c# - 为什么当我将类转换为它未实现的接口(interface)时没有编译器错误？

Question

如果我尝试从一个类到一个接口(interface)的无效转换，那么编译器不会报错(错误发生在运行时)；但是，如果我尝试对抽象类进行类似的转换，它确实会提示。

class Program
{
    abstract class aBaz
    {
        public abstract int A { get; }
    }

    interface IBar
    {
        int B { get; }
    }

    class Foo
    {
        public int C { get; }
    }

    static void Main()
    {
        Foo foo = new Foo();

        // compiler error, as expected, since Foo doesn't inherit aBaz
        aBaz baz = (aBaz)foo;

        // no compiler error, even though Foo doesn't implement IBar
        IBar bar = (IBar)foo;
    }
}

为什么编译器不拒绝从 Foo 到 IBar 的转换，当它(看似？)无效时？或者，换个问题，如果编译器允许对接口(interface) IBar 进行这种“无效”转换，为什么不允许对抽象类 aBaz ?

Answer 1

您需要了解 .Net 的继承系统才能明白为什么这是有道理的。在.Net 中，一个类只能继承自一个基类，但可以实现任意数量的接口(interface)。

class Program
{
    abstract class aBaz
    {
        public abstract int A { get; }
    }

    interface IBar
    {
        int B { get; }
    }

    class Foo
    {
        public int C { get; }
    }

    class BarableFoo : Foo, IBar
    {
        public int C { get; }
    }

    static void Main()
    {
        // This is why the compiler doesn't error on the later cast
        Foo foo = new BarableFoo();

        // compiler error: aBaz is a class and the compiler knows that
        // Foo is not a _subclass_ of aBaz.
        aBaz baz = (aBaz)foo;

        // no compiler error: the class Foo does not implement IBar, however at runtime
        // this instance, "foo", might be a subclass of Foo that _implements_ IBar.
        // This is perfectly valid, and succeeds at runtime.
        IBar bar = (IBar)foo;

        // On the other hand...
        foo = new Foo();

        // This fails at runtime as expected. 
        bar = (IBar)foo;
    }

}

在问题中极其简单的原始示例中，编译器似乎可以检测到 foo 的这个实例永远不会转换为 IBar，但这更像是一个“nice to have”警告，而不是一个问题语言正确性。