Why does data binding to DynamicResource not work?

The following code does not work. How do I make it work?开发者_StackOverflow中文版

<Image Source="{DynamicResource {Binding VM.ImageKey}}" />

This is an incorrect usage of the DynamicResource MarkupExtension. Correct it would be:

<Image Source="{DynamicResource VM.ImageKey}" />

Assuming you have a resource with a key "VM.ImageKey" defined somewhere like this:

<Bla.Resources>
    <BitmapImage x:Key="VM.ImageKey" UriSource="C:\Uri\To\Image.jpg" />
</Bla.Resources>

However if you want to bind against some property form the current DataContext you must not use DynamicResource but Binding:

<Image Source="{Binding VM.ImageKey}" />

Assuming your current DataContext is an instance that has a property called VM wich again has a property called ImageKey wich is a derived type of ImageSource.

This behaviour is by design. Binding works only on dependency properties of dependency objects and MarkupExtension is not dependency object.

It cannot work since the DyamicResource is a MarkupExtension and not a dependency property. Databinding only works with dependendcy properties.

However, there is a semi smooth workaround. Create a DynamicTextBlock class that extends a TextBlock.

The xaml:

<TextBlock x:Class="Rebtel.Win.App.Controls.DynamicTextBlock"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"/>

The codebehind:

public partial class DynamicTextBlock : TextBlock
{
    public static readonly DependencyProperty TextKeyProperty = DependencyProperty.Register(
        "TextKey", typeof(string), typeof(DynamicTextBlock), new PropertyMetadata(string.Empty, OnTextKeyChanged));

    private static void OnTextKeyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        var me = ((DynamicTextBlock)d);
        if (e.NewValue != null)
        {
            me.TextKey = (string) e.NewValue;
        }
    }

    public string TextKey
    {
        set { SetResourceReference(TextProperty, value); }
    }

    public DynamicTextBlock()
    {
        InitializeComponent();
    }
}

Usage:

  <local:DynamicTextBlock TextKey="{Binding TextKeyProperty}" />

The TextKeyProperty then returns a key that can be found in the ResourceDictionary. The same approach can be taken for an Image and its Source property.

If you want to specify the resource key dynamically you should specify it using the ResourceKey markup extension - not sure if it supports bindings in the way you want it to however. See here for more details.

I'm assuming that in this case, VM.ImageKey refers to a property on a data source whose value you wish to use as a resource dictionary key. The idea being that your data source can determine which image is used by supplying a resource key. (Most of the other answers on this page are unhelpful, because they have unfortunately missed what you're trying to do, assume that you want to use the literal text "VM.ImageKey" as a resource key, which I'm pretty sure isn't what you're asking for.)

This doesn't seem to be supported, but there's another approach that can enable you to select an image resource through a key determined by databinding: https://stackoverflow.com/a/20566945/497397