F# Custom operators that return default value

Question

I've defined a custom operator in F# like this:

static member (&?) value defaultValue =
    if value = null then
        defaultValue
    else
        value

The operator is defined within a type and should be called in the following scenario:
I'm retrieving information about the system processors using WMI.
Now i want to get the VoltageCaps property, but since that might be null, i want to assign a default value in that case.

let voltage = (m.["VoltageCaps"] &? (0 :> obj))

Visual Studio tells me "The type 'obj' does not support the operator '&?'"

Am i missing something here or is the operator itself faulty?

Answer 1

Your operator takes an arg of type obj so you should define it as a standalone function, not a type member.

let (&?) (value:obj) (defaultValue:'a) =
    match value with
    | null -> defaultValue
    | _ -> unbox value

Now you can do this:

let voltage = m.["VoltageCaps"] &? 0

I changed it to pattern match null due to much better performance than generic equality.