c# - 如何推断具有继承类型的类型参数？

Question

我有以下设置:

public abstract class super { }

public class sub : super { }

public static void Foo<T>(T element, Action<T> action)
    where T : new()
{ }

我想这样做:

Action<super> superAction = (s) => { };
Foo(new sub(), superAction);

但是这失败了，因为第二行试图调用 Foo<super>而不是 Foo<sub> .这会起作用:

Foo<sub>(new sub(), superAction);

1. 在这种情况下有没有办法推断类型参数？
2. 为什么它没有首先推断出正确的类型？

编辑:
问题归结为一个事实，这是可能的:

Action<sub> subAction = superAction;

但是编译器不使用这个事实作为推理逻辑。
所以问题1的答案是:

Foo(new sub(), superAction as Action<sub>);

问题 2，为什么编译器不自己执行此操作，但仍未解决。

编辑2:
问题 2 的简短答案是:
根据给定的参数，编译器确定此方法调用可能的泛型类型。如果有多个选项，它会选择“最高”(或派生最少)的一个。它出于或多或少的任意原因这样做。
此处不考虑通用类型约束(where 之后的约束，如 T : new())。

Answer 1

Why does it not infer the correct type in the first place?

编译器会推断出正确的类型。给定方法的正确类型是 super .为什么？因为这就是类型推断算法的工作原理。

让我们看看。

给定以下方法签名:

public static void Foo<T>(T element, Action<T> action) where T : new()

算法从第一阶段开始，归结为:

• Otherwise, if Ei has a type U (Ei in our example is T element) and xi (xi is the method argument Foo<T> is a value parameter then a lower-bound inference is made from U to Ti.

所以现在我们来看下限推理:

• If V is one of the unfixed Xi then U is added to the set of exact bounds for Xi.

这发生在第一个类型参数 T 上, 以及 Action<T> , 以下适用:

• Otherwise, if V is C then inference depends on the i-th type parameter of C:
• If it is covariant then a lower-bound inference is made.
• If it is contravariant then an upper-bound inference is made.
• If it is invariant then an exact inference is made.

现在，第二阶段开始:

• All unfixed type variables Xi which do not depend on (§7.5.2.5) any Xj are fixed (§7.5.2.10).

现在让我们看看固定意味着什么:

An unfixed type variable Xi with a set of bounds is fixed as follows:

• The set of candidate types Uj starts out as the set of all types in the set of bounds for Xi.

• We then examine each bound for Xi in turn: For each exact bound U of Xi all types Uj which are not identical to U are removed from the candidate set. For each lower bound U of Xi all types Uj to which there is not an implicit conversion from U are removed from the candidate set. For each upper bound U of Xi all types Uj from which there is not an implicit conversion to U are removed from the candidate set.

• If among the remaining candidate types Uj there is a unique type V from which there is an implicit conversion to all the other candidate types, then Xi is fixed to V.

• Otherwise, type inference fails.

这里发生的是我们基本上同时拥有super和 sub在类型 T 的有效候选集中.现在，当边界集都产生“最佳匹配”时，类型推断算法会选择“较大”的类型。 Eric Lippert talks about this in a blog post :

A "bound" is nothing more than a type, and a bound can be "upper", "lower" or "exact". For example, suppose we have a type parameter T with three bounds: a lower bound of Giraffe, an exact bound of Mammal, and an upper bound of Animal. Let's say that Animal is a "larger" type than Mammal (because all Mammals are Animals but not all Animals are Mammals, thus Animal must be the larger type), and Giraffe is a "smaller" type than Mammal. Given this set of bounds we know that T must be inferred to be first, either Giraffe or a type larger than Giraffe, because Giraffe is a lower bound; you can't infer a type smaller than Giraffe. Second, we know that T must be Mammal, exactly. And third, we know that T must be either Animal or a type smaller than Animal, because Animal is an upper bound. We cannot infer a type larger than Animal. The C# compiler deduces that Mammal is the only type in the set that meets all three requirements, and so T would be fixed to Mammal. If there are multiple types in the set that meet all the requirements (which of course cannot happen if there are any exact bounds!) then we pick the largest such type. (*)

Eric 还解释了为什么选择“更大”的类型:

There is an argument to be made for picking the smallest, but picking the largest seems to match more people's intuitions of what the right choice is.