[UWP] Resources.UnloadUnusedAssets() behaves differently against WeakReference

Let me share a problem I got hit. Though I fixed it already for my app, I think better share because it is difficult to find.

PROBLEM
WeakReference.Target becomes null on UWP earlier than mono.
As far as I researched, Resources.UnloadUnusedAssets() behaves differently against WeakReference as follows:

• WeakReference.Target becomes null on GC.Collect() after Resources.UnloadUnusedAssets() on mono
• WeakReference.Target becomes null on Resources.UnloadUnusedAssets() on UWP

STEPS TO REPRODUCE
Use this code or attached project.

Code (CSharp):

1. using UnityEngine;
2. using System;
3. using System.Collections;
4. using System.Collections.Generic;
5.  
6. public class NewBehaviourScript : MonoBehaviour {
7.     List<WeakReference> m_weakReferenceList;
8.  
9.     void Start()
10.     {
11.         Clear();
12.     }
13.  
14.     public void FireGC()
15.     {
16.         GC.Collect();
17.         Debug.Log("GC.Collect() is executed.");
18.     }
19.  
20.     public void UnloadUnusedAssets()
21.     {
22.         Resources.UnloadUnusedAssets();
23.         Debug.Log("Resources.UnloadUnusedAssets() is called.");
24.     }
25.  
26.     public void Clear()
27.     {
28.         m_weakReferenceList = new List<WeakReference>();
29.         Debug.Log("WeakReference list cleared.");
30.     }
31.  
32.     public void Add()
33.     {
34.         GameObject go = Resources.Load("Prefab") as GameObject;
35.         m_weakReferenceList.Add(new WeakReference(go, true));
36.         Debug.Log("WeakReference added to WeakReference list.");
37.     }
38.  
39.     public void ShowInfo()
40.     {
41.         for (int i = 0; i < m_weakReferenceList.Count; i++)
42.         {
43.             if (m_weakReferenceList[i].Target != null)
44.                 Debug.LogFormat("m_weakReferenceList[{0}].Target is NOT null.", i);
45.             else
46.                 Debug.LogFormat("m_weakReferenceList[{0}].Target is null.", i);
47.         }
48.     }
49. }
50.  

• Build it to UWP VS project.
• Call Add() -> UnloadUnusedAssets() -> ShowInfo() on both mono (Unity Editor) and UWP.
• On mono, m_weakReferenceList.Target is NOT null.
• On UWP, m_weakReferenceList.Target is null. (behaves different than mono).
• On mono, Call additionally FireGC() -> ShowInfo(). m_weakReferenceList.Target will be null.

NOTES
Though I read through the reference, I can't tell which behavior should be. However, this difference is a breaking change and caused various problems to my app.

UNITY VERSION
5.3.5p7

 

I thought that internally, Resources.UnloadUnusedAssets called GC.Collect