Populate one combobox based on selection of another

Question

I am learning wpf mvvm and have been struggling with what I feel is probably simple but have not been able to resolve on my own.

What I want is to be able to select an item in a populated combobox and then populate another combobox based on that selection. I can't seem to get the second combobox loaded in response to a selection.

I've included an example with 2 comboboxes and one textblock. When I run the app and select an item in the first combo, the textblock is updated based on the binding, but I don't know where to call the code to update the second combobox.

I tried adding the call to the SelectedString property setter, but that does not seem to ever be called. I'm sure I'm missing something simple, but I need someone to help lift the veil!

I've tried suggestions on other posts but so far I have been unsuccessful.

Here is the xaml for the view:

<Window x:Class="ThrowAwayMVVMApp.Views.MainView"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="Main Window" Height="400" Width="800">

    <DockPanel>
        <Grid>
            <Grid.RowDefinitions>
                <RowDefinition Height="60*" />
                <RowDefinition Height="282*" />
            </Grid.RowDefinitions>
            <!-- Add additional content here -->
            <ComboBox ItemsSource="{Binding MyStrings}" SelectedItem="{Binding Path=SelectedString, Mode=TwoWay}" Height="23" HorizontalAlignment="Left" Margin="18,24,0,0" Name="comboBox1" VerticalAlignment="Top" Width="204" />
            <TextBlock Text="{Binding SelectedString}" Height="23" HorizontalAlignment="Left" Margin="276,24,0,0" Name="textBlock1" VerticalAlignment="Top" Width="227" Background="#FFFAE7E7" />
            <ComboBox ItemsSource="{Binding ResultStrings}"  Height="23" HorizontalAlignment="Left" Margin="543,25,0,0" Name="comboBox2" VerticalAlignment="Top" Width="189" />
        </Grid>
    </DockPanel>
</Window>

Here is the view model:

public class MainViewModel : ViewModelBase
{
    public MainViewModel()
    {
        this.MyStrings = new ObservableCollection<string>
            {
                "One",
                "Two",
                "Three",
                "Four",
                "Five"
            };
    }

    public ObservableCollection<string> MyStrings { get; set; }
    public ObservableCollection<string> ResultStrings { get; set; }

    public string SelectedString
    {
        get { return (string)GetValue(SelectedStringProperty); }
        set 
        { 
            SetValue(SelectedStringProperty, value);
            this.ResultStrings = getResultStrings(value);
        }
    }

    // Using a DependencyProperty as the backing store for SelectedString.  This enables animation, styling, binding, etc...
    public static readonly DependencyProperty SelectedStringProperty =
        DependencyProperty.Register("SelectedString", typeof(string), typeof(MainViewModel), new UIPropertyMetadata(""));


    private ObservableCollection<string> getResultStrings(string input)
    {
        ObservableCollection<string> result = null;

        if (input == "Three")
        {
            result = new ObservableCollection<string> { "Six", "Seven", "Eight" };
        }
        else
        {
            result = new ObservableCollection<string> { "Nine", "Ten", "Eleven" };
        }

        return result;
    }
}

Answer 1

The SelectedString dependency property implementation is wrong: you should register PropertyChangedCallback to be notified when the DP is accessed directly and not with the CLR property set (see http://msdn.microsoft.com/en-us/library/system.windows.propertychangedcallback.aspx); this way you can change the related collection even if the DP is used directly.

When binding against a dependency property (like SelectedString), the WPF binding doesn't use the CLR property setter, so you never get getResultStrings called.

By the way, I would consider using a POCO approach on View Models, implementing INotifyPropertyChanged: DP are a pain to write and add a lot of noise to VMs (other than a nasty dependency on System.Windows).

Have a look at this blog post for a extensive comparison: http://kentb.blogspot.com/2009/03/view-models-pocos-versus.html

Answer 2

Try

private string selectedString;
public string SelectedString
{
    get { return selectedString; }
    set 
    { 
        if (selectedString == value) return;
        selectedString = value;
        // Required for the UI to know the change was successful
        RaisePropertyChanged("SelectedString");
        LoadStringResults(value);
    }
}

private ObservableCollection<string> resultStrings;
public ObservableCollection<string> ResultStrings
{
    get { return resultStrings; }
    set 
    { 
        if (resultStrings== value) return;
        resultStrings= value;
        // Required for databinding to know that ResultStrings changed
        // Previously you changed this property without updating the UI
        RaisePropertyChanged("ResultStrings");
    }
}

private void LoadStringResults(string input)
{
    ObservableCollection<string> result = null;

    if (input == "Three")
    {
        result = new ObservableCollection<string> { "Six", "Seven", "Eight" };
    }
    else
    {
        result = new ObservableCollection<string> { "Nine", "Ten", "Eleven" };
    }

    ResultStrings = result;
}