ComboBox with IEnumerable<Brush> as ItemsSource and SelectedItem exception

Question

Hi all,

I have this issue with ComboBox that uses IEnumerable<Brush> as ItemsSource; the problem lies when I (programmatically) try to set SelectedItem. Here is the code that describes the problem:

private readonly List<Brush> colors_;
private Brush white_;

ViewModelConstructor()
{
    colors_ = (from p in brushes_type.GetProperties()
               select (Brush)converter.ConvertFromString(p.Name)).ToList();

    white_ = colors_.Single(b => b.ToString() == "#FFFFFFFF");
}

public IEnumerable<Brush> Colors
{
    get { return colors_; }
}

public Brush White
{
    get { return white_; }
    set
    {
        if (white_ != value)
            white_ = value;
    }
}

And here is xaml code:

<ComboBox ItemsSource="{Binding Path=Colors}"
          SelectedItem="{Binding Path=White}">

    <ComboBox.ItemTemplate>
        <DataTemplate>
            <StackPanel Orientation="Horizontal">

                <Border BorderThickness="1"
                        BorderBrush="Black"
                        Width="20"
                        Height="12"
                        SnapsToDevicePixels="True"
                        Margin="0,0,4,0">

                    <Border Background="{Binding}"/>

                </Border>

                <TextBlock Text="{Binding}"/>

            </StackPanel>
        </DataTemplate>
    </ComboBox.ItemTemplate>
</ComboBox>

After the get on the White property is called I get an Exception: Cannot set a property on object '#FFFFFFFF' because it is in a read-only state. If i leave White (white_) as null, everything works just fine.

Answer 1

Just a shot in the dark here (can't currently try this), but how about binding SelectedValue instead of SelectedItem?

Answer 2

Using Reflector I've traced down where the problem appears to be occuring. Inside Selector.ItemSetIsSelected, it takes the new SelectedItem and does the following:

• If the container of the element is a DependencyObject it sets the IsSelectedProperty on the container to true.
• Otherwise if the element is a DependencyObject it sets the IsSelectedProperty on the element to true.

That second part is where the failure comes into play, the Brush objects you've chosen are Read-Only. Because the Selector.ItemSetIsSelected is somewhat broken in this case, you have two options.

Option 1, you just call .Clone() on the Brush object returned from the Converter.

colors_ = (from p in typeof(Brushes).GetProperties()
    select ((Brush)converter.ConvertFromString(p.Name)).Clone()).ToList();

---

EDIT: You should go with Option 1...Option 2 is the longwinded way to fix the problem

Option 2, you could wrap the Brush objects into one more object:

public class BrushWrapper
{
    public Brush Brush { get; set; }
}

You then update the data template paths:

<Border Background="{Binding Path=Brush}" />

and

<TextBlock Text="{Binding Path=Brush}" />

Finally, you update the ViewModel:

private readonly List<BrushWrapper> colors_;
private BrushWrapper white_;

public ColorViewModel()
{
    colors_ = (from p in typeof(Brushes).GetProperties()
               select new BrushWrapper {
                   Brush = (Brush)converter.ConvertFromString(p.Name)
               }).ToList();

    white_ = colors_.Single(b => b.Brush.ToString() == "#FFFFFFFF");
}

public List<BrushWrapper> Colors
{
    get { return colors_; }
}

public BrushWrapper White
{
    get { return white_; }
    set
    {
        if (white_ != value)
            white_ = value;
    }
}