c# - 是否可以在不强制转换的情况下在 C# 中实现 "virtual constructor"模式？

Question

我正在编写一个由 C# 编写的程序，该程序最终会被编译成一个应用程序。我希望每个生成的类型都提供一个“深度克隆”功能来复制整个数据树。也就是说，我希望有人能够做到:

var x = new Base(); // Base has public virtual Base DeepClone() { ... }
var y = new Derived(); // Derived overrides DeepClone
Base a = x.DeepClone();
Base b = y.DeepClone();
// Derived c = x.DeepClone(); // Should not compile
Derived d = y.DeepClone(); // Does not compile, DeepClone returns Base

代替

var x = new Base();
var y = new Derived();
Base a = x.DeepClone();
Base b = y.DeepClone();
// Derived c = x.DeepClone(); // Should not compile
Derived d = (Derived)y.DeepClone();

但是，C# 不允许您在简单的覆盖中执行此操作；覆盖必须返回与基上声明的类型相同的类型。

既然我编写的代码无论如何都会消除样板，那么我可以生成一些东西来允许第一个 block 编译吗？我尝试了类似于以下内容的操作:

abstract class Base
{
    public abstract Base DeepClone();
}

class Base2 : Base
{
    int Member { get; set; }

    public Base2() { /* empty on purpose */ }
    public Base2(Base2 other)
    {
        this.Member = other.Member;
    }

    public override Base2 DeepClone()
    {
        return new Base2(this);
    }
}

sealed class Derived : Base2
{
    string Member2 { get; set; }

    public Derived() { /* empty on purpose */ }
    public Derived(Derived other)
        : base(other)
    {
        this.Member2 = other.Member2;
    }

    public override Derived DeepClone()
    {
        return new Derived(this);
    }
}

但这不会编译，因为覆盖不匹配。我还尝试从基础覆盖该方法并使用“new”关键字将其隐藏，但这也不起作用。

Answer 1

是的，它是可行的，但是您必须将抽象方法从公共(public)方法转移到 protected 方法，然后创建一个只调用 protected 方法的公共(public)非抽象函数。派生类只需要实现 protected 函数并可以隐藏公共(public)函数，执行本应由客户端执行的转换。

abstract class Base
{
    public Base DeepClone()
    {
        return CloneInternal();
    }

    protected abstract Base CloneInternal();
}

class Base2 : Base
{
    int Member { get; set; }

    public Base2() { /* empty on purpose */ }
    public Base2(Base2 other)
    {
        this.Member = other.Member;
    }

    new public Base2 DeepClone()
    {
        return (Base2)CloneInternal();
    }

    protected override Base CloneInternal()
    {
        return new Base2(this);
    }
}

sealed class Derived : Base2
{
    string Member2 { get; set; }

    public Derived() { /* empty on purpose */ }
    public Derived(Derived other)
        : base(other)
    {
        this.Member2 = other.Member2;
    }

    new public Derived DeepClone()
    {
        return (Derived)CloneInternal();
    }

    protected override Base CloneInternal()
    {
        return new Derived(this);
    }
}