Downcast, `upcast`

With upcast and downcast, we traverse the hierarchy of types. An int is a subtype of obj. So we can move up and down between int and obj.

In this language, we do not have the same syntax for casting that C# has. Instead, we have keywords like upcast and downcast for the same purpose.

An example

Let us begin with a simple example that uses upcast and downcast. We create a variable with value 5. Then we cast it.

Start We use upcast on the int to move upwards in the type hierarchy to obj (which is the root object type).

Next We move back down from obj to int with the downcast operator. It works from less derived, to more derived.

let value = 5
printfn "%A" value

// With upcast we move up the type hierarchy.
// ... And int inherits from obj to this cast is valid.
let valueObj = upcast value : obj
printfn "%A" valueObj

// With downcast we move down the type hierarchy.
// ... We move down from obj to its derived type int.
let valueInt = downcast valueObj : int
printfn "%A" valueInt5
5
5

Type match

It is wonderful to be able to cast an object. But how do we know what type an obj is? We can use a match construct with a special type-testing operator.

Here Think of text() as a function that receives and object and returns a special string message if that object is of string type.

let value = "carrot"
// Cast the string to an obj.
let valueObj = upcast value : obj

// Match the type of the obj.
// ... See if it is a string or not.
let text (v : obj) =
    match v with
    | :? string -> "Is a string"
    | _ -> "Not a string"

// Call text function with object.
printfn "%A" (text valueObj)
"Is a string"

An error

Be careful with the syntax when casting in F#. If you omit a type or omit the ":" part of a downcast or upcast statement, a scary problem will appear.

However To fix the problem, be careful to review the syntax of your casting expressions to match a compiling example.

Error FS0013: The static coercion from type 'a to 'b
involves an indeterminate type based on information
prior to this program point.
Static coercions are not allowed on some types.
Further type annotations are needed.

Types, and casting types to other types, is a complex process. With upcast and downcast and type pattern-matching operators, we traverse the type tree.

Downcast, upcast

An example

Type match

An error

Downcast, `upcast`