Binding to resource key, WPF

Question

I have a ResourceDictionary with some images:

<BitmapImage UriSource="..\Images\Bright\folder-bright.png"
             x:Key="FolderItemImage" />

I've create a HierarchicalTemplate for treeview items like the following:

<HierarchicalDataTemplate ItemsSource="{Binding VisibleChildren}"
                          DataType="{x:Type st:StructureTreeItem}">
    <StackPanel Orientation="Horizontal">
        <TextBlock Text="{Binding ImageResourceKey}" />
        <Image x:Name="iIcon2" Source="{DynamicResource FolderItemImage}"/>
        <Image x:Name="iIcon"
               Source="{DynamicResource {Binding ImageResourceKey}}"/>
    </StackPanel>
</HierarchicalDataTemplate>

Now, when item is displayed:

• Textblock displays FolderItemImage
• First image is shown
• Second image is not shown.

---

The whole idea is to set item images to ones stored in resources, but the technique presented above unfortunately won't work and now I know, why:

<Image x:Name="iIcon3" Width="16" Height="16" Margin="0, 1, 3, 1" >
    <Image.Source>
        <DynamicResource ResourceKey="{Binding ImageResourceKey}" />
    </Image.Source>
</Image>

Result:

An unhandled exception of type 'System.Windows.Markup.XamlParseException' occurred in PresentationFramework.dll

Additional information: A 'Binding' cannot be set on the 'ResourceKey' property of type 'DynamicResourceExtension'. A 'Binding' can only be set on a DependencyProperty of a DependencyObject.

So I have to rephrase my question: how can I convert some data (a resource key, perhaps?) stored in model to a dynamic resource? It has to be a dynamic resource, because I'm certain it may change during runtime.

Answer 1

I write following markup extension to allow binding to resourceKey in general case

using System;
using System.ComponentModel;
using System.Globalization;
using System.Windows;
using System.Windows.Data;
using System.Windows.Markup;

namespace Mersoft.Mvvm.MarkupExtensions
{

    public class ResourceBinding : MarkupExtension
    {
        #region Helper properties

        public static object GetResourceBindingKeyHelper(DependencyObject obj)
        {
            return (object)obj.GetValue(ResourceBindingKeyHelperProperty);
        }

        public static void SetResourceBindingKeyHelper(DependencyObject obj, object value)
        {
            obj.SetValue(ResourceBindingKeyHelperProperty, value);
        }

        // Using a DependencyProperty as the backing store for ResourceBindingKeyHelper.  This enables animation, styling, binding, etc...
        public static readonly DependencyProperty ResourceBindingKeyHelperProperty =
            DependencyProperty.RegisterAttached("ResourceBindingKeyHelper", typeof(object), typeof(ResourceBinding), new PropertyMetadata(null, ResourceKeyChanged));

        static void ResourceKeyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            var target = d as FrameworkElement;
            var newVal = e.NewValue as Tuple<object, DependencyProperty>;

            if (target == null || newVal == null)
                return;

            var dp = newVal.Item2;

            if (newVal.Item1 == null)
            {
                target.SetValue(dp, dp.GetMetadata(target).DefaultValue);
                return;
            }

            target.SetResourceReference(dp, newVal.Item1);

        }

        #endregion

        public ResourceBinding()
        {

        }

        public ResourceBinding(string path)
        {
            this.Path = new PropertyPath(path);
        }

        public override object ProvideValue(IServiceProvider serviceProvider)
        {
            var provideValueTargetService = (IProvideValueTarget)serviceProvider.GetService(typeof(IProvideValueTarget));
            if (provideValueTargetService == null)
                return null;

            if (provideValueTargetService.TargetObject != null &&
                provideValueTargetService.TargetObject.GetType().FullName == "System.Windows.SharedDp")
                return this;


            var targetObject = provideValueTargetService.TargetObject as FrameworkElement;
            var targetProperty = provideValueTargetService.TargetProperty as DependencyProperty;
            if (targetObject == null || targetProperty == null)
                return null;



            var binding = new Binding();

            #region binding

            binding.Path = this.Path;
            binding.XPath = this.XPath;
            binding.Mode = this.Mode;
            binding.UpdateSourceTrigger = this.UpdateSourceTrigger;
            binding.Converter = this.Converter;
            binding.ConverterParameter = this.ConverterParameter;
            binding.ConverterCulture = this.ConverterCulture;

            if (this.RelativeSource != null)
                binding.RelativeSource = this.RelativeSource;

            if (this.ElementName != null)
                binding.ElementName = this.ElementName;

            if (this.Source != null)
                binding.Source = this.Source;

            binding.FallbackValue = this.FallbackValue;

            #endregion

            var multiBinding = new MultiBinding();
            multiBinding.Converter = HelperConverter.Current;
            multiBinding.ConverterParameter = targetProperty;

            multiBinding.Bindings.Add(binding);

            multiBinding.NotifyOnSourceUpdated = true;

            targetObject.SetBinding(ResourceBindingKeyHelperProperty, multiBinding);

            return null;

        }


        #region Binding Members

        /// <summary> The source path (for CLR bindings).</summary>
        public object Source
        {
            get;
            set;
        }

        /// <summary> The source path (for CLR bindings).</summary>
        public PropertyPath Path
        {
            get;
            set;
        }

        /// <summary> The XPath path (for XML bindings).</summary>
        [DefaultValue(null)]
        public string XPath
        {
            get;
            set;
        }

        /// <summary> Binding mode </summary>
        [DefaultValue(BindingMode.Default)]
        public BindingMode Mode
        {
            get;
            set;
        }

        /// <summary> Update type </summary>
        [DefaultValue(UpdateSourceTrigger.Default)]
        public UpdateSourceTrigger UpdateSourceTrigger
        {
            get;
            set;
        }

        /// <summary> The Converter to apply </summary>
        [DefaultValue(null)]
        public IValueConverter Converter
        {
            get;
            set;
        }

        /// <summary>
        /// The parameter to pass to converter.
        /// </summary>
        /// <value></value>
        [DefaultValue(null)]
        public object ConverterParameter
        {
            get;
            set;
        }

        /// <summary> Culture in which to evaluate the converter </summary>
        [DefaultValue(null)]
        [TypeConverter(typeof(System.Windows.CultureInfoIetfLanguageTagConverter))]
        public CultureInfo ConverterCulture
        {
            get;
            set;
        }

        /// <summary>
        /// Description of the object to use as the source, relative to the target element.
        /// </summary>
        [DefaultValue(null)]
        public RelativeSource RelativeSource
        {
            get;
            set;
        }

        /// <summary> Name of the element to use as the source </summary>
        [DefaultValue(null)]
        public string ElementName
        {
            get;
            set;
        }


        #endregion

        #region BindingBase Members

        /// <summary> Value to use when source cannot provide a value </summary>
        /// <remarks>
        ///     Initialized to DependencyProperty.UnsetValue; if FallbackValue is not set, BindingExpression
        ///     will return target property's default when Binding cannot get a real value.
        /// </remarks>
        public object FallbackValue
        {
            get;
            set;
        }

        #endregion



        #region Nested types

        private class HelperConverter : IMultiValueConverter
        {
            public static readonly HelperConverter Current = new HelperConverter();

            public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
            {
                return Tuple.Create(values[0], (DependencyProperty)parameter);
            }
            public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
            {
                throw new NotImplementedException();
            }
        }

        #endregion
    }
}

Usage

<Image Source="{local:ResourceBinding ImageResourceKey}"/>