Converter as a Markup Extension

I was stuck on a gnarly WPF problem the other day (to do with inheritance context for 3rd party controls and UI elements not in the logical or visual tree)*. Well fortunately for me, I happen to be just an IM away from some the smartest WPF people in ANZ, not the least of which is Mabster

Here's a (trivialised) snippet that represents the code I sent him:

<TextBlock
    Text="{Binding Foo, Converter={local:DebugConverter}}" />

His first comment was 'Wait, the converter is a MarkupExtension?'

It sure is - It's something I've done since I read this article by Dr Wpf a couple of years ago...

The converter itself looks like this:

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

namespace SomeNamespace
{
    public class DebugConverter : MarkupExtension, IValueConverter
    {
        public override object ProvideValue(IServiceProvider serviceProvider)
        {
            return this;
        }

        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            //  No conversion needed - just want to break here to see what's coming into the Binding
            return value;
        }

        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
        {
            throw new NotImplementedException();
        }
    }
}

Basically, if the converter object is a markup extension, then you remove the ceremony for declaring an instance somewhere in the xaml resources, and then having to refer to it by key when you use it - you can just create a fresh instance inline in the Binding...

In other words, we change from:

<Window.Resources>
    <local:DebugConverter x:Key="debugConverter" />
</Window.Resources>

<TextBlock
    Text="{Binding Foo, Converter={StaticResource debugConverter}}" />

to (as we saw earlier):

<TextBlock
    Text="{Binding Foo, Converter={local:DebugConverter}}" />

It's certainly cleaner xaml, for only a minor investment in the code (we just need to implement an override for ProvideValue)... so what's the catch?

Well, if we have a converter being used in multiple places across a view, and from a performance perspective, we just wanted to instantiate (and re-use) a single instance, then this isn't going to work for us.

But in the 99 times out of 100 where we don't care about that, this is a useful tool to have in your bag of tricks.

Enjoy :)

* Never did solve it - had to chuck in some code-behind, but that's not the end-of-the-world :)

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