attributes - F# 属性中的位置参数

Question

我尝试在 F# 中使用位置参数创建属性，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，但这是我最终选择的最接近的方法。

Answer 1

我发现您的代码存在 2 个问题

1. 为了使其正常工作，您需要为 ColumnAttribute 定义一个无参构造函数。
2. Index 和 Name 的类型分别是 int option 和 string option，但您需要它是int 和 string。

尝试以下操作

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 }