为什么c#是这样设计的?
根据我的理解,一个接口只描述行为,并且服务于描述实现接口的类的契约义务。
如果类希望在共享方法中实现这种行为,为什么不应该呢?
以下是我想到的一个例子:
// These items will be displayed in a list on the screen.
public interface IListItem {
string ScreenName();
...
}
public class Animal: IListItem {
// All animals will be called "Animal".
public static string ScreenName() {
return "Animal";
}
....
}
public class Person: IListItem {
private string name;
// All persons will be called by their individual names.
public string ScreenName() {
return name;
}
....
}
事实上,确实如此。
截至2022年年中,当前版本的c#完全支持所谓的静态抽象成员:
interface INumber<T>
{
static abstract T Zero { get; }
}
struct Fraction : INumber<Fraction>
{
public static Fraction Zero { get; } = new Fraction();
public long Numerator;
public ulong Denominator;
....
}
请注意,根据你的Visual Studio版本和你安装的。net SDK,你必须至少更新其中一个(或两个),或者你必须启用预览功能(参见在Visual Studio中使用预览功能和预览语言)。
看到更多:
https://learn.microsoft.com/en-us/dotnet/csharp/whats-new/tutorials/static-virtual-interface-members
https://blog.ndepend.com/c-11-static-abstract-members/
https://khalidabuhakmeh.com/static-abstract-members-in-csharp-10-interfaces: ~:文本=静态% 20文摘% 20成员% 20允许% 20,像% 20 % 20其他% 20接口% 20的定义。
Regarding static methods used in non-generic contexts I agree that it doesn't make much sense to allow them in interfaces, since you wouldn't be able to call them if you had a reference to the interface anyway. However there is a fundamental hole in the language design created by using interfaces NOT in a polymorphic context, but in a generic one. In this case the interface is not an interface at all but rather a constraint. Because C# has no concept of a constraint outside of an interface it is missing substantial functionality. Case in point:
T SumElements<T>(T initVal, T[] values)
{
foreach (var v in values)
{
initVal += v;
}
}
这里没有多态性,泛型使用对象的实际类型并调用+=操作符,但这失败了,因为它不能确定该操作符是否存在。简单的解决方案是在约束中指定它;简单的解决方案是不可能的,因为操作符是静态的,静态方法不能在接口中,(这就是问题所在)约束被表示为接口。
c#需要的是一个真正的约束类型,所有的接口都是约束,但不是所有的约束都是接口,然后你可以这样做:
constraint CHasPlusEquals
{
static CHasPlusEquals operator + (CHasPlusEquals a, CHasPlusEquals b);
}
T SumElements<T>(T initVal, T[] values) where T : CHasPlusEquals
{
foreach (var v in values)
{
initVal += v;
}
}
已经有很多关于为所有数字类型创建i算术来实现的讨论,但是存在效率问题,因为约束不是多态构造,所以创建算术约束可以解决这个问题。