Types 如何在F#中键入cast？

我必须枚举集合的成员，并使用成员的特定属性创建一个数组：

  let ops: int array = [| for x in app.Operations ->
                            let op=  x : IAzOperation
                            op.OperationID |] 

这里的

app.Operations

是iazooperation的集合，但在枚举时，将每个成员作为

Obj

返回。所以我想输入cast每个成员并访问属性。但我不知道怎么打字。 如果我按照我在这里提到的方式进行打字，它会给我以下错误：

This espression was expected to have type IAzOPeration but here has type obj.

---

我在这里遗漏了什么？

您需要下行操作符

：？>

：

type Base1() =
    abstract member F : unit -> unit
    default u.F() =
     printfn "F Base1"

type Derived1() =
    inherit Base1()
    override u.F() =
      printfn "F Derived1"


let d1 : Derived1 = Derived1()

// Upcast to Base1.
let base1 = d1 :> Base1

// This might throw an exception, unless
// you are sure that base1 is really a Derived1 object, as
// is the case here.
let derived1 = base1 :?> Derived1

// If you cannot be sure that b1 is a Derived1 object,
// use a type test, as follows:
let downcastBase1 (b1 : Base1) =
   match b1 with
   | :? Derived1 as derived1 -> derived1.F()
   | _ -> ()

downcastBase1 base1

let ops: int array = [| for x in app.Operations do
                          let op =  x :?> IAzOperation
                          yield op.OperationID |] 

正如其名称中的符号

？

所示，向下转换可能会失败并导致运行时异常

对于序列，您可以使用另一个选项：

let ops:int数组=
[|对于应用程序中的操作|>Seq.cast->op.OperationID |]

为确保完整性，请选择具有模式匹配的版本[|用于：？iazooperation作为op in app.Operations->op.OperationID |]

let ops: int array = 
    [| for op in app.Operations |> Seq.cast<IAzOperation> -> op.OperationID |]