CODEWITHSUNDEEP
f#garbage-collection
Thomas
Thomas

Reputation: 12107

Will this object survive garbage collection in F#?

I have a F# object like this:

type myObject() = 
    ...
    do
        // calling a 3rd party lib that will do
        // callbacks into this object's methods

and, in my main initialization, I do this:

let _ = myObject()

and go on with the rest of the code.

So, this object is setting up callbacks from a 3rd party library; the callbacks are going to land in this object but the object itself will not be referenced anywhere.

Question 1: will this object survive garbage collection

Question 2: if not, how to make it permanent

Upvotes: 0

Views: 112

Answers (1)

Tomas Petricek
Tomas Petricek

Reputation: 243051

I think the question whether an object assigned to a placeholder _ will remain referenced from the location where it is declared is not the important question here. The important question is whether the external library will keep a reference to the object. For most normal libraries that I can think of this would be the case. For example, consider something like this:

let register f = 
  let tmr = new System.Timers.Timer(1000.)
  tmr.Elapsed.Add f
  tmr.Start()

type MyObject() as this = 
  do register this.Foo
  member x.Foo(_) = printfn "hi"

let _ = MyObject()

Here, the implementation of .NET timers will need to keep a reference to all the timers that you created (so that it can trigger them) and the timer we create keeps reference to the instance of MyObject. Regardless of whether _ creates a reference or not, the object will be referenced through the callback.

I can think of only a few ways in which you could register a callback without keeping a reference to the object - and those would most likely be by using native interop and unsafe code. If that's what you're doing, then it's better to create a global ResizeArray somewhere in a static field and keep a reference to your object there. In all other normal cases, you don't need to worry about this.

Upvotes: 2

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