Monotouch crashes with NullReferenceException on non nullable object

Question

Here is the crash log ( short ):

2013-04-24 19:56:24 +0000 Memotion Unhandled managed exception: Object reference not set to an instance of an object (System.NullReferenceException)
at Pipedream.UI.UIElement.set_Size (Vector2 value) [0x00000] in :0
at Pipedream.UI.StageLayer..ctor (Pipedream.ComponentID id) [0x00000] in :0
at Pipedream.UI.Stage.CreateLayer (Pipedream.ComponentID id) [0x00000] in :0

Actually the code of the constructor looks like this:

internal StageLayer(ComponentID id)
    : base(id, ComponentLayer.System)
{
    base.Size = ApplicationBase.Instance.ScreenSize;
    ApplicationBase.Instance.PropertyChanged += HandlePropertyChanged;
}

And the property looks like this:

public virtual Vector2 Size
{
    get
    {
        return _Size;
    }
    set
    {
        Vector2 old = _Size;
        if (SetData(SizeDeclaration, value, ref _Size, Invalidate))
        {
            CompareUpdate(WidthDeclaration, _Size.X, old.X);
            CompareUpdate(HeightDeclaration, _Size.Y, old.Y);
        }
    }
}

Well the Vector2 is a struct and cannot be null. Also this code works perfectly on desktop. I can't think of any reason why this code should crash but it also does not on iOS simulator, only on iOS-device ( I currently have no device so i'm not able to debug it directly ).

I start some tasks before this code is called, but they using their own data and cannot conflict with the current data, even then there should not be any NullReferenceException.

Edit

It turns out that the exception is thrown when the CompareUpdate-method should be called. Anyway i thought that non virtual generic methods should not make any issues?

protected Boolean CompareUpdate<T>(DependencyProperty property, T newValue, T oldValue)
{
    if (!Object.Equals(newValue, oldValue))
    {
        ForceUpdate(property, newValue, oldValue);
        return true;
    }
    return false;
}

Edit 2

After some more test cases i found out that that this might be a real compiler issue. The following test fails:

Log.Info(_Size.X.ToString()); // _Size is still a struct

With this exception:

System.NullReferenceException: Object reference not set to an instance of an object
at System.Single.ToString () [0x00000] in /Developer/MonoTouch/Source/mono/mcs/class/corlib/System/Single.cs:260
at Pipedream.UI.UIElement.set_Size (Vector2 value) [0x00031] in C:\WORK\00_PROJECTS\16 Pipedream\00_FRAMEWORK\trunk\Pipedream\UI\UIElement.cs:332

If i change the original code to the following there is also no error:

Vector2 old = _Size;
if (SetData(SizeDeclaration, value, ref _Size, Invalidate))
{
    CompareUpdate(WidthDeclaration, 0f, 0f);
    CompareUpdate(HeightDeclaration, 0f, 0f);
}

This error also occurs when i remove the SetData-method, so this can't be the error cause. I've checked the this reference so the stack seems to be ok, but if i try to access the _Sizes variable X and try to print it to the console the NullReferenceException occurs again.

Hard to say... the stack trace could be incomplete (e.g. something caller in the setter). In any case such issues are better solved by filing a bug report @ http://bugzilla.xamarin.com and attach a test case that shows the issue (so we can duplicate it ourselves). — poupou, Apr 24 '13 at 23:44
@poupou Yeah maybe there is a bug in the compiler of monotouch. There is a huge engine in the back which has been written by me so i can't attach it to a bugreport. I try to replicate it later theese days. — Felix K., Apr 24 '13 at 23:55
@poupou Updated my answer, i think i've much more issues with generics which should not occur. — Felix K., Apr 27 '13 at 12:15
No doubt something is wrong but `Single.ToString()` works so there's something contextual in your project (or options) that makes this fails. We still need a bug report (for a test case and for all your project options) along with all the versions numbers of the software you're using. — poupou, Apr 27 '13 at 14:23
@poupou I could send you this and i already created a test case ( not working, seems to be contextual ) but i won't upload the hole code to your server especially if everyone can download it. What options do we've? — Felix K., Apr 27 '13 at 14:31
You can mark the bug (or the attachement) as private. Only you and Xamarin will see (and download) private bugs/attachments. If it's too big (more than 5MB) you can give a link (private bug or comment) to download (e.g. from dropbox) and remove the test case once the bug is fixed. You can also contact support@xamarin.com to get NDA paperwork. — poupou, Apr 27 '13 at 14:50
an other reference to a same kind of bug http://stackoverflow.com/q/22252819/3314372 — Jan, Mar 07 '14 at 15:46

score 0 · Accepted Answer · edited Jul 09 '13 at 12:26

0

As it turns out this has an architecture difference bug. The [StructLayout(LayoutKind.Sequential, Pack = 1)] line of the Vector2 causes the Vector2 to be unaliagned which is illegal in some architectures.

Removing the line for now removed the issue.

edited Jul 09 '13 at 12:26

miguel.de.icaza

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answered Jun 01 '13 at 21:33

Felix K.

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Monotouch crashes with NullReferenceException on non nullable object

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