原文 How do you create a DynamicResourceBinding that supports Converters, StringFormat?

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In the past I've resorted to using several hard-to-follow/hard-to-implement hacks and workarounds to achieve what I feel is missing functionality. That's why to me this is close to (but not quite) the Holy Grail of solutions... being able to use this in XAML exactly like you would a binding, but having the source be a DynamicResource.

Note: I say 'Binding' but technically it's a DynamicResourceExtension on which I've defined a Converter, ConverterParameter, ConverterCulture and StringFormat but which does use a Binding internally. As such, I named it based on its usage model, not its actual type.

The key to making this work is a unique feature of the Freezable class:

If you add a Freezable to the resources of a FrameworkElement, any DependencyProperties on that Freezable which are set to a DynamicResource will resolve those resources relative to that FrameworkElement's position in the Visual Tree.

Using that bit of 'magic sauce', the trick is to set a DynamicResource on a Freezable's DependencyProperty, add the Freezable to the resource collection of the target FrameworkElement, then use that Freezable's DependencyProperty as the source for an actual binding.

That said, here's the solution.

DynamicResourceBinding
public class DynamicResourceBinding : DynamicResourceExtension
{
    #region Internal Classes

        private class DynamicResourceBindingSource : Freezable
        {
            public static readonly DependencyProperty ResourceReferenceExpressionProperty = DependencyProperty.Register(
                nameof(ResourceReferenceExpression),
                typeof(object),
                typeof(DynamicResourceBindingSource),
                new FrameworkPropertyMetadata());

            public object ResourceReferenceExpression
            {
                get { return GetValue(ResourceReferenceExpressionProperty); }
                set { SetValue(ResourceReferenceExpressionProperty, value); }
            }

            protected override Freezable CreateInstanceCore()
            {
                return new DynamicResourceBindingSource();
            }
        }

    #endregion Internal Classes

    public DynamicResourceBinding(){}

    public DynamicResourceBinding(string resourceKey)
    : base(resourceKey){}

    public IValueConverter Converter          { get; set; }
    public object          ConverterParameter { get; set; }
    public CultureInfo     ConverterCulture   { get; set; }
    public string          StringFormat       { get; set; }

    public override object ProvideValue(IServiceProvider serviceProvider)
    {
        // Get the expression representing the DynamicResource
        var resourceReferenceExpression = base.ProvideValue(serviceProvider);

        // If there's no converter, nor StringFormat, just return it (Matches standard DynamicResource behavior}
        if(Converter == null && StringFormat == null)
            return resourceReferenceExpression;

        // Create the Freezable-based object and set its ResourceReferenceExpression property directly to the 
        // result of base.ProvideValue (held in resourceReferenceExpression). Then add it to the target FrameworkElement's
        // Resources collection (using itself as its key for uniqueness) so it participates in the resource lookup chain.
        var dynamicResourceBindingSource = new DynamicResourceBindingSource(){ ResourceReferenceExpression = resourceReferenceExpression };

        // Get the target FrameworkElement so we have access to its Resources collection
        // Note: targetFrameworkElement may be null in the case of setters. Still trying to figure out how to handle them.
        // For now, they just fall back to looking up at the app level
        var targetInfo = (IProvideValueTarget)serviceProvider.GetService(typeof(IProvideValueTarget));
        var targetFrameworkElement = targetInfo.TargetObject as FrameworkElement;
        targetFrameworkElement?.Resources.Add(dynamicResourceBindingSource, dynamicResourceBindingSource);

        // Now since we have a source object which has a DependencyProperty that's set to the value of the
        // DynamicResource we're interested in, we simply use that as the source for a new binding,
        // passing in all of the other binding-related properties.
        var binding = new Binding()
        {
            Path               = new PropertyPath(DynamicResourceBindingSource.ResourceReferenceExpressionProperty),
            Source             = dynamicResourceBindingSource,
            Converter          = Converter,
            ConverterParameter = ConverterParameter,
            ConverterCulture   = ConverterCulture,
            StringFormat       = StringFormat,
            Mode               = BindingMode.OneWay
        };

        // Now we simply return the result of the new binding's ProvideValue
        // method (or the binding itself if the target is not a FrameworkElement)
        return (targetFrameworkElement != null)
            ? binding.ProvideValue(serviceProvider)
            : binding;
    }
}
And just like with a regular binding, here's how you use it (assuming you've defined a 'double' resource with the key 'MyResourceKey')...

<TextBlock Text="{drb:DynamicResourceBinding ResourceKey=MyResourceKey, Converter={cv:MultiplyConverter Factor=4}, StringFormat='Four times the resource is {0}'}" />
or even shorter, you can omit 'ResourceKey=' thanks to constructor overloading to match how 'Path' works on a regular binding...

<TextBlock Text="{drb:DynamicResourceBinding MyResourceKey, Converter={cv:MultiplyConverter Factor=4}, StringFormat='Four times the resource is {0}'}" />
Awesomesausage! (Well, ok, AwesomeViennasausage thanks to the small 'setter' caveat I uncovered after writing this. I updated the code with the comments.)

As I mentioned, the trick to get this to work is using a Freezable. Thanks to its aforementioned 'magic powers' of participating in the Resource Lookup relative to the target, we can use it as the source of the internal binding where we have full use of all of a binding's facilities.

Note: You must use a Freezable for this to work. Inserting any other type of DependencyObject into the target FrameworkElement's resources--ironically even including another FrameworkElement--will resolve DynamicResources relative to the Application and not the FrameworkElement where you used the DynamicResourceBinding since they don't participate in localized resource lookup (unless they too are in the Visual Tree of course.) As a result, you lose any resources which may be set within the Visual Tree.

The other part of getting that to work is being able to set a DynamicResource on a Freezable from code-behind. Unlike FrameworkElement (which we can't use for the above-mentioned reasons) you can't call SetResourceReference on a Freezable. Actually, I have yet to figure out how to set a DynamicResource on anything but a FrameworkElement.

Fortunately, here we don't have to since the value provided from base.ProvideValue() is the result of such a call anyway, which is why we can simply set it directly to the Freezable's DependencyProperty, then just bind to it.

So there you have it! Binding to a DynamicResource with full Converter and StringFormat support.

For completeness, here's something similar but for StaticResources...

StaticResourceBinding
public class StaticResourceBinding : StaticResourceExtension
{
    public StaticResourceBinding(){}

    public StaticResourceBinding(string resourceKey)
    : base(resourceKey){}

    public IValueConverter Converter          { get; set; }
    public object          ConverterParameter { get; set; }
    public CultureInfo     ConverterCulture   { get; set; }
    public string          StringFormat       { get; set; }

    public override object ProvideValue(IServiceProvider serviceProvider)
    {
        var staticResourceValue = base.ProvideValue(serviceProvider);

        if(Converter == null)
            return (StringFormat != null)
                ? string.Format(StringFormat, staticResourceValue)
                : staticResourceValue;

        var targetInfo               = (IProvideValueTarget)serviceProvider.GetService(typeof(IProvideValueTarget));
        var targetFrameworkElement   = (FrameworkElement)targetInfo.TargetObject;
        var targetDependencyProperty = (DependencyProperty)targetInfo.TargetProperty;

        var convertedValue = Converter.Convert(staticResourceValue, targetDependencyProperty.PropertyType, ConverterParameter, ConverterCulture);

        return (StringFormat != null)
            ? string.Format(StringFormat, convertedValue)
            : convertedValue;
    }
}
Anyway, that's it! I really hope this helps other devs as it has really simplified our control templates, especially around common border thicknesses and such.

Enjoy!