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

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