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 |]