Question

I am trying to create a list box that displays a set of data that will be updated over time. I have a simple list box:

<ListBox Name="lbRegisters" ItemsSource="{Binding}" />

And I have defined a class for my objects:

public class register : INotifyPropertyChanged

{
    private int address;
    public int Address { get { return address; } }

    private int value;
    public int Value
    {
        get{ return value; }
        set{
            this.value = value;

            OnValueChanged("Value");
        }
    }

    public register(int a)
    {
        address = a;
        value = 0;
    }

    public event PropertyChangedEventHandler PropertyChanged;

    private void OnValueChanged(string name){
        if (PropertyChanged != null) 
            PropertyChanged(this, new PropertyChangedEventArgs(name));
    }

    public override string ToString()
    {
        return address + ": " + value;
    }
}

And I hold a list of these in an ObservableCollection<register>. Then set the ListBox.ItemsSource=registerslist; in the CodeBehind. The list is initialized and the inital data displays correctly.

Now what do I need to do to get my ListBox to update when a “register.Value” changes. The event handler is called but there nothing is subscribed to the event.

I guess I need to trigger something in the ListBox or ObservableCollection to tell the GUI to update. I have read dozens of posts of a similar problem but they all seem to indicate that once you have implemented INotofyPropertyChanged it just automagicaly works.

What is the next step?

Answer 1

The problem is on your ToString() function. Yes it could be used to display complex string in ListView items but it is not a proper way to bindings because ListView does not knows when part of this string was changed.

Do the following:
1. Declare property on register class like

    public string AddressValue
    {
        get { return address + ": " + value; }
    }

2. Add OnValueChanged("AddressValue") in value and address setters like:

public int Value
{
    get{ return value; }
    set{
        this.value = value;

        OnValueChanged("Value");
        OnValueChanged("AddressValue")
    }
}

3. Declare you ListBox with ItemTemplate like:

    <ListBox x:Name="lbRegisters" ItemsSource="{Binding}">
        <ListBox.ItemTemplate>
            <DataTemplate>
                    <TextBlock  Text="{Binding AddressValue}"  />
            </DataTemplate>
        </ListBox.ItemTemplate>
    </ListBox>

So the idea is adding new property to register class which will be updated when address and value changed. And bind list item text to this property.