Question

我正在尝试使用位置参数在F＃中创建一个属性，但始终失败。

type ColumnAttribute(?index:int,?name:string) =
    inherit Attribute()
    let mutable index = index
    let mutable name = name
    member x.Index 
        with get() = index
        and set value = index <- value
        member x.Name 
        with get() = name
        and set value = name <- value

type Substance = {
    [<Column(Index=1)>] Name : string
    [<Column(Index=0)>] Id : int
    [<Column(Name="sequence")>] Sequence : string
}

我尝试了几种不同的方法，但这是我最接近的方法。

Answer 1

我在您的代码中看到了两个问题

为了使其正常工作，您需要为ColumnAttribute定义无参数构造函数。
Index和Name的类型分别为int option和string option，但您需要int和string。

尝试以下

type ColumnAttribute(index:int option,name:string option) =     
    inherit Attribute()     
    let mutable index = index
    let mutable name = name
    new () = ColumnAttribute (None, None)
    member x.Index          
        with get() = match index with | Some i -> i | None -> 0
        and set value = index <- Some value         
    member x.Name          
        with get() = match name with | Some n -> n | None -> ""
        and set value = name <- Some value  

type Substance = {     
    [<Column(Index=1)>] Name : string     
    [<Column(Index=0)>] Id : int     
    [<Column(Name="sequence")>] Sequence : string }

Answer 2

这有效：

type ColumnAttribute() =
    inherit System.Attribute()
    let mutable index = 0
    let mutable name = ""
    member x.Index 
        with get() = index
        and set value = index <- value
    member x.Name 
        with get() = name
        and set value = name <- value

type Substance = {
    [<Column(Index=1)>] Name : string
    [<Column(Index=0)>] Id : int
    [<Column(Name="sequence")>] Sequence : string
}

属性已经支持类似于命名参数的属性设置器语法。为了得到你想要的东西，使用无参数构造函数并依赖于属性语法，而不是命名参数（无论如何它都不适用于属性，因为它们被推断为option ... ... t有资格作为常数表达式）。

Answer 3

你走了：

open System

type ColumnAttribute() =
    inherit Attribute()
    let mutable index = 0
    let mutable name = ""
    member x.Index         
        with get() = index        
        and set value = index <- value        
    member x.Name         
        with get() = name        
        and set value = name <- value

type Substance = {    
    [<Column(Index=1)>] Name : string    
    [<Column(Index=0)>] Id : int    
    [<Column(Name="sequence")>] Sequence : string
}

请注意，原始尝试中的可选参数（例如?index）导致类型为例如int option而不是int。

属性中的F＃位置参数

3 个答案: