Using a Markup Extension as value of property Setter in XAML. {0} is not valid for Setter.Value. The only supported MarkupExtension types are

Question

Below is a very small example how to get this exception . This is already a known issue (for more than 7 years!). Is there already a solution for that?

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Exception

FooExtension is not valid for Setter.Value. The only supported MarkupExtension types are
DynamicResourceExtension and BindingBase or derived types.

C# Class

public class FooExtension : MarkupExtension
{
    public string Key { get; set; }

    public override object ProvideValue(IServiceProvider serviceProvider)
    {
        return new SolidColorBrush((Color)ColorConverter.ConvertFromString(Key));
    }
}

This is not the original class/MarkupExtension I'm using, but the logic (return SolidColorBrush ) are identical and it gives exactly the same error/behavior.

• Exception during design time
• Exception in designer gone after rebuild solution
• No problems in runtime

XAML usage

Background="{extensions:Foo Key=#FFF37C21}"

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The extension worked until yesterday for more than 1 year! Including every Windows, VisualStudio or any other update! And now since 1 day I got this bug behavior during the design time!

Im using:

• VS15.7.3
• .NET Framework 4.7.02556

Sources:

• https://social.msdn.microsoft.com/Forums/vstudio/en-US/931d7bff-90b6-4a70-bb0b-3a097e1301a1/net-40-breaking-change-using-a-markup-extension-as-value-of-property-setter-in-xaml-style?forum=wpf

• .NET 4.0 Breaking Change - Using a Markup Extension as Value of Property Setter in XAML Style

• {0} is not valid for Setter.Value. The only supported MarkupExtension types are DynamicResourceExtension and BindingBase or derived types

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Binding Workaround (not working)

On some pages it is said that you should use binding . But there I got the same error/behavior!

btw. I use this markup extension in a Storyboard , so I can't return a binding anyway.

    public override object ProvideValue(IServiceProvider serviceProvider)
    {            
        Binding binding = new Binding();
        binding.Source = new SolidColorBrush((Color)ColorConverter.ConvertFromString(Key));

        return binding.ProvideValue(serviceProvider);
    }

Answer 1

I solved the problem! I was a kind of XAML Resources relationship. I defined a ResourceDictionary inside of App.xaml which was already defined in the AssemblyInfo.cs as a XmlnsPrefix / XmlnsDefinition !