C# 9 新特性――record的相关总结

时间：2022-02-10 13:49:59|栏目：.NET代码|点击：次

Intro

C# 9 中引入了 record，record 是一个特殊类，用它来实现 model 在有些情况下会非常的好用

Sample

record RecordPerson
{
 public string Name { get; init; }

 public int Age { get; init; }
}

record RecordPerson2(string Name, int Age);

public static void MainTest()
{
 var p1 = new RecordPerson()
 {
  Name = "Tom",
  Age = 12,
 };
 Console.WriteLine(p1);

 var p2 = p1 with { Age = 10 };
 Console.WriteLine(p2);

 var p3 = new RecordPerson() { Name = "Tom", Age = 12 };
 Console.WriteLine(p3);
 Console.WriteLine($"p1 Equals p3 =:{p1 == p3}");

 RecordPerson2 p4 = new("Tom", 12);
 Console.WriteLine(p4);
}

这里的示例，用 record 声明了两个 model，第二个 model 声明的时候使用了简化的写法，

record RecordPerson2(string Name, int Age); 这样的声明意味着，构造方法有两个参数，分别是 string Name 和 int Age，并对应着两个属性，属性的声明方式和 RecordPerson 一样 public string Name { get; init; } 都是一个 get 一个 init

对于 record 支持一个 with 表达式，来修改某几个属性的值，这对于有很多属性都相同的场景来说是及其方便的，来看一下上面示例的输出结果

What inside

那么 record 内部发生了什么呢，我们来反编译看一下，我们看一下使用 DnSpy 反编译的结果

RecordPerson

private class RecordPerson : IEquatable<RecordSample.RecordPerson>
{
 // Token: 0x17000007 RID: 7
 // (get) Token: 0x06000027 RID: 39 RVA: 0x000025F4 File Offset: 0x000007F4
 [Nullable(1)]
 protected virtual Type EqualityContract
 {
  [NullableContext(1)]
  [CompilerGenerated]
  get
  {
   return typeof(RecordSample.RecordPerson);
  }
 }

 // Token: 0x17000008 RID: 8
 // (get) Token: 0x06000028 RID: 40 RVA: 0x00002600 File Offset: 0x00000800
 // (set) Token: 0x06000029 RID: 41 RVA: 0x00002608 File Offset: 0x00000808
 public string Name
 {
  [CompilerGenerated]
  get
  {
   return this.<Name>k__BackingField;
  }
  [CompilerGenerated]
  set
  {
   this.<Name>k__BackingField = value;
  }
 }

 // Token: 0x17000009 RID: 9
 // (get) Token: 0x0600002A RID: 42 RVA: 0x00002611 File Offset: 0x00000811
 // (set) Token: 0x0600002B RID: 43 RVA: 0x00002619 File Offset: 0x00000819
 public int Age
 {
  [CompilerGenerated]
  get
  {
   return this.<Age>k__BackingField;
  }
  [CompilerGenerated]
  set
  {
   this.<Age>k__BackingField = value;
  }
 }

 // Token: 0x0600002C RID: 44 RVA: 0x00002624 File Offset: 0x00000824
 public override string ToString()
 {
  StringBuilder stringBuilder = new StringBuilder();
  stringBuilder.Append("RecordPerson");
  stringBuilder.Append(" { ");
  if (this.PrintMembers(stringBuilder))
  {
   stringBuilder.Append(" ");
  }
  stringBuilder.Append("}");
  return stringBuilder.ToString();
 }

 // Token: 0x0600002D RID: 45 RVA: 0x00002678 File Offset: 0x00000878
 [NullableContext(1)]
 protected virtual bool PrintMembers(StringBuilder builder)
 {
  builder.Append("Name");
  builder.Append(" = ");
  builder.Append(this.Name);
  builder.Append(", ");
  builder.Append("Age");
  builder.Append(" = ");
  builder.Append(this.Age.ToString());
  return true;
 }

 // Token: 0x0600002E RID: 46 RVA: 0x000026EA File Offset: 0x000008EA
 [NullableContext(2)]
 public static bool operator !=(RecordSample.RecordPerson r1, RecordSample.RecordPerson r2)
 {
  return !(r1 == r2);
 }

 // Token: 0x0600002F RID: 47 RVA: 0x000026F6 File Offset: 0x000008F6
 [NullableContext(2)]
 public static bool operator ==(RecordSample.RecordPerson r1, RecordSample.RecordPerson r2)
 {
  return r1 == r2 || (r1 != null && r1.Equals(r2));
 }

 // Token: 0x06000030 RID: 48 RVA: 0x0000270C File Offset: 0x0000090C
 public override int GetHashCode()
 {
  return (EqualityComparer<Type>.Default.GetHashCode(this.EqualityContract) * -1521134295 + EqualityComparer<string>.Default.GetHashCode(this.<Name>k__BackingField)) * -1521134295 + EqualityComparer<int>.Default.GetHashCode(this.<Age>k__BackingField);
 }

 // Token: 0x06000031 RID: 49 RVA: 0x0000274C File Offset: 0x0000094C
 [NullableContext(2)]
 public override bool Equals(object obj)
 {
  return this.Equals(obj as RecordSample.RecordPerson);
 }

 // Token: 0x06000032 RID: 50 RVA: 0x0000275C File Offset: 0x0000095C
 [NullableContext(2)]
 public virtual bool Equals(RecordSample.RecordPerson other)
 {
  return other != null && this.EqualityContract == other.EqualityContract && EqualityComparer<string>.Default.Equals(this.<Name>k__BackingField, other.<Name>k__BackingField) && EqualityComparer<int>.Default.Equals(this.<Age>k__BackingField, other.<Age>k__BackingField);
 }

 // Token: 0x06000033 RID: 51 RVA: 0x000027B0 File Offset: 0x000009B0
 [NullableContext(1)]
 public virtual RecordSample.RecordPerson <Clone>$()
 {
  return new RecordSample.RecordPerson(this);
 }

 // Token: 0x06000034 RID: 52 RVA: 0x000027B8 File Offset: 0x000009B8
 protected RecordPerson([Nullable(1)] RecordSample.RecordPerson original)
 {
  this.Name = original.<Name>k__BackingField;
  this.Age = original.<Age>k__BackingField;
 }

 // Token: 0x06000035 RID: 53 RVA: 0x000027D9 File Offset: 0x000009D9
 public RecordPerson()
 {
 }

 // Token: 0x0400000C RID: 12
 [CompilerGenerated]
 [DebuggerBrowsable(DebuggerBrowsableState.Never)]
 private readonly string <Name>k__BackingField;

 // Token: 0x0400000D RID: 13
 [CompilerGenerated]
 [DebuggerBrowsable(DebuggerBrowsableState.Never)]
 private readonly int <Age>k__BackingField;
}

从上面的反编译结果可以看的出来，record 其实就是一个 class，只是编译器会帮我们做一些事情，编译器帮我们做了哪些事呢？

实现了基于属性值的相等性比较，不再使用默认的引用，并且重写了 ==/!= operator 和 GetHashCode
为了方便调试，重写了 ToString 方法，也提供了 PrintMembers 方法来实现比较方便只显示某些比较重要的参数
实现了 EqualityContract 方法来指定类型比较的类型，默认是当前类型
实现了 <Clone>$ 方法和一个特殊的构造方法，用来克隆一个对象，相当于 record 帮我们实现了一个浅复制的原型模式，还是强类型的，这个方法在代码里不能直接调用，当我们使用 with 表达式的时候，编译器会调用这个方法，并对某些属性进行赋值

再来看一下 RecordPerson2

private class RecordPerson2 : IEquatable<RecordSample.RecordPerson2>
{
 // Token: 0x06000036 RID: 54 RVA: 0x000027E2 File Offset: 0x000009E2
 public RecordPerson2(string Name, int Age)
 {
  this.Name = Name;
  this.Age = Age;
  base..ctor();
 }

 // Token: 0x1700000A RID: 10
 // (get) Token: 0x06000037 RID: 55 RVA: 0x000027F9 File Offset: 0x000009F9
 [Nullable(1)]
 protected virtual Type EqualityContract
 {
  [NullableContext(1)]
  [CompilerGenerated]
  get
  {
   return typeof(RecordSample.RecordPerson2);
  }
 }

 // Token: 0x1700000B RID: 11
 // (get) Token: 0x06000038 RID: 56 RVA: 0x00002805 File Offset: 0x00000A05
 // (set) Token: 0x06000039 RID: 57 RVA: 0x0000280D File Offset: 0x00000A0D
 public string Name
 {
  [CompilerGenerated]
  get
  {
   return this.<Name>k__BackingField;
  }
  [CompilerGenerated]
  set
  {
   this.<Name>k__BackingField = value;
  }
 }

 // Token: 0x1700000C RID: 12
 // (get) Token: 0x0600003A RID: 58 RVA: 0x00002816 File Offset: 0x00000A16
 // (set) Token: 0x0600003B RID: 59 RVA: 0x0000281E File Offset: 0x00000A1E
 public int Age
 {
  [CompilerGenerated]
  get
  {
   return this.<Age>k__BackingField;
  }
  [CompilerGenerated]
  set
  {
   this.<Age>k__BackingField = value;
  }
 }

 // Token: 0x0600003C RID: 60 RVA: 0x00002828 File Offset: 0x00000A28
 public override string ToString()
 {
  StringBuilder stringBuilder = new StringBuilder();
  stringBuilder.Append("RecordPerson2");
  stringBuilder.Append(" { ");
  if (this.PrintMembers(stringBuilder))
  {
   stringBuilder.Append(" ");
  }
  stringBuilder.Append("}");
  return stringBuilder.ToString();
 }

 // Token: 0x0600003D RID: 61 RVA: 0x0000287C File Offset: 0x00000A7C
 [NullableContext(1)]
 protected virtual bool PrintMembers(StringBuilder builder)
 {
  builder.Append("Name");
  builder.Append(" = ");
  builder.Append(this.Name);
  builder.Append(", ");
  builder.Append("Age");
  builder.Append(" = ");
  builder.Append(this.Age.ToString());
  return true;
 }

 // Token: 0x0600003E RID: 62 RVA: 0x000028EE File Offset: 0x00000AEE
 [NullableContext(2)]
 public static bool operator !=(RecordSample.RecordPerson2 r1, RecordSample.RecordPerson2 r2)
 {
  return !(r1 == r2);
 }

 // Token: 0x0600003F RID: 63 RVA: 0x000028FA File Offset: 0x00000AFA
 [NullableContext(2)]
 public static bool operator ==(RecordSample.RecordPerson2 r1, RecordSample.RecordPerson2 r2)
 {
  return r1 == r2 || (r1 != null && r1.Equals(r2));
 }

 // Token: 0x06000040 RID: 64 RVA: 0x00002910 File Offset: 0x00000B10
 public override int GetHashCode()
 {
  return (EqualityComparer<Type>.Default.GetHashCode(this.EqualityContract) * -1521134295 + EqualityComparer<string>.Default.GetHashCode(this.<Name>k__BackingField)) * -1521134295 + EqualityComparer<int>.Default.GetHashCode(this.<Age>k__BackingField);
 }

 // Token: 0x06000041 RID: 65 RVA: 0x00002950 File Offset: 0x00000B50
 [NullableContext(2)]
 public override bool Equals(object obj)
 {
  return this.Equals(obj as RecordSample.RecordPerson2);
 }

 // Token: 0x06000042 RID: 66 RVA: 0x00002960 File Offset: 0x00000B60
 [NullableContext(2)]
 public virtual bool Equals(RecordSample.RecordPerson2 other)
 {
  return other != null && this.EqualityContract == other.EqualityContract && EqualityComparer<string>.Default.Equals(this.<Name>k__BackingField, other.<Name>k__BackingField) && EqualityComparer<int>.Default.Equals(this.<Age>k__BackingField, other.<Age>k__BackingField);
 }

 // Token: 0x06000043 RID: 67 RVA: 0x000029B4 File Offset: 0x00000BB4
 [NullableContext(1)]
 public virtual RecordSample.RecordPerson2 <Clone>$()
 {
  return new RecordSample.RecordPerson2(this);
 }

 // Token: 0x06000044 RID: 68 RVA: 0x000029BC File Offset: 0x00000BBC
 protected RecordPerson2([Nullable(1)] RecordSample.RecordPerson2 original)
 {
  this.Name = original.<Name>k__BackingField;
  this.Age = original.<Age>k__BackingField;
 }

 // Token: 0x06000045 RID: 69 RVA: 0x000029DD File Offset: 0x00000BDD
 public void Deconstruct(out string Name, out int Age)
 {
  Name = this.Name;
  Age = this.Age;
 }

 // Token: 0x0400000E RID: 14
 [CompilerGenerated]
 [DebuggerBrowsable(DebuggerBrowsableState.Never)]
 private readonly string <Name>k__BackingField;

 // Token: 0x0400000F RID: 15
 [CompilerGenerated]
 [DebuggerBrowsable(DebuggerBrowsableState.Never)]
 private readonly int <Age>k__BackingField;
}

RecordPerson2 相比 RecordPerson 的区别在于构造器不同：

看上面反编译的结果，可以看出：

RecordPeron2 和 RecordPerson 都声明了两个属性，都是 public string Name { get; init; }/public int Age { get; init; }
RecordPerson 的构造方法是无参构造方法，而 RecordPerson2 的构造方法是 RecordPerson2(string Name, int Age)
多出来一个 Deconstruct 方法，使得我们可以比较方便的解析一个对象的值，可以参考下面这个示例

foreach (var (name, age) in new[] { p4 })
{
 Console.WriteLine($"{name}={age}");
}

再来看一下测试方法的反编译结果：

RecordSample.RecordPerson p = new RecordSample.RecordPerson
{
 Name = "Tom",
 Age = 12
};
Console.WriteLine(p);
RecordSample.RecordPerson recordPerson = p.<Clone>$();
recordPerson.Age = 10;
RecordSample.RecordPerson p2 = recordPerson;
Console.WriteLine(p2);
RecordSample.RecordPerson p3 = new RecordSample.RecordPerson
{
 Name = "Tom",
 Age = 12
};
Console.WriteLine(p3);
Console.WriteLine(string.Format("p1 Equals p3 =:{0}", p == p3));
RecordSample.RecordPerson2 p4 = new RecordSample.RecordPerson2("Tom", 12);
Console.WriteLine(p4);

这里主要可以看到 with 表达式的实现，其实就是调用 <Clone>$ 方法复制了一个对象，并修改指定的属性值

record 实现了基于值的相等性比较，并且实现了原型模式，可以比较方便的创建一个新的值完全相等的对象，这对于有一些业务场景来说是非常适合使用 record 来代替原来的实现的

Reference

https://docs.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-9
https://github.com/WeihanLi/SamplesInPractice/tree/master/CSharp9Sample
https://github.com/WeihanLi/SamplesInPractice/blob/master/CSharp9Sample/RecordSample.cs

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