Refreshing a binding that uses a value converter

Question

I have a WPF UI that is bound to an object. I'm using a ValueConverter to convert a property to a specific image by a business rule:

    public class ProposalStateImageConverter : IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            var proposal = value as Proposal;
            var basePath = "pack://application:,,,/ePub.Content;component/Images/General/Flag_{0}.png";
            string imagePath;

            if(proposal.Invoice != null)
            {
                imagePath = string.Format(basePath, "Good");
            }
            else
            {
                imagePath = string.Format(basePath, "Warning");
            }

            var uri = new Uri(imagePath);
            var src = uri.GetImageSource(); //Extention method

            return src;
        }
    }

The element is a TreeView where the image is on the 2nd level:

    <TreeView x:Name="tree"
              ItemsSource="{Binding People}" 
              SelectedItemChanged="OnTreeItemChanged">
        <TreeView.Resources>
            <HierarchicalDataTemplate DataType="{x:Type dmn:Person}" 
                                      ItemsSource="{Binding Proposals}">
                <StackPanel Orientation="Horizontal" ToolTip="{Binding Path=Fullname}" Margin="3">
                    <Image Margin="5,0,5,0" Width="16" Height="16" Source="pack://application:,,,/ePub.Content;component/Images/General/Person_Active.png" />
                    <TextBlock Text="{Binding Path=Firstname}" />
                    <TextBlock Text="{Binding Path=Lastname}" Margin="5,0,0,0" />
                </StackPanel>
            </HierarchicalDataTemplate>
            <HierarchicalDataTemplate DataType="{x:Type dmn:Proposal}">
                <StackPanel Orientation="Horizontal" Margin="3">
                    <Image x:Name="invoiceImage" Width="16" Height="16" Margin="5,0,5,0" Source="{Binding, Converter={StaticResource ProposalStateImageConverter}, UpdateSourceTrigger=PropertyChanged}" />
                    <TextBlock Text="{Binding DeliveryDate, Converter={StaticResource textCulturedDateConverter}}" />
                </StackPanel>
            </HierarchicalDataTemplate>
        </TreeView.Resources>
    </TreeView>

It is working fine, but later, when the object's state changes, I want to refresh the image and make the value converter reevaluate. How is this possible?

Answer 1

Assuming you can't use INotifyPropertyChanged because you're binding to the whole object, you need to call BindingExpression.UpdateTarget.

The slight subtlety is in getting hold of the binding expression. This requires you to have a fairly intimate knowledge of the view: as far as I know, the only way to do this is to call BindingOperations.GetBindingExpression, passing the control and property whose binding you want to update, e.g.:

BindingOperations.GetBindingExpression(myImage, Image.SourceProperty).UpdateTarget();