How to use WeakReference in F#

[xp44mm]

This is a piece of C# code:

var obj = new object();
var weak = new WeakReference(obj);
GC.Collect();
Console.WriteLine("IsAlive: {0} obj!=null is {1}", weak.IsAlive,obj!=null);

obj = null;
GC.Collect();
Console.WriteLine("IsAlive: {0}", weak.IsAlive);

This is the output you'll see when running the example:

IsAlive: True obj!=null is True
IsAlive: False

This is the F# code I translated:

    let mutable obj = new obj()
    let weak = new WeakReference(obj)
    GC.Collect()
    Console.WriteLine("IsAlive: {0} obj!=null is {1}", weak.IsAlive, obj<>null)

    obj <- null
    GC.Collect()
    Console.WriteLine("IsAlive: {0}", weak.IsAlive)

The problem is that the reference is still alive after obj becomes null.

[vzarytovskii]

I would ask this question on stackoverflow, since not that many people read this repository's discussions.

[jl0pd]

This happens because of how F# compiles top-level statements. Instead of placing local variables into locals, it places them into static fields. Moving all code into function solves the issue

open System

let main() =
    let mutable obj = new obj()
    let weak = new WeakReference(obj)
    GC.Collect()
    Console.WriteLine("IsAlive: {0} obj!=null is {1}", weak.IsAlive, obj<>null)

    obj <- null
    GC.Collect()
    Console.WriteLine("IsAlive: {0}", weak.IsAlive)
    
main()

There's awesome tool for inspecting compilation result: https://sharplab.io/

[xp44mm]

The code is placed in a local function, and running it is still an unexpected result.

[Xyncgas]

GC.Collect() doesn't always collect, Maybe that's why