Silverlight MVVM Combobox Binding getting broken

Question

I have a combo box defined as such

<ComboBox Name="RoomDropDown" Visibility="{Binding Path=RoomDropDownVisible,Mode=OneWay,Converter={StaticResource BoolVisibilityConvertor}}"
                          ItemsSource="{Binding Path=RoomList,Mode=OneWay}" DisplayMemberPath="display" SelectedValuePath="display" SelectedValue="{Binding Path=Room,Mode=TwoWay}"/>

There are 2 properties defined in the ViewModel, RoomList which is List and the Room property which is a string.

First time when i run the app everything works fine, and the Drop Down gets the correct values as well as the correct values is selected. However on a certain conditions the RoomList property is changed to a different source & the Room property is also changed. The problem that is now happening is the Combo Box is showing the correct values but the selected value is not getting selected. Worse, we can live with that, but the setter is also not firing when the value is manually changed in the DropDown.

Any pointers on what is going wrong here?

Followup: Don't think I managed to get the exact problem across, here is some sample code that I wanted to add to illustrate the problem:

<Grid x:Name="LayoutRoot" Background="White">
    <StackPanel VerticalAlignment="Center" Width="100">
        <ComboBox Name="TestBox" Height="20" Width="100" ItemsSource="{Binding Path=ComboSource}" DisplayMemberPath="display" SelectedValuePath="code" 
                  SelectedValue="{Binding Path=ComboSelection,Mode=TwoWay}"/>
        <Button Content="Click Here" Click="Button_Click" />
    </StackPanel>
 </Grid>

Code:-

public MainPage()
    {
        InitializeComponent();
        this.Loaded += (s, e) =>
            {
                var temp = new List<Binding>();
                temp.Add(new Binding() { code = "1", display = "One" });
                temp.Add(new Binding() { code = "2", display = "Two" });
                this.ComboSource = temp;
                this.ComboSelection = "1";
                this.DataContext = this;
            };
    }

    private static readonly DependencyProperty ComboSelectionProperty =
        DependencyProperty.Register("ComboSelectionProperty", typeof(string), typeof(MainPage), new PropertyMetadata(null));

    public string ComboSelection
    {
        get { return (string)GetValue(ComboSelectionProperty); }
        set 
        { 
            SetValue(ComboSelectionProperty, value);
            this.RaisePropertyChanged("ComboSelection");
        }
    }

    private static readonly DependencyProperty ComboSourceProperty =
        DependencyProperty.Register("ComboSourceProperty", typeof(List<Binding>), typeof(MainPage), new PropertyMetadata(null));

    public List<Binding> ComboSource
    {
        get
        {
            return (List<Binding>)GetValue(ComboSourceProperty);
        }
        set
        {
            SetValue(ComboSourceProperty, value);
            this.RaisePropertyChanged("ComboSource");
        }
    }

    private void Button_Click(object sender, RoutedEventArgs e)
    {
        var temp = new List<Binding>();
        temp.Add(new Binding() { code = "3", display = "Three" });
        temp.Add(new Binding() { code = "4", display = "Four" });

        this.ComboSource = temp;
        this.ComboSelection = "3";

    }

    #region INotifyPropertyChanged Members

    public event PropertyChangedEventHandler PropertyChanged;

    private void RaisePropertyChanged(string propertyName)
    {
        if (this.PropertyChanged != null)
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
    }

    #endregion
 }

 public class Binding
{
    public string code {get; set;}
    public string display { get; set; }
}

Not strictly MVVM, but to explain the problem, when the button click event is fired, the Combosource is changed with a new selection being made, however that selection does not bind and the problem i mentioned above starts happening.

Answer 1

Not sure about your actual sourcecode, but in your example your dependency registrations are not quite right.

Should have been:

DependencyProperty.Register("ComboSelection"

and

DependencyProperty.Register("ComboSource"

not "xxxProperty". Then it all fires the change correctly.

Basically you are registering the dependency object itself, instead of the get/set property methods. The parameter documentation/naming may seem a little misleading.

*Note: As per Dan Bryant's comment - the INotifyPropertyChange stuff is also not needed and I ripped it out of your sample code (I can assure you it works perfectly fine without it).