Bug 6715 - System.WeakReference problem
Summary: System.WeakReference problem
Status: RESOLVED FIXED
Alias: None
Product: Android
Classification: Xamarin
Component: Mono runtime / AOT Compiler ()
Version: 4.2.x
Hardware: All All
: --- normal
Target Milestone: ---
Assignee: Bugzilla
URL:
Depends on:
Blocks:
 
Reported: 2012-08-27 16:50 UTC by Alexey Subbotin
Modified: 2012-10-03 14:32 UTC (History)
3 users (show)

Tags:
Is this bug a regression?: ---
Last known good build:

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Status:
RESOLVED FIXED

Description Alexey Subbotin 2012-08-27 16:50:07 UTC
System.WeakReference.Target not always correctly cleared during gc. In the sample below the second garbage collection leaves object referenced by Target property in incorrect state (the managed peer is alive while a corresponding native peer in Java has already been removed). The provided sample throws NRE deep inside ToString call:

	static WeakReference CreateBitmap()
	{
		return new WeakReference(Bitmap.CreateBitmap(10, 10, Bitmap.Config.Argb8888));
	}

	static void Test()
	{
		var bitmap = CreateBitmap();

		GC.Collect();
		Console.WriteLine(bitmap.Target);

		GC.Collect();
		Console.WriteLine(bitmap.Target); // NRE inside ToString(), after successfull dereferencing

		GC.Collect();
		Console.WriteLine(bitmap.Target);
	}

Exception info:
System.NullReferenceException: Object reference not set to an instance of an object
  at Android.Runtime.JNIEnv.CallObjectMethod (intptr,intptr) [0x00000] in /Users/builder/data/lanes/monodroid-mac-monodroid-4.2.4-branch/9f7cbd60/source/monodroid/src/Mono.Android/src/Runtime/JNIEnv.g.cs:129

  at Java.Lang.Object.ToString () [0x0003e] in /Users/builder/data/lanes/monodroid-mac-monodroid-4.2.4-branch/9f7cbd60/source/monodroid/src/Mono.Android/platforms/android-8/src/generated/Java.Lang.Object.cs:189

  at System.IO.TextWriter.Write (object) [0x00006] in /Users/builder/data/lanes/monodroid-mac-monodroid-4.2.4-branch/9f7cbd60/source/mono/mcs/class/corlib/System.IO/TextWriter.cs:163

  at System.IO.TextWriter.WriteLine (object) [0x00000] in /Users/builder/data/lanes/monodroid-mac-monodroid-4.2.4-branch/9f7cbd60/source/mono/mcs/class/corlib/System.IO/TextWriter.cs:273

  at System.IO.SynchronizedWriter.WriteLine (object) [0x0000c] in /Users/builder/data/lanes/monodroid-mac-monodroid-4.2.4-branch/9f7cbd60/source/mono/mcs/class/corlib/System.IO/TextWriter.cs:563

  at System.Console.WriteLine (object) [0x00000] in /Users/builder/data/lanes/monodroid-mac-monodroid-4.2.4-branch/9f7cbd60/source/mono/mcs/class/corlib/System/Console.cs:432

  at TestWeaksAndroid.Activity1.Test () [0x0001f] in d:\Projects\TestWeaksAndroid\TestWeaksAndroid\Activity1.cs:38

  at TestWeaksAndroid.Activity1.OnCreate (Android.OS.Bundle) [0x00016] in d:\Projects\TestWeaksAndroid\TestWeaksAndroid\Activity1.cs:48

  at Android.App.Activity.n_OnCreate_Landroid_os_Bundle_ (intptr,intptr,intptr) [0x00010] in /Users/builder/data/lanes/monodroid-mac-monodroid-4.2.4-branch/9f7cbd60/source/monodroid/src/Mono.Android/platforms/android-8/src/generated/Android.App.Activity.cs:1490

  at (wrapper dynamic-method) object.6388eb25-5ff0-4675-9fed-ebae070e8844 (intptr,intptr,intptr) <IL 0x00012, 0x00033>
Comment 1 Alexey Subbotin 2012-08-27 17:02:10 UTC
WeakReference.Target property becomes null only after third garbage collection.
Comment 2 Mark Probst 2012-09-17 17:16:44 UTC
A fix is on the way.

As it is right now, weak references to objects which have Java peers will stay around for one more garbage collection, during which time their Java peers will be gone, i.e. they will act as if they have been disposed.

If you need the correct behaviour right now, you could let the WeakReference reference the object through an intermediary managed-only object.
Comment 3 Mark Probst 2012-10-03 14:32:53 UTC
Fixed.