INotifyPropertyChanged and Threading

Question

I have a base class implementing INotifyPropertyChanged :

protected void OnNotifyChanged(string pName)
{
    if (PropertyChanged != null)
    {
        PropertyChanged(this, new PropertyChangedEventArgs(pName));
    }
}

public event PropertyChangedEventHandler PropertyChanged;

I have a derived class with a property Latitude like so:

private double latitude;

public double Latitude
{
    get { return latitude; }
    set { latitude = value; OnNotifyChanged("Latitude"); }
}

My derived class also has a method Fly that manipulates Latitude .

I also have a Form with a TextBox bound to Latitude of my derived class:

txtLat.DataBindings.Clear();    
txtLat.DataBindings.Add("Text", bindSrc, "Latitude");

A thread is used to kick off Fly like so:

Thread tFly = new Thread(f.Fly);
tFly.IsBackground = true;
tFly.Start();

When Latitude changes, an exception is thrown:

DataBinding cannot find a row in the list that is suitable for all bindings.

Answer 1

This seems to be an odd issue with thread affinity. Ultimately, the code is trying to do the update from a non-UI thread - I'm unclear why it isn't just displaying the cross-thread exception, though - I wonder whether this is actually a catch-all exception handler. If I remove the BindingSource (and bind directly to the object, which is valid) you do get a cross-thread exception (which I expected).

Personally , I would be inclined to handle this manually, ie subscribe to the event with a method that does an Invoke to the UI thread and updates the Text manually. However, I'm just checking if some previous cross-threaded binding code might help...

---

Here's an example using Invoke :

using System;
using System.ComponentModel;
using System.Threading;
using System.Windows.Forms;

class FlightUav : INotifyPropertyChanged
{
    protected void OnNotifyChanged(string pName)
    {
        if (PropertyChanged != null)
            PropertyChanged(this, new PropertyChangedEventArgs(pName));
    }
    public event PropertyChangedEventHandler PropertyChanged;
    private double _latitude;
    public double Latitude
    {
        get { return _latitude; }
        set { _latitude = value; OnNotifyChanged("Latitude"); }
    }
    public void Fly()
    {
        for (int i = 0; i < 100; i++)
        {
            Latitude++;
            Thread.Sleep(10);
        }
    }
    [STAThread]
    static void Main()
    {
        using (Form form = new Form())
        {
            FlightUav currentlyControlledFlightUav = new FlightUav();

            currentlyControlledFlightUav.PropertyChanged += delegate
            { // this should be in a *regular* method so that you can -= it when changing bindings...
                form.Invoke((MethodInvoker)delegate
                {
                    form.Text = currentlyControlledFlightUav.Latitude.ToString();
                });
            };


            using (Button btn = new Button())
            {
                btn.Text = "Fly";
                btn.Click += delegate
                {
                    Thread tFly = new Thread(currentlyControlledFlightUav.Fly);
                    tFly.IsBackground = true;
                    tFly.Start();
                };
                form.Controls.Add(btn);
                Application.Run(form);
            }
        }
    }


}

---

Here's an example using a (modified) version of some old threading code of mine:

using System;
using System.ComponentModel;
using System.Threading;
using System.Windows.Forms;

class FlightUav : INotifyPropertyChanged
{
    protected void OnNotifyChanged(string pName)
    {
        if (PropertyChanged != null)
            PropertyChanged(this, new PropertyChangedEventArgs(pName));
    }
    public event PropertyChangedEventHandler PropertyChanged;
    private double _latitude;
    public double Latitude
    {
        get { return _latitude; }
        set { _latitude = value; OnNotifyChanged("Latitude"); }
    }
    public void Fly()
    {
        for (int i = 0; i < 100; i++)
        {
            Latitude++;
            Thread.Sleep(10);
        }
    }
    [STAThread]
    static void Main()
    {
        using (Form form = new Form())
        {
            FlightUav currentlyControlledFlightUav = new FlightUav();
            BindingSource bindSrc = new BindingSource();
            var list = new ThreadedBindingList<FlightUav>();
            list.Add(currentlyControlledFlightUav);
            bindSrc.DataSource = list;

            form.DataBindings.Clear();
            form.DataBindings.Add("Text", list, "Latitude");

            using (Button btn = new Button())
            {
                btn.Text = "Fly";
                btn.Click += delegate
                {
                    Thread tFly = new Thread(currentlyControlledFlightUav.Fly);
                    tFly.IsBackground = true;
                    tFly.Start();
                };
                form.Controls.Add(btn);
                Application.Run(form);
            }
        }
    }


}
public class ThreadedBindingList<T> : BindingList<T>
{
    private readonly SynchronizationContext ctx;
    public ThreadedBindingList()
    {
        ctx = SynchronizationContext.Current;
    }
    protected override void OnAddingNew(AddingNewEventArgs e)
    {
        SynchronizationContext ctx = SynchronizationContext.Current;
        if (ctx == null)
        {
            BaseAddingNew(e);
        }
        else
        {
            ctx.Send(delegate
            {
                BaseAddingNew(e);
            }, null);
        }
    }
    void BaseAddingNew(AddingNewEventArgs e)
    {
        base.OnAddingNew(e);
    }
    protected override void OnListChanged(ListChangedEventArgs e)
    {
        if (ctx == null)
        {
            BaseListChanged(e);
        }
        else
        {
            ctx.Send(delegate
            {
                BaseListChanged(e);
            }, null);
        }
    }
    void BaseListChanged(ListChangedEventArgs e)
    {
        base.OnListChanged(e);
    }
}