C# 9 新特性 — record 解讀

Intro

C# 9 中引入了 record，record 是一個特殊類，用它來實現 model 在有些情況下會非常的好用

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);
}

這里的示例，用 record 聲明了兩個 model，第二個 model 聲明的時候使用了簡化的寫法，

record RecordPerson2(string Name, int Age); 這樣的聲明意味着，構造方法有兩個參數，分別是 string Name 和 int Age，並對應着兩個屬性，屬性的聲明方式和 RecordPerson 一樣 public string Name { get; init; } 都是一個 get 一個 init

對於 record 支持一個 with 表達式，來修改某幾個屬性的值，這對於有很多屬性都相同的場景來說是及其方便的，來看一下上面示例的輸出結果

What inside

那么 record 內部發生了什么呢，我們來反編譯看一下，我們看一下使用 DnSpy 反編譯的結果

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;
}

從上面的反編譯結果可以看的出來，record 其實就是一個 class，只是編譯器會幫我們做一些事情，編譯器幫我們做了哪些事呢？

• 實現了基於屬性值的相等性比較，不再使用默認的引用，並且重寫了 ==/!= operator 和 GetHashCode
• 為了方便調試，重寫了 ToString 方法，也提供了 PrintMembers 方法來實現比較方便只顯示某些比較重要的參數
• 實現了 EqualityContract 方法來指定類型比較的類型，默認是當前類型
• 實現了 <Clone>$ 方法和一個特殊的構造方法，用來克隆一個對象，相當於 record 幫我們實現了一個淺復制 的 原型模式，還是強類型的，這個方法在代碼里不能直接調用，當我們使用 with 表達式的時候，編譯器會調用這個方法，並對某些屬性進行賦值

再來看一下 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 的區別在於構造器不同：

看上面反編譯的結果，可以看出：

• RecordPeron2 和 RecordPerson 都聲明了兩個屬性，都是 public string Name { get; init; }/public int Age { get; init; }
• RecordPerson 的構造方法是無參構造方法，而 RecordPerson2 的構造方法是 RecordPerson2(string Name, int Age)
• 多出來一個 Deconstruct 方法，使得我們可以比較方便的解析一個對象的值，可以參考下面這個示例

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

再來看一下測試方法的反編譯結果：

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);

這里主要可以看到 with 表達式的實現，其實就是調用 <Clone>$ 方法復制了一個對象，並修改指定的屬性值

More

record 實現了基於值的相等性比較，並且實現了 原型模式，可以比較方便的創建一個新的值完全相等的對象，這對於有一些業務場景來說是非常適合使用 record 來代替原來的實現的

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