为什么不可变对象是线程安全的?

class Unit {
    private readonly string name;
    private readonly double scale;

    public Unit(string name, double scale) {
        this.name = name;
        this.scale = scale,
    }

    public string Name { get { return name; } }
    public string Scale { get { return scale; } }

    private static Unit gram = new Unit("Gram", 1.0);

    public Unit Gram { get { return gram; } }
}
class Unit {
    private readonly string name;