Markup extensions in EventSetter do not work

[mikernet]

• .NET Core Version: .NET 5
• Windows version: 21H1 19043.1237
• Does the bug reproduce also in WPF for .NET Framework 4.8?: N/A - library only supports netcore3+

Problem description:

While attempting to make my Method Binding Extension work with EventSetter, it turned out that setting the Handler property to a markup extension does not appear to work. The markup extension is never instantiated (so ProvideValue() is not called). Instead, the following exception is thrown on InitializeComponent():

System.Windows.Markup.XamlParseException: ''XAML Node Stream: Missing EndMember for 'WpfApp2.MainWindow.System.Windows.FrameworkElement.Resources' before EndObject.' 

Sample code:

    <Window.Resources>
        <Style x:Key="ButtonStyle" TargetType="Button">
            <EventSetter Event="Click" Handler="{s:MethodBinding OnButtonClick}" />
        </Style>
    </Window.Resources>

The issue is 100% NOT the XAML structure.

[vishalmsft]

@mikernet Markup extension only works for Dependency Property. EventSetter.Handler is not a Dependency Property. Hence, this won't work.
Getting this working is a Feature Request.

[mikernet]

Oh, oops...that cryptic error message threw me off, didn't even think to check if it was a dependency property. A nicer error message here would be helpful, me thinks.

[this-is-your-do]

@vishalmsft Thanks for taking a look, but there seems to be more going on here - I was able to successfully workaround this limitation by creating an empty derived class from EventSetter:
class EventSetter : System.Windows.EventSetter { }
And then simply using this derived class instead of the usual EventSetter in xaml:
<local:EventSetter Event="Click" Handler="{s:MethodBinding OnButtonClick}" />
Here, MethodBindingExtension is successfully called and I was able to get it (with some code changes) to return a working event handler as well.

Note, in particular, that this works despite Handler still not being a dependency property. Based on this and other evidence, I think markup extensions usually work just fine with regular properties. (Bindings don't, but not all markup extensions are bindings!)

Rather, it seems like there's something in xaml parsing that's special casing the EventSetter.Handler property in a way that prevents markup extensions from being used with it.

(Interestingly enough, if I were to change from EventSetter to local:EventSetter but not use an extension for the Handler property, things would break.)

[mikernet]

That makes more sense tbh...I could have sworn markup extensions worked for regular properties, but I've been out of WPF land for a few months so I'm a but rusty there, haha.

Just tested it and they do indeed work on normal properties.