Csharp 避免只读记录 struct 中的防御副本

这里没有防御副本;只读记录被标记为[IsReadOnly]，get-only属性也是如此-这足以告诉编译器它们是aren't needed;你可以在IL中看到这一点，例如，如果我们这样做:

using System;

void NoCopies(in S obj)
{   // no copies because the *type* is [IsReadOnly]
    Console.WriteLine(obj.A);
    Console.WriteLine(obj.B);
}

void AlsoHasNoCopies(in T obj)
{   // no copies because the *members* are [IsReadOnly]
    Console.WriteLine(obj.A);
    Console.WriteLine(obj.B);
}

void StillNoCopies(in LazyManual obj)
{   // no copies because the *members* are [IsReadOnly]
    Console.WriteLine(obj.A);
    Console.WriteLine(obj.B);
}


void DoesHaveCopies(in BasicManual obj)
{   // finally, we have defensive copies
    Console.WriteLine(obj.A);
    Console.WriteLine(obj.B);
}


public readonly record struct S(int A, int B)
{
   
}

public record struct T(int A, int B) // not marked readonly
{
   
}

public struct LazyManual
{
    public int A {get;} // auto-marked [IsReadOnly]
    public int B {get;}
}

public struct BasicManual
{
    private int _a, _b;
    public int A => _a; // not auto-marked [IsReadOnly]
    public int B => _b;
}

编译/反编译这段代码，我们可以看到:

[CompilerGenerated]
internal class Program
{
    private static void <Main>$(string[] args)
    {
    }

    [CompilerGenerated]
    internal static void <<Main>$>g__NoCopies|0_0([In][IsReadOnly] ref S obj)
    {
        Console.WriteLine(obj.A);
        Console.WriteLine(obj.B);
    }

    [CompilerGenerated]
    internal static void <<Main>$>g__AlsoHasNoCopies|0_1([In][IsReadOnly] ref T obj)
    {
        Console.WriteLine(obj.A);
        Console.WriteLine(obj.B);
    }

    [CompilerGenerated]
    internal static void <<Main>$>g__StillNoCopies|0_2([In][IsReadOnly] ref LazyManual obj)
    {
        Console.WriteLine(obj.A);
        Console.WriteLine(obj.B);
    }

    [CompilerGenerated]
    internal static void <<Main>$>g__DoesHaveCopies|0_3([In][IsReadOnly] ref BasicManual obj)
    {
        BasicManual basicManual = obj;
        Console.WriteLine(basicManual.A);
        basicManual = obj;
        Console.WriteLine(basicManual.B);
    }
}

只有手动实现的类型的最后一个选项not readonly使用手动属性:具有防御性副本.

对于完整的背景，我们有:

[IsReadOnly]
public struct S : IEquatable<S>
{ ... }

public struct T : IEquatable<T>
{
    // ...
    public int A
    {
        [IsReadOnly]
        [CompilerGenerated]
        get
        {
            return <A>k__BackingField;
        }
        [CompilerGenerated]
        set
        {
            <A>k__BackingField = value;
        }
    }

    public int B
    {
        [IsReadOnly]
        [CompilerGenerated]
        get
        {
            return <B>k__BackingField;
        }
        [CompilerGenerated]
        set
        {
            <B>k__BackingField = value;
        }
    }
}

public struct LazyManual
{
    [CompilerGenerated]
    private readonly int <A>k__BackingField;

    [CompilerGenerated]
    private readonly int <B>k__BackingField;

    public int A
    {
        [IsReadOnly]
        [CompilerGenerated]
        get
        {
            return <A>k__BackingField;
        }
    }

    public int B
    {
        [IsReadOnly]
        [CompilerGenerated]
        get
        {
            return <B>k__BackingField;
        }
    }
}

public struct BasicManual
{
    private int _a;

    private int _b;

    public int A
    {
        get
        {
            return _a;
        }
    }

    public int B
    {
        get
        {
            return _b;
        }
    }
}