Are C# weak references in fact soft?

Question

The basic difference is that weak references are supposed to be claimed on each run of the GC (keep memory footprint low) while soft references ought to be kept in memory until the GC actually requires memory (they try to expand lifetime but may fail anytime, which is useful for e.g. caches especially of rather expensive objects).

To my knowledge, there is no clear statement as to how weak references influence the lifetime of an object in .NET. If they are true weak refs they should not influence it at all, but that would also render them pretty useless for their, I believe, main purpose of caching (am I wrong there?). On the other hand, if they act like soft refs, their name is a little misleading.

Personally, I imagine them to behave like soft references, but that is just an impression and not founded.

Implementation details apply, of course. I'm asking about the mentality associated with .NET's weak references - are they able to expand lifetime, or do they behave like true weak refs?

(Despite a number of related questions I could not find an answer to this specific issue yet.)

Answer 1

Are C# weak references in fact soft?

No.

am I wrong there?

You are wrong there. The purpose of weak references is absolutely not caching in the sense that you mean. That is a common misconception.

are they able to expand lifetime, or do they behave like true weak refs?

No, they do not expand lifetime.

Consider the following program (F# code):

do
  let x = System.WeakReference(Array.create 0 0)
  for i=1 to 10000000 do
    ignore(Array.create 0 0)
  if x.IsAlive then "alive" else "dead"
  |> printfn "Weak reference is %s"

This heap allocates an empty array that is immediately eligible for garbage collection. Then we loop 10M times allocating more unreachable arrays. Note that this does not increase memory pressure at all so there is no motivation to collect the array referred to by the weak reference. Yet the program prints "Weak reference is dead" because it was collected nevertheless. This is the behaviour of a weak reference. A soft reference would have been retained until its memory was actually needed.

Here is another test program (F# code):

open System

let isAlive (x: WeakReference) = x.IsAlive

do
  let mutable xs = []
  while true do
    xs <- WeakReference(Array.create 0 0)::List.filter isAlive xs
    printfn "%d" xs.Length

This keeps filtering out dead weak references and prepending a fresh one onto the front of a linked list, printing out the length of the list each time. On my machine, this never exceeds 1,000 surviving weak references. It ramps up and then falls to zero in cycles, presumably because all of the weak references are collected at every gen0 collection. Again, this is the behaviour of a weak reference and not a soft reference.

Note that this behaviour (aggressive collection of weakly referenced objects at gen0 collections) is precisely what makes weak references a bad choice for caches. If you try to use weak references in your cache then you'll find your cache getting flushed a lot for no reason.