTime(DateTime.UtcNow); }
     }

     public static YearMonth FromDateOnly(DateOnly dateOnly)
     {
         return new YearMonth(dateOnly.Year, dateOnly.Month);
     }

     public static YearMonth FromDateTime(DateTime dateTime)
     {
         return new YearMonth(dateTime.Year, dateTime.Month);
     }
        
     public static YearMonth FromString(string s)
     {
         if (DateTime.TryParse(s, out var date))
         {
             return FromDateTime(date);
         }
         throw new ArgumentException("format is error", nameof(s));
     }

     public override string ToString()
     {
         return $"{Year.ToString().PadLeft(4, '0')}-{Month.ToString().PadLeft(2, '0')}";
     }

     public static bool operator ==(YearMonth left, YearMonth right)
     {
         return left.Year == right.Year && left.Month == right.Month;
     }

     public static bool operator !=(YearMonth left, YearMonth right)
     {
         return !(left.Year == right.Year && left.Month == right.Month);
     }

     public static bool operator >=(YearMonth left, YearMonth right)
     {
         return (left.Year > right.Year) || (left.Year == right.Year && left.Month >= right.Month);
     }

     public static bool operator <=(YearMonth left, YearMonth right)
     {
         return (left.Year < right.Year) || (left.Year == right.Year && left.Month <= right.Month);
     }

     public static bool operator >(YearMonth left, YearMonth right)
     {
         return (left.Year > right.Year) || (left.Year == right.Year && left.Month > right.Month);
     }

     public static bool operator <(YearMonth left, YearMonth right)
     {
         return (left.Year < right.Year) || (left.Year == right.Year && left.Month < right.Month);
     }
               
     public override bool Equals(object obj)
     {
         return base.Equals(obj);
     }

     public override int GetHashCode()
     {
         return base.GetHashCode();
     }       
}

其中特性 [JsonConverter(typeof(YearMonthJsonConverter))]用于Json序列化和反序列化。

public class YearMonthJsonConverter : JsonConverter<YearMonth>
{
     public override YearMonth Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
     {
         return YearMonth.FromString(reader.GetString());
     }

     public override void Write(Utf8JsonWriter writer, YearMonth value, JsonSerializerOptions options)
     {
         writer.WriteStringValue(value.ToString());
     }
}

 YearMonth的一些用法示例：

var ym = new YearMonth(2023, 2);
int n = ym.DaysInMonth();     //n：28
DateOnly d1 = ym.FirstDay();  //d1：2023/2/1
DateOnly d2 = ym.LastDay();   //d2：2023/2/28
string str = ym.ToString();   //str：2023-02
YearMonth ym2 = ym.AddMonths(1);  //ym2: 2023-03
YearMonth ym3 = YearMonth.FromDateOnly(new DateOnly(2023, 2, 8)); //ym3: 2023-02 
YearMonth ym4 = YearMonth.FromDateTime(new DateTime(2023, 2, 8, 12, 23, 45)); //ym4: 2023-02
bool b = new YearMonth(2023, 3) > new YearMonth(2023, 2);  //b: true

  至此，上面的YearMonth时间类型已经满足小编的开发需要，当然也可以根据需求继续扩展其它功能。