c# - 扩展方法的继承

Question

继承如何与 C# 中的扩展方法一起工作。

假设您有一个接口(interface) IA、IB : IA 和 IC，以及一个类 Foo : IB, IC，现在一个定义扩展方法:

public static class Extensions {

    public static void Bar (this IA instance) {
        //Some code
    }

    public static void Bar (this IB instance) {
        //Some code
    }

    public static void Bar (this IC instance) {
        //Some code
    }

    public static void Bar (this Foo instance) {
        //Some code
    }

}

编译器如何确定 Foo.Bar() 的行为？根据经验测试，编译器总是选择最具体的实例(如普通调用)并且不使用动态绑定(bind)(因为 this 注释)是更“语法”的糖我想……

如果两个或多个类从继承层次结构中的不同分支定义一个方法，则调用是不明确的。在这种情况下，有没有一种方法可以定义一种方法相对于另一种方法的优先级？

以上说法正确吗？

Answer 1

继承确实很重要，但更重要的实际上是命名空间，来自 C# 规范:

if the normal processing of the invocation finds no applicable methods, an attempt is made to process the construct as an extension method invocation.

可以在C# spec, 7.6.5.2 Extension method invocations 中找到确切的过程和示例。解析过程(其中C是可能的类型解析集，M是扩展方法集):

The search for C proceeds as follows:
• Starting with the closest enclosing namespace declaration, continuing with each enclosing namespace declaration, and ending with the containing compilation unit, successive attempts are made to find a candidate set of extension methods:

o If the given namespace or compilation unit directly contains non-generic type declarations Ci with eligible extension methods Mj, then the set of those extension methods is the candidate set.

o If namespaces imported by using namespace directives in the given namespace or compilation unit directly contain non-generic type declarations Ci with eligible extension methods Mj, then the set of those extension methods is the candidate set.

• If no candidate set is found in any enclosing namespace declaration or compilation unit, a compile-time error occurs.

• Otherwise, overload resolution is applied to the candidate set as described in (§7.5.3). If no single best method is found, a compile-time error occurs.

• C is the type within which the best method is declared as an extension method. Using C as a target, the method call is then processed as a static method invocation (§7.5.4).

如果我理解正确的话，这意味着它会在编译时选择最近命名空间中的最佳扩展方法。理解没有进行动态调用非常重要，解析是静态完成的(即在编译时)。