这是传递给带有in关键字的方法的只读结构的防御性副本

Question

我正在尝试将a传递readonly struct给带有in修饰符的方法。当我查看生成的IL代码时，似乎已经构造了只读结构的防御性副本。

在readonly struct被定义为

public readonly struct ReadonlyPoint3D
{
    public ReadonlyPoint3D(double x, double y, double z)
    {
        this.X = x;
        this.Y = y;
        this.Z = z;
    }

    public double X { get; }
    public double Y { get; }
    public double Z { get; }
}

接受的方法 ReadonlyPoint3D

private static double CalculateDistance(in ReadonlyPoint3D point1, in ReadonlyPoint3D point2)
{
    double xDifference = point1.X - point2.X;
    double yDifference = point1.Y - point2.Y;
    double zDifference = point1.Z - point2.Z;

    return Math.Sqrt(xDifference * xDifference + yDifference * yDifference + zDifference * zDifference);
}

以及我调用此方法的方式：

static void Main(string[] args)
{
    var point1 = new ReadonlyPoint3D(0, 0, 0);
    var point2 = new ReadonlyPoint3D(1, 1, 1);

    var distance = CalculateDistance(in point1, in point2);
}

如果查看为CalculateDistance方法调用生成的IL ，我会看到ReadonlyPoint3D实例是通过引用传递的：

IL_0045: ldloca.s     point1
IL_0047: ldloca.s     point2
IL_0049: call         float64 CSharpTests.Program::CalculateDistance(valuetype CSharpTests.ReadonlyPoint3D&, valuetype CSharpTests.ReadonlyPoint3D&)
IL_004e: stloc.2      // distance

但是，CalculateDistance方法的IL似乎会复制point1＆point2参数：

// [25 9 - 25 10]
IL_0000: nop

// [26 13 - 26 54]
IL_0001: ldarg.0      // point1
IL_0002: call         instance float64 CSharpTests.ReadonlyPoint3D::get_X()
IL_0007: ldarg.1      // point2
IL_0008: call         instance float64 CSharpTests.ReadonlyPoint3D::get_X()
IL_000d: sub
IL_000e: stloc.0      // xDifference

// the resit is omitted for the sake of brevity, essentially same code repeated for Y & Z

ldarg.0＆ldarg.1in CalculateDistance方法生成的IL使我认为是point1＆的副本point2。我期望在这里看到的是ldloca.s我认为要加载point1＆地址的指令point2。

我是否正确理解并制作了防御性副本？还是我对IL代码的解释是错误的？

我在C＃7.3中使用.NET Core 2.1

---

编辑

根据Microsoft文档，使用in修饰符传递的可变结构将创建防御性副本。

如果我定义了可变结构

public struct MutablePoint3D
{
    public MutablePoint3D(double x, double y, double z)
    {
        this.X = x;
        this.Y = y;
        this.Z = z;
    }

    public double X { get; set; }
    public double Y { get; set; }
    public double Z { get; set; }
}

并通过 in

private static double CalculateDistance(in MutablePoint3D point1, in MutablePoint3D point2)
{
    double xDifference = point1.X - point2.X;
    double yDifference = point1.Y - point2.Y;
    double zDifference = point1.Z - point2.Z;

    return Math.Sqrt(xDifference * xDifference + yDifference * yDifference + zDifference * zDifference);
}

我可以看到生成的IL代码类似于生成的代码readonly struct：

// [26 13 - 26 54]
IL_0001: ldarg.0      // point1
IL_0002: call         instance float64 CSharpTests.MutablePoint3D::get_X()
IL_0007: ldarg.1      // point2
IL_0008: call         instance float64 CSharpTests.MutablePoint3D::get_X()
IL_000d: sub
IL_000e: stloc.0      // xDifference
// the resit is omitted for the sake of brevity

另一个观察结果是，如果我in从CalculateDisctance接受方法中删除修饰符ReadonlyPoint3D，则生成的IL代码正是我所期望的

// [35 13 - 35 54]
IL_0001: ldarga.s     point1
IL_0003: call         instance float64 CSharpTests.ReadonlyPoint3D::get_X()
IL_0008: ldarga.s     point2
IL_000a: call         instance float64 CSharpTests.ReadonlyPoint3D::get_X()
IL_000f: sub
IL_0010: stloc.0      // xDifference

但这似乎与Microsoft Docs中的建议不符

---

编辑2

正如@PetSerAl在评论中所建议的，sharplab.io为该代码生成了不同的IL。差异- ldobj仅针对的指令CalculateDistance(in MutablePoint3D point1, in MutablePoint3D point2)将说明仅针对这种情况进行了防御性复制。

但是，问题中张贴的IL指令取自ReSharper的IL Viewer，并通过ILDASM.exe工具进行了验证（用于Release配置，如sharplab.io）。因此，我不确定这种差异来自何处以及哪个输出值得信任。

Answer 1

长篇讨论可以在相关的 GitHub 问题上找到。

本质上，这是一个已修复的Roslyn bug，最新版本的 VS 2019（16.2 及更高版本）已修复。