如何获取CRTP类型的通用类型定义

问题描述

在C#中提供以下 CRTP 类型:

Given the following CRTP type in C#:

public abstract class DataProviderBase<TProvider>
    where TProvider : DataProviderBase<TProvider> { }

如何在F#中获取其通用类型定义?

How would I get its generic type definition in F#?

let typeDef = typedefof<DataProviderBase<_>>

产生错误:

在C#中，它将是:

var typeDef = typeof(DataProviderBase<>);

更新

我找到了一种解决方法:

UPDATE

I found a workaround:

[<AbstractClass>]
type DummyProvider() =
  inherit DataProviderBase<DummyProvider>()

let typeDef = typeof<DummyProvider>.BaseType.GetGenericTypeDefinition()

还有没有其他类型的方法吗?

Is there another way to do it, without the extra type?

推荐答案

我认为这实际上是一个很好的问题.我没有找到更好的解决方法.您可以像这样使用typedefof来稍微简化您的解决方法:

I think this is actually a very good question. I didn't find a better workaround for this.You can slightly simplify your workaround by using typedefof like this:

let typeDef = typedefof<DataProviderBase<DummyProvider>>

技术细节

问题在于F#的typedefof<'T>只是一个接受类型参数的普通函数(与C#中的typeof不同，后者是一个运算符).为了调用它，您需要给它一个实际的类型，然后该函数将在幕后调用GetGenericTypeDefinition.

The problem is that F#'s typedefof<'T> is just an ordinary function that takes a type argument (unlike typeof in C#, which is an operator). In order to call it, you need to give it an actual type and the function will then call GetGenericTypeDefinition under the cover.

typedefof<option<_>>起作用的原因是F#指定了 default 类型作为参数(在本例中为obj).通常，F#选择与约束匹配的不太具体的类型.对于您的情况:

The reason why typedefof<option<_>> works is that F# specifies a default type as an argument (in this case obj). In general, F# chooses the less concrete type that matches the constraints. In your case:

DataProviderBase<_>将变为DataProviderBase<DataProviderBase<_>>，依此类推.

除非您定义新类型(如在解决方法中那样)，否则没有任何具体类型可用作typedefof<...>的类型参数.在这种情况下，默认机制根本不起作用...

Unless you define a new type (as in your workaround), there is no concrete type that could be used as a type argument of typedefof<...>. In this case, the defaulting mechanism simply doesn't work...

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