《ASP.NET Core技术内幕与项目实战》精简集-DDD准备5.3：值对象

//实体类-充血模型================================================================================
public class Region
{
    public long Id { get; init; }  //Id标识符
    public MultiName Name { get; init; } //名称
    public Area Area { get; init; } //面积
    public Geo Location { get; init; } //地理位置
    public RegionLevel Level { get; private set; } //地区级别
    public long? Population { get; private set; } //人口
    
    //私有无参构造函数，EFCore默认使用
    private Region(){}
    //公有有参构造函数，外部创建对象时初始化
    public Region(MultiName name, Area area, Geo location, RegionLevel level)
    {
        Name = name;
        Area = area;
        Location = location;
        Level = level;
    }

    public void ChangeLevel(RegionLevel level)
    {
        this.Level= level;
    }

    public void ChangePopulation(long population)
    {
        this.Population = population;
    }
}


//相关的值对象和枚举类型。值对象类型为不可变类型，推荐使用record==========================
public record MultiName(string Chinese, string? English); //多语言名称
public record Area(double Value, AreaType Unit); //面积
public enum AreaType { SquareKM, Hectare, CnMu}; //面积单位，平方公里、公顷、亩
public enum RegionLevel { Province, City, County, Town}; //地区级别
public record Geo //地理位置
{
    public double Longitude { get; init; }
    public double Latitude { get; init; }
    public Geo(double longitude, double latitude)
    {
        if (longitude<-180 || longitude>180)
        {
            throw new ArgumentException("纬度值无效！");
        }
        if (latitude < -90 || latitude > 90)
        {
            throw new ArgumentException("经度值无效！");
        }
        this.Longitude= longitude;
        this.Latitude= latitude;
    }
}

public class MyDbContext:DbContext
{
    public DbSet<Region> Regions { get; set; }
    ......
    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {     
        modelBuilder.Entity<Region>(b =>
        {
            b.ToTable("T_Regions");            
            //设置值对象Name
            b.OwnsOne(r => r.Name, vo =>
            {
                vo.Property(n => n.English).HasMaxLength(20).IsUnicode(false);
                vo.Property(n => n.Chinese).HasMaxLength(20).IsUnicode(true);
            });
            //设置值对象Area，同时将值对象Area类中Unit属性的枚举值映射为字符串
            b.OwnsOne(r => r.Area, vo =>
            {
                vo.Property(a => a.Unit).HasMaxLength(20).IsUnicode(false).HasConversion<string>();
            });
            //设置值对象Location
            b.OwnsOne(r => r.Location);
            //设置枚举值Level，映射为字符串
            b.Property(r => r.Level).HasMaxLength(20).IsUnicode(false).HasConversion<string>();
        });
    }
}

using var ctx = new MyDbContext();

//新增一条数据
//var region1 = new Region(name1,area1,geo1,RegionLevel.City);
//region1.ChangePopulation(20000000);
//ctx.Regions.Add(region1);
//ctx.SaveChanges();
//查询数据
var region = ctx.Regions.FirstOrDefault(x => x.Name.Chinese == "广州");
Console.WriteLine(region); //将Region类修改为record类型，可打印字符串