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)