C# 9.0中引入的新特性init和record的使用思考

写在前面

.NET 5.0已经发布,C# 9.0也为我们带来了许多新特性,其中最让我印象深刻的就是init和record type,很多文章已经把这两个新特性讨论的差不多了,本文不再详细讨论,而是通过使用角度来思考这两个特性。

init

init是C# 9.0中引入的新的访问器,它允许被修饰的属性在对象初始化的时候被赋值,其他场景作为只读属性的存在。直接使用的话,可能感受不到init的意义,所以我们先看看之前是如何设置属性为只读的。

private set设置属性为只读

设置只读属性有很多种方式,本文基于private set来讨论。
首先声明一个产品类,如下代码所示,我们把Id设置成了只读,这个时候也就只能通过构造函数来赋值了。在通常情况下,实体的唯一标识是不可更改的,同时也要防止Id被意外更改。

   1:  public class Product
   2:  {
   3:      public Product(int id)
   4:      {
   5:          this.Id = id;
   6:      }
   7:   
   8:      public int Id { get; private set; }
   9:      //public int Id { get; }
  10:   
  11:      public string ProductName { get; set; }
  12:   
  13:      public string Description { get; set; }
  14:  }
  15:   
  16:  class Program
  17:  {
  18:      static void Main(string[] args)
  19:      {
  20:          Product product = new Product(1)
  21:          {
  22:              ProductName = "test001",
  23:              Description = "Just a description"
  24:          };
  25:   
  26:          Console.WriteLine($"Current Product Id: {product.Id},\n\rProduct Name: {product.ProductName}, \n\rProduct Description: {product.Description}");
  27:          
  28:          //运行结果
  29:          //Current Product Id: 1,
  30:          //Product Name: test001,
  31:          //Product Description: Just a description
  32:          
  33:          Console.ReadKey();
  34:      }
  35:  }

record方式设置只读

使用init方式,是非常简单的,只需要把private set改成init就行了:

   1:  public int Id { get; init; }

两者比较

为了方便比较,我们可以将ProductName设置成了private set,然后通过ILSpy来查看一下编译后的代码,看看编译后的Id和ProductName有何不同

C# 9.0中引入的新特性init和record的使用思考

咋一看,貌似没啥区别,都使用到了initonly来修饰。但是如果仅仅只是替换声明方式,那么这个新特性似乎就没有什么意义了。
接下来我们看第二张图:

C# 9.0中引入的新特性init和record的使用思考

如图标记的那样,区别还是很明显的,通过init修饰的属性并没有完全替换掉set,由此看来微软在设计init的时候,还是挺用心思的,也为后面的赋值留下了入口。

   1:  instance void modreq([System.Runtime]System.Runtime.CompilerServices.IsExternalInit) set_Id (
   2:     int32 'value'
   3:    )

另外在赋值的时候,使用private set修饰的属性,需要定义构造函数,通过构造函数赋值。而使用了init修饰的属性,则不需要定义构造函数,直接在对象初始化器中赋值即可。

   1:  Product product = new Product
   2:  {
   3:      Id = 1,
   4:      ProductName = "test001",
   5:      Description = "Just a description"
   6:  };
   7:   
   8:  product.Id = 2;//Error CS8852 Init-only property or indexer 'Product.Id' can only be assigned in an object initializer, or on 'this' or 'base' in an instance constructor or an 'init' accessor.

如上代码所示,只读属性Id的赋值并没有在构造函数中赋值,毕竟当一个类的只读字段十分多的时候,构造函数也变得复杂。而且在赋值好之后,无法修改,这和我们对只读属性在通常情况下的理解是一致的。另外通过init修饰的好处便是省却了一部分只读属性在操作上的复杂性,使得对象的声明与赋值更加直观。
在合适的场景下选择最好的编程方式,是程序员的一贯追求,千万不要为了炫技而把init当成了茴字的第N种写法到处去问。


record

record是一个非常有用的特性,它是不可变类型,其相等性是通过内部的几个属性来确定的,同时它支持我们以更加方便的方式、像定义值类型那样来定义不可变引用类型。
我们把之前的Product类改成record类型,如下所示:

   1:  public record Product
   2:  {
   3:      public Product(int id, string productName, string description) => (Id, ProductName, Description) = (id, productName, description);
   4:   
   5:      public int Id { get; }
   6:   
   7:      public string ProductName { get; }
   8:   
   9:      public string Description { get; }
  10:  }

然后查看一下IL,可以看到record会被编译成类,同时继承了System.Object,并实现了IEquatable泛型接口。
编译器为我们提供的几个重要方法如下:

  • Equals
  • GetHashCode()
  • Clone
  • PrintMembers和ToString()

比较重要的三个方法

Equals:

C# 9.0中引入的新特性init和record的使用思考

通过图片中的代码,我们知道比较两个record对象,首先需要比较类型是否相同,然后再依次比较内部属性。

GetHashCode():

C# 9.0中引入的新特性init和record的使用思考

record类型通过基类型以及所有的属性及字段的方式来计算HashCode,这在整个继承层次结构中增强了基于值的相等性,也就意味着两个同名同姓的人不会被认为是同一个人

Clone:

C# 9.0中引入的新特性init和record的使用思考

这个方法貌似非常简单,实在看不出有什么特别的地方,那么我们通过后面的内容再来解释这个方法。

record在DDD值对象中的应用

record之前的定义方式:

了解DDD值对象的小伙伴应该想到了,record类型的特性非常像DDD中关于值对象的描述,比如不可变性、其相等于是基于其内部的属性的等等,我们先来看下值类型的定义方式。

   1:  public abstract class ValueObject
   2:  {
   3:      public static bool operator ==(ValueObject left, ValueObject right)
   4:      {
   5:          if (ReferenceEquals(left, null) ^ ReferenceEquals(right, null))
   6:          {
   7:              return false;
   8:          }
   9:          return ReferenceEquals(left, null) || left.Equals(right);
  10:      }
  11:   
  12:      public static bool operator !=(ValueObject left, ValueObject right)
  13:      {
  14:          return !(left == right);
  15:      }
  16:   
  17:      protected abstract IEnumerable<object> GetEqualityComponents();
  18:   
  19:   
  20:      public override bool Equals(object obj)
  21:      {
  22:          if (obj == null || obj.GetType() != GetType())
  23:          {
  24:              return false;
  25:          }
  26:   
  27:          var other = (ValueObject)obj;
  28:   
  29:          return this.GetEqualityComponents().SequenceEqual(other.GetEqualityComponents());
  30:      }
  31:   
  32:      public override int GetHashCode()
  33:      {
  34:          return GetEqualityComponents()
  35:              .Select(x => x != null ? x.GetHashCode() : 0)
  36:              .Aggregate((x, y) => x ^ y);
  37:      }
  38:      // Other utility methods
  39:  }
  40:  public class Address : ValueObject
  41:  {
  42:      public string Street { get; private set; }
  43:      public string City { get; private set; }
  44:      public string State { get; private set; }
  45:      public string Country { get; private set; }
  46:      public string ZipCode { get; private set; }
  47:   
  48:      public Address(string street, string city, string state, string country, string zipcode)
  49:      {
  50:          Street = street;
  51:          City = city;
  52:          State = state;
  53:          Country = country;
  54:          ZipCode = zipcode;
  55:      }
  56:   
  57:      protected override IEnumerable<object> GetEqualityComponents()
  58:      {
  59:          // Using a yield return statement to return each element one at a time
  60:          yield return Street;
  61:          yield return City;
  62:          yield return State;
  63:          yield return Country;
  64:          yield return ZipCode;
  65:      }
  66:   
  67:      public override string ToString()
  68:      {
  69:          return $"Street: {Street}, City: {City}, State: {State}, Country: {Country}, ZipCode: {ZipCode}";
  70:      }
  71:  }

main方法如下:

   1:  static void Main(string[] args)
   2:  {
   3:      Address address1 = new Address("aaa", "bbb", "ccc", "ddd", "fff");
   4:      Console.WriteLine($"address1: {address1}");
   5:   
   6:      Address address2 = new Address("aaa", "bbb", "ccc", "ddd", "fff");
   7:      Console.WriteLine($"address2: {address2}");
   8:   
   9:      Console.WriteLine($"address1 == address2: {address1 == address2}");
  10:   
  11:      string jsonAddress1 = address1.ToJson();
  12:      Address jsonAddress1Deserialize = jsonAddress1.FromJson<Address>();
  13:      Console.WriteLine($"jsonAddress1Deserialize == address1: {jsonAddress1Deserialize == address1}");
  14:   
  15:      Console.ReadKey();
  16:  }

运行结果如下:

   1:  基于class:
   2:  address1: Street: aaa, City: bbb, State: ccc, Country: ddd, ZipCode: fff
   3:  address2: Street: aaa, City: bbb, State: ccc, Country: ddd, ZipCode: fff
   4:  address1 == address2: True
   5:  jsonAddress1Deserialize == address1: True
采用record方式定义:

如果有大量的值对象需要我们编写,这无疑是加重我们的开发量的,这个时候record就派上用场了,最简洁的record风格的代码如下所示,只有一行:

   1:  public record Address(string Street, string City, string State, string Country, string ZipCode);

IL代码如下图所示,从图中我们也可以看到record类型的对象,默认情况下用到了init来限制属性的只读特性。

C# 9.0中引入的新特性init和record的使用思考

main方法代码不变,运行结果也没有因为Address从class变成record而发生改变

   1:  基于record:
   2:  address1: Street: aaa, City: bbb, State: ccc, Country: ddd, ZipCode: fff
   3:  address2: Street: aaa, City: bbb, State: ccc, Country: ddd, ZipCode: fff
   4:  address1 == address2: True
   5:  jsonAddress1Deserialize == address1: True

如此看来我们的代码节省的不止一点点,而是太多太多了,是不是很爽啊。

record对象属性值的更改

使用方式如下:

   1:  class Program
   2:  {
   3:      static void Main(string[] args)
   4:      {
   5:          Address address1 = new Address("aaa", "bbb", "ccc", "ddd", "fff");
   6:          Console.WriteLine($"1. address1: {address1}");
   7:   
   8:          Address addressWith = address1 with { Street = "############" };
   9:   
  10:          Console.ReadKey();
  11:      }
  12:  }
  13:   
  14:  public record Address(string Street, string City, string State, string Country, string ZipCode);

通过ILSpy查看如下所示:

   1:  private static void Main(string[] args)
   2:  {
   3:      Address address1 = new Address("aaa", "bbb", "ccc", "ddd", "fff");
   4:      Console.WriteLine($"1. address1: {address1}");
   5:      Address address2 = address1.<Clone>$();
   6:      address2.Street = "############";
   7:      Address addressWith = address2;
   8:      Console.ReadKey();
   9:  }

由此可以看到record在更改的时候,实际上是通过调用Clone而产生了浅拷贝的对象,这也非常符合DDD ValueObject的设计理念。

参考:

  • https://docs.microsoft.com/en-us/dotnet/architecture/microservices/microservice-ddd-cqrs-patterns/implement-value-objects
  • https://deviq.com/value-object/
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