Singleton Property Value is wrong when called from different thread

Question

I have the following Singleton Pattern for the ViewModel of my Options:

    private static volatile GeneralOptionsViewModel instance;
    private static object syncRoot = new object();
    /// <summary>
    /// threadsave singleton
    /// </summary>
    public static GeneralOptionsViewModel Instance
    {
        get
        {
            if (instance == null)
            {
                lock (syncRoot)
                {
                    if (instance == null)
                        instance = new GeneralOptionsViewModel();
                }
            }

            return instance;
        }
    }

In my XAML I have a color picker from the extended toolkit package:

<xctk:PropertyGridEditorColorPicker Background="Transparent" Name="face" 
Margin="5,0" Width="50" BorderBrush="#32FFFFFF" BorderThickness="1" 
SelectedColor="{Binding FaceRectColor, Mode=OneWayToSource, 
UpdateSourceTrigger=PropertyChanged}"/>

As you can see it is bound to FaceRectColor property of the GeneralOptionsViewModel class which is defined like follows. Within the setter there is a conversion to MCvScalar (also a property of the same class), the format I later need for my application:

    public Color FaceRectColor
    {
        get
        {
            return faceRectColor;
        }
        set
        {
            if (faceRectColor != value)
            {
                faceRectColor = value;
                FaceRectColorScalar = new MCvScalar(value.B, value.R, value.G, value.A);
                SetProperty(ref faceRectColor, value);
            }
        }
    }

My problem now is, that the binding works and also the correct values are written to the variable, however when I call the singleton with the property from a different class - and from a different thread - it always shows zero for all color channels. However, if I break the program directly within the singleton class I can see the correct values. AFAIK the singleton should be threadsafe, so I'm looking for the reason of this behavior. My guess is some threading issue, since other properties from the singleton class are displayed correctly, but they are only called in the main thread.

Edit: In my case all property values of the singleton class are set before the worker thread is active. This means no changes during the time the worker thread is active.

Edit II: Here is the complete project for code evaluation. In the class CameraViewModel in line 202 is the relevant call for a function, where I want to pass the values from the singleton.

Answer 1

When your property changes and it does so on a different thread, the calls that are made to notify everybody (in particular the UI) of this change are running in the calling thread. Accessing the UI in a thread that is not the UI thread is a bad idea. It might sometimes work. But it will fail sooner or later.

The solution to your current problem is changing the property in the UI thread.

That said, maybe you should think about whether you need a Singleton. That's a huge red flag that something is wrong with the structure of your program. You don't need a Singleton. Nothing bad would happen if some other context had a second settings viewmodel. You seem to want a Singleton because it's so nice and easy to have a global variable. That is the drawback of a Singleton. It's disadvantage that you buy into because you need something from this pattern. If you find you are using this pattern only because it's disadvantage gives you an excuse to have a global variable, you are doing patterns wrong. It's an anti-pattern.

Answer 2

Your "singleton" contains a public constructor which effectively makes it a non-singleton. And you are not binding to the singleton in your GeneralOptionsView.

If you really want GeneralOptionsViewModel to be a singleton, you should implement it like this:

public sealed class GeneralOptionsViewModel : ViewModelBase
{
    private static readonly GeneralOptionsViewModel _instance = new GeneralOptionsViewModel();
    private GeneralOptionsViewModel()
    {
        GetAvailableCameraList();
        DetectorTypeList = new List<string>() { "Cascade Detector" };
        SelectedDetectorTypeIndex = 0;
    }

    public static GeneralOptionsViewModel Instance => _instance;

    //...
}

You should then set the DataContext of your view to the singleton:

<Grid DataContext="{Binding Source={x:Static local:GeneralOptionsViewModel.Instance}}">