C# and F# casting - specifically the 'as' keyword

Question

In C# I can do:

var castValue = inputValue as Type1 

In F#, I can do:

let staticValue = inputValue :> Type1 let dynamicValue = inputValue :?> Type1 

But neither of them is the equivalent of the C#'s keyword as.

I guess I need to do a match expression for the equivalent in F#

match inputValue with | :? Type1 as type1Value -> type1Value | _ -> null 

Is this correct?

Answer 1

As far as I know, F# doesn't have any built-in operator equivalent to C# as so you need to write some more complicated expression. Alternatively to your code using match, you could also use if, because the operator :? can be use in the same way as is in C#:

let res = if (inputValue :? Type1) then inputValue :?> Type1 else null 

You can of course write a function to encapsulate this behavior (by writing a simple generic function that takes an Object and casts it to the specified generic type parameter):

let castAs<'T when 'T : null> (o:obj) =    match o with   | :? 'T as res -> res   | _ -> null 

This implementation returns null, so it requires that the type parameter has null as a proper value (alternatively, you could use Unchecked.defaultof<'T>, which is equivalent to default(T) in C#). Now you can write just:

let res = castAs<Type1>(inputValue) 

Answer 2

I would use an active pattern. Here is the one I use:

let (|As|_|) (p:'T) : 'U option =     let p = p :> obj     if p :? 'U then Some (p :?> 'U) else None 

Here is a sample usage of As:

let handleType x =      match x with     | As (x:int) -> printfn "is integer: %d" x     | As (s:string) -> printfn "is string: %s" s     | _ -> printfn "Is neither integer nor string"  // test 'handleType' handleType 1 handleType "tahir" handleType 2. let stringAsObj = "tahir" :> obj handleType stringAsObj