Why the box here?

Question

While browsing StackOverflow I stumbled upon the following answer:

https://stackoverflow.com/a/3817367/162694

// ... removed unneeded code
/// This type is intended for private use within Singleton only.
type private SyncRoot = class end

type Singleton =
    [<DefaultValue>]
    static val mutable private instance: Singleton

    private new() = { }

    static member Instance = 
        lock typeof<SyncRoot> (fun() ->
            // vvv
            if box Singleton.instance = null then
            // ^^^
                Singleton.instance <- Singleton())
        Singleton.instance  

Can somebody elaborate why the box here is needed?

Answer 1

The given Singleton type doesn't have null as a proper value. In other words, it's not nullable and normally shouldn't have the value null. Hence, it is not sensible to compare a value of type Singleton with null, even though the use of an uninitialized variable via [<DefaultValue>] may create a null-valued variable of this type. Boxing turns anything into obj, which is nullable and therefore valid in this context.

Using Unchecked.defaultof<Singleton> instead of null would make the boxing unnecessary and compile. (There's also the [<AllowNullLiteral>] attribute, which can be added to the Singleton type to specify that instances of this type can be null.)