为什么F＃在C＃中不允许多个属性？(Why does F# not allow multiple attributes where C# does?)

为什么F＃在C＃中不允许多个属性？(Why does F# not allow multiple attributes where C# does?)

以下代码在C＃中编译：

[ContentType("text")] [ContentType("projection")] public class Class1 { }

F＃中的以下代码不能编译：

[<ContentType("text")>] [<ContentType("projection")>] type Class1() = class end

F＃中的编译错误是： “属性类型'ContentTypeAttribute'具有'AllowMultiple = false'。该属性的多个实例不能附加到单个语言元素。”

通过反编译ContentType，我可以看到ContentType继承自AttributeUsage中'AllowMultiple = true'的 MultipleBaseMetadataAttribute。

事实上，似乎F＃不会继承父类的AttributeUsage。

[<AttributeUsage(AttributeTargets.Class, AllowMultiple = true)>] type FooAttribute() = inherit Attribute() type BarAttribute() = inherit FooAttribute() [<Foo>] [<Foo>] type MyClassCompiles() = class end

哪里

[<Bar>] [<Bar>] type MyClassDoesNotCompile() = class end

The following code compiles in C#:

[ContentType("text")] [ContentType("projection")] public class Class1 { }

The following code in F# does not compile:

[<ContentType("text")>] [<ContentType("projection")>] type Class1() = class end

The compile error in F# is: "The attribute type 'ContentTypeAttribute' has 'AllowMultiple=false'. Multiple instances of this attribute cannot be attached to a single language element."

By decompiling ContentType, I can see that ContentType inherits from MultipleBaseMetadataAttribute which has 'AllowMultiple=true' in the AttributeUsage.

In fact, it seems like F# does not inherit the AttributeUsage from the parent class.

[<AttributeUsage(AttributeTargets.Class, AllowMultiple = true)>] type FooAttribute() = inherit Attribute() type BarAttribute() = inherit FooAttribute() [<Foo>] [<Foo>] type MyClassCompiles() = class end

where

[<Bar>] [<Bar>] type MyClassDoesNotCompile() = class end

最满意答案

看起来像一个错误。 电子邮件fsbugs [at] microsoft.com。 这是另一个显而易见的错误：它看起来并不尊重AttributeTargets ：

[<AttributeUsage(AttributeTargets.Enum)>]
type FooAttribute() = 
  inherit Attribute()

[<Foo>]
type T = struct end //happily compiles
Looks like a bug. Email fsbugs[at]microsoft.com. Here's another apparent bug: it doesn't appear to honor AttributeTargets:
 
[<AttributeUsage(AttributeTargets.Enum)>]
type FooAttribute() = 
  inherit Attribute()

[<Foo>]
type T = struct end //happily compiles