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Upcasting a collection
f#

Was wondering if there is an easy way to upcast all of the elements in an array.

Basically, I have a function that expects an array of type A, and have an array filled with many objects which are subtypes of A. I can write:

[| (B :> A); (C :> A); (D :> A); ...etc |]

but would rather cast them all in one go if possible. Any suggestions? Or maybe I'm going at it the wrong way.

The context is I'm rewriting a C# windows forms app in F# for kicks, and dealing with UI designer code. Doing it to get comfortable banging out F# code, since before now I've mostly just read up on it.

Thanks

.

Don't shoot me, I'm new to this as well, but would this be what you want?

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open System.Windows.Forms


let objControls = [|box (new Button()); box (new ListBox()); box (new Label())|]


let controls = objControls |> Array.map (fun oc -> oc :?> Control)


let form = new Form()
form.Controls.AddRange(controls)

Where objControls is your array of objects-that-are-actually-controls.

By on 4/23/2009 1:49 AM ()

I believe the best you can do is use the upcast keyword:

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let arr : Controls array = [| upcast b; upcast c; upcast d; |]
By on 4/22/2009 5:23 PM ()

Seq.cast works fine for me:

> [""] |> Seq.cast<System.Object>;;
val it : seq<System.Object> = seq [""]

or:

> let tmp : System.Object seq = [""] |> Seq.cast;;

val tmp : seq<System.Object>

As I understand it, Seq.cast does a downcast, but it seems to cover upcasts as well. You might get null pointer exceptions at run time in the case where the types aren't correctly related, though.

I wonder, would it be hard to make a type-safe Seq.upcast?

By on 4/23/2009 1:06 PM ()

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