Question

I’m trying to bind a ComboBox to an ObservableCollection<MyType> but it’s not working as I expect. When I create an instance of ComboData, e.g.

ComboData = new ObservableCollection<MyType>();

and add data to it, e.g.

ComboData.Add(new MyType(string.Format("{0}", i.ToString()), i));

I can see that the Value property of MyType is being set and the PropertyChangedEventHandler event handler executes. This is correct functionality.

The problem is when I select a different item in the ComboBox later. I expect to see Value being set again and firing the event handler, but nothing happens. However, I can see that the property’s getter is being fired and returning _value.

Is there something I’m doing wrong?

MyType is defined as:

public class MyType : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    public MyType(string key, int value)
    {
        Key = key;
        Value = value;
    }

    public string Key { get; set; }
    private int _value;
    public int Value
    {
        get { return _value; }
        set
        {
            if (_value != value)
            {
                _value = value;

                OnPropertyChanged("Value");
            }
        }
    }

    internal void OnPropertyChanged(string prop)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(prop));
        }
    }

    private void IntegerItem_PropertyChanged(object sender, PropertyChangedEventArgs e)
    {
        // this never executes *after* the ComboBox has been populated!!!
    }

}

The User Control’s XAML is:

<Grid>
    <ComboBox x:Name="cboItems"
      DisplayMemberPath="Key"
      SelectedValuePath="Value"
      ItemsSource="{Binding Path=ComboData}">
    </ComboBox>
</Grid>

And the code behind for the User Control is

public partial class ComboBoxControl : UserControl
{
    public ObservableCollection<MyType> ComboData { get; set; }

    public ComboBoxControl()
    {
        InitializeComponent();

        ComboData = new ObservableCollection<MyType>();
        int min = 0;
        int max = 10;
        for (int i = min; i <= max; i++)
        {
            ComboData.Add(new MyType(string.Format("{0}", i.ToString()), i));
        }

        cboItems.DataContext = this;            
    }
}

What do I need to do to fire the event handler when I change the selected item in the ComboBox?

Answer 1

Selecting a different option in your combobox isn’t changing any of the items in ComboData, it’s just changing the selected item, so it won’t use the setter in MyType. It uses the getter to retrieve the value to display.

The ComboBox has a SelectedItemChanged event which will trigger when a new item is selected.