c# – 为什么子类变量在父类成员变量之前初始化

发布时间：2020-12-16 01:59:15 所属栏目：百科来源：网络整理

导读：请考虑以下代码.字段i和j在m和n之前初始化.我们知道父对象是在子对象之前创建的,但是在我的程序中,编译器正在为基类之前的子类的成员变量分配和初始化内存.这是为什么？ class X{ private int m = 0; private int n = 90; public X() { }}class Y:X{ private

请考虑以下代码.字段i和j在m和n之前初始化.我们知道父对象是在子对象之前创建的,但是在我的程序中,编译器正在为基类之前的子类的成员变量分配和初始化内存.这是为什么？

class X
{
    private int m = 0;
    private int n = 90;
    public X() { }
}

class Y:X
{
    private int i = 8;
    private int j = 6;
    public Y()
    { }
    public static void Main(string []args)
    {
        Y y1 = new Y();  
    }
}

解决方法

这在 Eric Lippert’s blog中解释：

[…] an initialized readonly field is always observed in its initialized state,and we cannot make that guarantee unless we run all the initializers first,and then all of the constructor bodies.

不确定为什么readonly在这里提到,但是例如,这确保了以下场景,尽管是愚蠢的,工作：

class Base
{
    public Base()
    {
        if (this is Derived) (this as Derived).Go();
    }
}

class Derived : Base
{
    X x = new X();

    public void Go()
    {
        x.DoSomething(); // !
    }
}

class Base
{
    public Base()
    {
        Go();
    }

    public virtual Go() {}
}

class Derived : Base
{
    X x = new X();

    public override void Go()
    {
        x.DoSomething(); // !
    }
}

该命令在C# Language Specification(17.10.2)中明确说明：

[…] constructor implicitly performs the initializations specified by the variable-initializers of the instance fields declared in its class. This corresponds to a sequence of assignments that are executed immediately upon entry to the constructor and before the implicit invocation of the direct base class constructor.

（编辑：李大同）

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