Question

I have an XML document that describes how to build UI elements for user input, and I have data objects that contain some data and an XPath expression. I have a DataTemplate for the data object type that uses a HierarchicalDataTemplate to build the UI based on the XML, but I only want to use a subset of the XML. If the XPath expression were static, I could just write:

<TreeView ItemsSource="{Binding Source={StaticResource dataProvider},
    XPath=/Static/XPath/Expression/*}" />

Since the XPath expression comes from a data object, it would be convenient to use data binding:

<TreeView ItemsSource="{Binding Source={StaticResource dataProvider},
    XPath={Binding Path=Data.XPathExpression}}" />

Unfortunately, the Binding MarkupExtension doesn’t inherit from DependencyObject, so it’s properties aren’t DependencyProperty and don’t support data binding.

How can I apply a dynamic XPath expression when binding to XML data?

Answer 1

You can create a IMultiValueConverter that converts the XML data and XPath expression into new XML data:

public object Convert(object[] values, Type targetType, object parameter,
    CultureInfo culture)
{
    try
    {
        if (values.Length < 2) { return null; }

        var data = values[0] as IEnumerable;
        if (data == null) { return null; }

        var xPathExpression = values[1] as string;
        if (xPathExpression == null) { return null; }

        XmlDocument xmlDocument = data.Cast<XmlNode>()
            .Where(node => node.OwnerDocument != null)
            .Select(node => node.OwnerDocument)
            .FirstOrDefault();

        return (xmlDocument == null) ? null : 
            xmlDocument.SelectNodes(xPathExpression);
    }
    catch (Exception) { return null; }
}

public object[] ConvertBack(object value, Type[] targetTypes,
    object parameter, System.Globalization.CultureInfo culture)
{
    throw new NotImplementedException();
}

Then, use MultiBinding to bind the ItemsSource to the XML data and a dynamic XPathExpression with the converter:

<TreeView>
    <TreeView.Resources>
        <this:XmlXPathConverter x:Key="xmlXPathConverter" />
    </TreeView.Resources>
    <TreeView.ItemsSource>
        <MultiBinding Converter="{StaticResource xmlXPathConverter}">
            <Binding Source="{StaticResource dataProvider}" />
            <Binding Path="Data.XPathExpression" />
        </MultiBinding>
    </TreeView.ItemsSource>
</TreeView>