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C# 9 新特性 — record 解读


C# 9 新特性 — record 解读
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


More

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

版权声明:本文为博主原创文章,遵循 CC 4.0 BY-SA 版权协议,转载请附上原文出处链接和本声明。
本文链接:https://www.cnblogs.com/weihanli/p/14227667.html

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