子类特定的静态成员数据

Question

我正在尝试实现一个类族，这些类跟踪每个类中存在的实例数。因为所有这些类都有这种行为，所以我想把它拉到一个超类中，这样我就不必对每个类重复实现了。考虑以下代码：

class Base
{
    protected static int _instances=0;
    protected int _id;

    protected Base()
    {
        // I would really like to use the instances of this's class--not
        // specifically Base._instances
        this._id = Base._instances;
        Base._instances++;
    }
}

class Derived : Base
{
                                // Values below are desired,
                                // not actual:
    Derived d1 = new Derived(); // d1._id = 0
    Derived d2 = new Derived(); // d2._id = 1
    Derived d3 = new Derived(); // d3._id = 2

    public Derived() : base() { }
}

class OtherDerived : Base
{
                                            // Values below are desired,
                                            // not actual:
    OtherDerived od1 = new OtherDerived();  // od1._id = 0
    OtherDerived od2 = new OtherDerived();  // od2._id = 1
    OtherDerived od3 = new OtherDerived();  // od3._id = 2

    public OtherDerived() : base() { }
}

如何实现每个类的实例计数器（与基类的计数器分开的计数器）？我试过混合静态和抽象（不编译）。请指教。

Answer 1

不，你不能那样做。但是您可以拥有一个静态的Dictionary<Type, int>，并通过调用GetType 在执行时找出类型。

class Base
{
    private static readonly IDictionary<Type, int> instanceCounterMap
        = new Dictionary<Type, int>();
    protected int _id;

    protected Base()
    {
        // I don't normally like locking on other objects, but I trust
        // Dictionary not to lock on itself
        lock (instanceCounterMap)
        {
            // Ignore the return value - we'll get 0 if it's not already there
            instanceCounterMap.TryGetValue(GetType(), out _id);
            instanceCounterMap[GetType()] = _id + 1;    
        }
    }
}

Answer 2

我建议不要试图让一个类来计算它自己的实例，而是创建一个工厂类来管理实例的生命周期。而不是新建一个“OtherDerived”，而是使用 Build 或 Create 方法实现 OtherDerivedFactory 来为它们提供服务。在内部，它可以对实例进行计数。

哎呀，使用接口对其进行更多抽象，然后您可以在运行时注入工厂。非常适合测试、替代实现等。