Covariance and Contravariance in C#, Part Ten: Dealing With Ambiguity

weixin_30519071

于 2009-03-05 15:13:00 发布

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原文链接：http://www.cnblogs.com/i2001she1u/archive/2009/03/05/1403901.html

本文通过一个具体的代码示例探讨了泛型协变在C#中的应用，特别是当一个类型同时实现了多个泛型接口时，如何处理不同类型的枚举器返回问题。作者提出了几种可能的行为选项，并邀请读者讨论最佳解决方案。

OK, I wasn’t quite done. One more variance post!

Smart people: suppose we made IEnumerable<T> covariant in T. What should this program fragment do?

class C : IEnumerable<Giraffe>, IEnumerable<Turtle> {
    IEnumerator<Giraffe> IEnumerable<Giraffe>.GetEnumerator() {
        yield return new Giraffe();
    }
    IEnumerator<Turtle> IEnumerable<Turtle>.GetEnumerator() {
        yield return new Turtle();
    }
// [etc.]
}

class Program {
    static void Main() {
        IEnumerable<Animal> animals = new C();
        Console.WriteLine(animals.First().GetType().ToString());
    }
}

Options:

1) Compile-time error.
2) Run-time error.
3) Always enumerate Giraffes.
4) Always enumerate Turtles.
5) Choose Giraffes vs Turtles at runtime.
6) Other, please specify.

And if you like any options other than (1), should this be a compile-time warning?

转载于:https://www.cnblogs.com/i2001she1u/archive/2009/03/05/1403901.html