如何保持init-only特性并将属性值传递给内部Wheel类？

我有一个Wheel类，包含两个init-only属性Spokes（辐条数）和DiskBrake（碟刹）：

public class Wheel
{
    internal Wheel() { }

    public int Spokes { get; init; }
    public bool DiskBrake { get; init; }
}

该类要求Wheel实例化后属性必须不可变，因此属性设为init-only类型，由readonly字段支持。

我还有一个Bicycle类，包含两个Wheel实例：

public class Bicycle
{
    private readonly Wheel _frontWheel = new();
    private readonly Wheel _rearWheel = new();

    public int FrontWheelSpokes
    {
        get => _frontWheel.Spokes;
        init => _frontWheel.Spokes = value;
    }

    public int RearWheelSpokes
    {
        get => _rearWheel.Spokes;
        init => _rearWheel.Spokes = value;
    }

    public bool FrontWheelDiskBrake
    {
        get => _frontWheel.DiskBrake;
        init => _frontWheel.DiskBrake = value;
    }

    public bool RearWheelDiskBrake
    {
        get => _rearWheel.DiskBrake;
        init => _rearWheel.DiskBrake = value;
    }
}

预期用法如下：

Bicycle bicycle = new()
{
    FrontWheelSpokes = 32,
    RearWheelSpokes = 24,
    FrontWheelDiskBrake = true,
    RearWheelDiskBrake = false
};

但这段代码无法编译，在init => _xxxWheel.Yyy = value;这几行出现四个编译错误：

Init-only property or indexer 'Wheel.Spokes' can only be assigned in an object initializer, or on 'this' or 'base' in an instance constructor or an 'init' accessor.

请问能否在不破坏Wheel和Bicycle两类所有属性的“init-only特性”的前提下，修复这些编译错误？

约束条件：

• 不暴露Wheel类构造函数给外部，仅Bicycle自身可实例化车轮。
• 不将init-only属性替换为构造函数参数。
• 不修改Bicycle的公共API结构。

核心思路是：不能直接修改已有Wheel实例的init-only属性，而是通过创建新的Wheel实例（在对象初始化器中设置属性），再将新实例赋值给readonly字段——因为readonly字段允许在init访问器中重新赋值。

修改后的Bicycle类代码如下：

public class Bicycle
{
    private readonly Wheel _frontWheel = new();
    private readonly Wheel _rearWheel = new();

    public int FrontWheelSpokes
    {
        get => _frontWheel.Spokes;
        init => _frontWheel = new Wheel { Spokes = value, DiskBrake = _frontWheel.DiskBrake };
    }

    public int RearWheelSpokes
    {
        get => _rearWheel.Spokes;
        init => _rearWheel = new Wheel { Spokes = value, DiskBrake = _rearWheel.DiskBrake };
    }

    public bool FrontWheelDiskBrake
    {
        get => _frontWheel.DiskBrake;
        init => _frontWheel = new Wheel { Spokes = _frontWheel.Spokes, DiskBrake = value };
    }

    public bool RearWheelDiskBrake
    {
        get => _rearWheel.DiskBrake;
        init => _rearWheel = new Wheel { Spokes = _rearWheel.Spokes, DiskBrake = value };
    }
}

方案说明 #

1. 完全满足所有约束：
   • Wheel的构造函数仍为internal，外部无法直接实例化；
   • 未将init-only属性替换为构造函数参数；
   • Bicycle的公共API结构完全保留，原有用法代码可正常编译运行。
2. 保持不可变性：Wheel实例一旦创建就无法修改，Bicycle的readonly字段确保内部的Wheel实例只能在初始化阶段替换，实例化后无法变更。

内容的提问来源于stack exchange，提问作者Theodor Zoulias