How to Bind Flags Enums To ListBox In MVVM

Question

I want to bind an enum which has flags attribute to a listbox with a check list box item template in mvvm pattern? How can I do this?

[Flags]
public enum SportTypes
{
   None = 0,
   Baseball = 1,
   Basketball = 2,
   Football = 4,
   Handball = 8,
   Soccer = 16,
   Volleyball = 32
}


<ListBox Name="checkboxList2"
                 ItemsSource="{Binding Sports}"

                 Margin="0,5" 
                 SelectionMode="Multiple">
            <ListBox.ItemTemplate>
                <DataTemplate>
                    <CheckBox IsChecked="{Binding Path=SportTypeEnum, Converter={StaticResource enumBooleanConverter}}" 
                  Content="{Binding Item}"/>
                </DataTemplate>

            </ListBox.ItemTemplate>

Hashish

@HCL - The solution with a value-converter does not work because ConverterParameter is not bindable. – Hashish

Answer 1

You can't easily bind the value directly, because the converter can't build the flag combination from a single flag. So you need to manage a collection of flags, and build the combination based on this collection. The difficulty is that the ListBox.SelectedItems property is readonly. However, this blog post gives a nice workaround, using an attached property.

Here's a complete example using this solution :

Code-behind

[Flags]
public enum MyEnum
{
    Foo = 1,
    Bar = 2,
    Baz = 4
}

public partial class TestEnum : Window, INotifyPropertyChanged
{
    public TestEnum()
    {
        InitializeComponent();
        _flags = (MyEnum[])Enum.GetValues(typeof(MyEnum));
        _selectedFlags = new ObservableCollection<MyEnum>();
        _selectedFlags.CollectionChanged += new System.Collections.Specialized.NotifyCollectionChangedEventHandler(_selectedFlags_CollectionChanged);
        this.DataContext = this;
    }

    void _selectedFlags_CollectionChanged(object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
    {
        if (_selectedFlags.Count == 0)
            Value = default(MyEnum);
        else
            Value = _selectedFlags.Aggregate((v, acc) => acc | v);
    }

    private MyEnum[] _flags;
    public MyEnum[] Flags
    {
        get { return _flags; }
        set
        {
            _flags = value;
            OnPropertyChanged("Flags");
        }
    }

    private ObservableCollection<MyEnum> _selectedFlags;
    public ObservableCollection<MyEnum> SelectedFlags
    {
        get { return _selectedFlags; }
        set
        {
            _selectedFlags = value;
            OnPropertyChanged("SelectedFlags");
        }
    }

    private MyEnum _value;
    public MyEnum Value
    {
        get { return _value; }
        set
        {
            _value = value;
            OnPropertyChanged("Value");
            var currentFlags = _flags.Where(f => _value.HasFlag(f));
            var addedFlags = currentFlags.Except(_selectedFlags).ToArray();
            var removedFlags = _selectedFlags.Except(currentFlags).ToArray();
            foreach (var f in addedFlags)
            {
                _selectedFlags.Add(f);
            }
            foreach (var f in removedFlags)
            {
                _selectedFlags.Remove(f);
            }
        }
    }

    private DelegateCommand<MyEnum> _setValueCommand;
    public ICommand SetValueCommand
    {
        get
        {
            if (_setValueCommand == null)
            {
                _setValueCommand = new DelegateCommand<MyEnum>(v => Value = v);
            }
            return _setValueCommand;
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;

    protected virtual void OnPropertyChanged(string propertyName)
    {
        var handler = PropertyChanged;
        if (handler != null)
            handler(this, new PropertyChangedEventArgs(propertyName));
    }
}

XAML

<Window x:Class="TestPad.TestEnum"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:my="clr-namespace:TestPad"
        Title="TestEnum" Height="300" Width="300">
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto" />
            <RowDefinition Height="*" />
            <RowDefinition Height="*" />
        </Grid.RowDefinitions>
        <TextBlock Text="{Binding Value}" />
        <ListBox Grid.Row="1"
                 ItemsSource="{Binding Flags}"
                 SelectionMode="Extended"
                 my:MultiSelectorBehavior.SynchronizedSelectedItems="{Binding SelectedFlags}">
            <ListBox.ItemTemplate>
                <DataTemplate>
                    <CheckBox Content="{Binding}" IsChecked="{Binding IsSelected, RelativeSource={RelativeSource Mode=FindAncestor, AncestorType=ListBoxItem}}" />
                </DataTemplate>
            </ListBox.ItemTemplate>
        </ListBox>
        <ItemsControl Grid.Row="2"
                      ItemsSource="{Binding Flags}">
            <ItemsControl.ItemTemplate>
                <DataTemplate>
                    <Button Command="{Binding DataContext.SetValueCommand, RelativeSource={RelativeSource Mode=FindAncestor, AncestorType=Window}}"
                            CommandParameter="{Binding}"
                            Content="{Binding}" />
                </DataTemplate>
            </ItemsControl.ItemTemplate>
        </ItemsControl>
    </Grid>
</Window>

NOTE: for the sake of simplicity, the ViewModel is the Window class. In a real MVVM application, you would of course use a separate ViewModel class...

Descendent

Answer 2

I see two solutions: One that is fully dynamic, and one that is static. The dynamic solution is a lot of work and IMO not trivial. The static one should be easy:

Create an Panel within your DataTemplate. There in, place for each Flag-Value a CheckBox. Then use the ConverterParameter to specifiy the flag, the converter should use. This would look something like this:

<StackPanel>    
   <CheckBox IsChecked="{Binding Path=SportTypeEnum, Converter={StaticResource enumBooleanConverter},ConverterParameter={x:Static local:SportTypes.Baseball}}" Content="Baseball"/>
   <CheckBox IsChecked="{Binding Path=SportTypeEnum, Converter={StaticResource enumBooleanConverter},ConverterParameter={x:Static local:SportTypes.Basketball}}" Content="Basketball"/>
   <CheckBox IsChecked="{Binding Path=SportTypeEnum, Converter={StaticResource enumBooleanConverter},ConverterParameter={x:Static local:SportTypes.Football}}" Content="Football"/>
   <CheckBox IsChecked="{Binding ..../>
</StackPanel/>

In your converter you only have to do some logical AND-comparisons and you will have what you're looking for. If you're interested in the dynamic solution, make a comment, I can give you some ideas where to start. But IMO this will really not be trivial.

Additonal info

If you want to have a list instead of the StackPanel, use a ScrollViewer in a Border or even a ListBox.

Highwayman

What exactly do you write to the ConvertBack method for two way binding? – Surrey

Answer 3

To expand on Chris's post, here's a more thorough explanation of how you can do this.

This isn't the most ideal scenario, as the property holding the enum has to be a bit more complex than usual, but it works.

Converter code:

public class EnumFlagConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        var theEnum = value as Enum;
        return theEnum.HasFlag(parameter as Enum);
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        var theEnum = parameter as Enum;
        return theEnum;
    }
}

XAML for converter:

<StackPanel>
    <StackPanel.Resources>
        <local:EnumFlagConverter x:Key="MyConverter" />
    </StackPanel.Resources>
    <CheckBox IsChecked="{Binding Path=SportTypeEnum, Converter={StaticResource MyConverter},ConverterParameter={x:Static local:SportTypes.Baseball}}" Content="Baseball"/>
    <CheckBox IsChecked="{Binding Path=SportTypeEnum, Converter={StaticResource MyConverter},ConverterParameter={x:Static local:SportTypes.Basketball}}" Content="Basketball"/>
    <CheckBox IsChecked="{Binding Path=SportTypeEnum, Converter={StaticResource MyConverter},ConverterParameter={x:Static local:SportTypes.Football}}" Content="Football"/>
    <CheckBox IsChecked="{Binding ..../>
</StackPanel>

Then, to bind to this, I did a bit of trickery to get the converter to work properly:

private SportTypeEnum _TheSportType;
public SportTypeEnum _TheSportType
{
    get { return _TheSportType; }
    set
    {
        if (_TheSportType.HasFlag(value))
            _TheSportType &= ~value;
        else
            _TheSportType |= value;
        NotifyPropertyChanged();
    }
}

Because of this special setter logic, you probably want to include a method like this to allow you to fully set the value from code:

private void ResetTheSportType()
{
    _TheSportType = _TheSportType.None;
    NotifyPropertyChanged(() => TheSportType);
}

Contexture