System.StackOverflowException WPF MVVM

Question

I am trying to create a simple digital clock in WPF using MVVM. I have a label which has a binding. The code behind is simple simple and raising a property changed event each second and I have a stackoverflow exception. Can someone please help ?

public partial class MainWindow : Window, INotifyPropertyChanged
{
    public MainWindow()
    {
        InitializeComponent();
        this.DataContext = this;
    }
    private string _lblValue;
    public string LabelValue
    {
        get
        {
            UpdateLabel();
            return _lblValue;
        }
        set
        {
            _lblValue = value;
            OnPropertyChanged(LabelValue);
        }
    }

    private void UpdateLabel()
    {
        _lblValue = System.DateTime.Now.ToString();
        //System.Threading.Thread.Sleep(2000);
        OnPropertyChanged("LabelValue");
    }

    public event PropertyChangedEventHandler PropertyChanged;
    protected void OnPropertyChanged(string propName)
    {
        var handler = PropertyChanged;
        if (handler != null)
        {
            handler(this, new PropertyChangedEventArgs(propName));
        }
    }
}

As a side notenode, what you are doing is a violation of MVVM pattern. Window is a View class, tightly coupled to WPF framework. Your ViewModels can't depend on Window, Component or other UI classes — Tseng
– Tseng, Commented Mar 21, 2014 at 11:53

Stephen Zeng · Accepted Answer · 2014-03-21 03:24:44Z

2

As har07 explained it is a infinite UI loop. Here is my fix for this issue.

public partial class MainWindow : Window, INotifyPropertyChanged
{
    public MainWindow()
    {
        InitializeComponent();
        this.DataContext = this;

        Task.Run(() => UpdateLabel());
    }

    private string _lblValue;
    public string LabelValue
    {
        get
        {
            return _lblValue;
        }
        set
        {
            _lblValue = value;
            OnPropertyChanged();
        }
    }

    private void UpdateLabel()
    {
        while (true)
        {
            LabelValue = System.DateTime.Now.ToString();
            System.Threading.Thread.Sleep(1000);
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;
    protected void OnPropertyChanged([CallerMemberName] string propName = null)
    {
        var handler = PropertyChanged;
        if (handler != null)
        {
            handler(this, new PropertyChangedEventArgs(propName));
        }
    }
}

answered Mar 21, 2014 at 3:24

Stephen Zeng

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1 Comment

yepimadotnetfreak Over a year ago

Thanks Stephen. This worked.. I was trying to come up with a simple example of binding and raising an event. I was stuck at that point. Appreciate your response.

har07 · Accepted Answer · 2014-03-21 02:58:36Z

2

This is what happens :

UpdateLabel() function invoked in the getter of LabelValue
UpdateLabel() call OnPropertyChanged("LabelValue");
Step 2 causes UI to check for updated value of property LabelValue, in other words it causes getter of LabelValue called
Back to step 1.

All of above steps repeated until stackoverflow exception thrown. Try to remove step 2 to fix the problem.

answered Mar 21, 2014 at 2:58

har07

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Comments

user3419305 · Accepted Answer · 2014-03-21 08:56:32Z

In your case it would be better to use a timer with an interval of one second.
For Example:

using System.Timers;

public partial class MainWindow : Window, INotifyPropertyChanged
{
    private Timer _clockTimer;

    public MainWindow()
    {
        InitializeComponent();
        this.DataContext = this;

       _clockTimer = new Timer(1000); // The interval is in milliseconds
       _clockTimer.Elapsed += (sender, e) =>
       {
           LabelValue = System.DateTime.Now.ToString();
       };
    }

    private string _lblValue;
    public string LabelValue
    {
        get
        {
            return _lblValue;
        }
        set
        {
            _lblValue = value;
            OnPropertyChanged("LabelValue");
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;
    protected void OnPropertyChanged(string propName)
    {
        var handler = PropertyChanged;
        if (handler != null)
        {
            handler(this, new PropertyChangedEventArgs(propName));
        }
    }
}

If you have further question on Timers, you can look on MSDN for more information.

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