How to type cast in F#?

Question

I have to enumerate through the members of a collection and create an array with a particular property of the members:

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

Here app.Operations is a collection of IAzOperation but on enumeration, returns each member as Obj. So I want to type cast each member and access the property. but not sure how to typecast it. If I typecast the way I've mentioned here, it gives me error that:

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

What am I missing here?

Answer 1

You need the downcasting operator :?>:

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

As the symbol ? in its name denotes, downcasting could fail and result in a runtime exception.

In case of sequences, you have another option to use Seq.cast:

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

Answer 2

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