WPF Quick Tip: Converters as MarkupExtensions

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Everybody's gotta love their IValueConverters, right? And not forgetting IMultiValueConverters too.

However, they're a bit of a pain when it comes to Xaml. I hate having to wonder all the way up to the resources section(!), create my converter, think of a name for the key and finally reference the converter through a StaticResource markup extension.

Another thing I hate is the parsing of Converter Parameters back into the appropriate type. Boo.

Not anymore with this quick tip. Hooray.

Take this MultiplyConverter (we've all written one of these):

public class MultiplyConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        double val = System.Convert.ToDouble(value);
        double factor = System.Convert.ToDouble(parameter);
        return val * factor;
    }

    public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        // left as an exercise for the reader
        // don't be a wimp, it's not hard
    }
}

Which might be used something like...


<local:MultiplyConverter x:Key="BarryTheConverter" />

<Rectangle Width="{Binding ElementName=otherElement, Path=(Canvas.Left), Converter={StaticResource BarryTheConverter}, ConverterParameter=-0.5}" />

Sort of yuck.

V2

I think it's a big improvement to forego the ConverterParameter and use a property on the Value Converter itself:

public class MultiplyConverter : IValueConverter
{
    public double Factor { get; set; }

    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        double val = System.Convert.ToDouble(value);
        return val * Factor;
    }

Which would be used like so...


<local:MultiplyConverter Factor="-0.5" x:Key="BarryTheConverter" />

<Rectangle Width="{Binding ElementName=otherElement, Path=(Canvas.Left), Converter={StaticResource BarryTheConverter}}" />

A minor improvement IMO. But better nonetheless, provided we don't need to create lots of converters for all the different 'Factors' we might use... unless... we implement the Converter as a markup extension.

V3

public class MultiplyConverter : MarkupExtension, IValueConverter
{
    public double Factor { get; set; }

    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        double val = System.Convert.ToDouble(value);
        return val * Factor;
    }

    public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        // left as an exercise for the reader
        // don't be a wimp, it's not hard
    }

    public override object ProvideValue(IServiceProvider serviceProvider)
    {
        return this;
    }
}

Which might be used like so



<Rectangle Width="{Binding ElementName=otherElement, Path=(Canvas.Left), Converter={local:MultiplyConverter Factor=-0.5}}" />

Much better and reads easier. Hooray!

Tags: WPF

Post By Josh Twist

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23:29
17 Apr 2009

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Comments:

Posted by Sven @ 18 Apr 2009 14:01
Sweet ! I see no reason not to "upgrade" all my existing converters to this state so I can choose whether to declare them as a resource or instantiate them inline withing the Converter={} clause.

I guess the disadvantage of this new instantiation method is that it creates a converter instance per Converter={} clause, right ?

Posted by josh @ 18 Apr 2009 23:48
Yes, if you think of that as a disadvantage. Of course, WPF is object heavy so any saving here (in terms of reducing the number of objects) is going to be negligible.

Posted by OAB @ 27 Apr 2009 07:26
Very cool. I had no idea it was that easy to write a markup extension. I will definitely be using this.

Posted by sinan @ 24 Jul 2009 12:18
hey guys,
There is far simpler method of doing V3.
I am not inheriting from MarkupExtension class also,
Regular value converter.
Just use tags!! Not inline coding with curly braces.
As the following:

<Window.Title>
<Binding>
<Binding.Converter>
<ConverterEx:MyConverter MyParam="Sinan" />
</Binding.Converter>
</Binding>
</Window.Title>

class MyConverter:IValueConverter
{
public string MyParam { get; set; }

public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
return MyParam;
}

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

Any ideas?

Posted by josh @ 25 Jul 2009 00:14
Hi sSinan,

I've always known of this approach but, to be brutally honest, I think it's even worse than using a StaticResource. I like the shorthand format that MarkupExtensions provide.

Thanks for posting though - it's a good point!

Josh

Posted by Oliver @ 08 Sep 2009 01:29
The MarkExtension worked brilliantly! Thank you.

Posted by Grant BlahaErath @ 15 Oct 2009 01:01
This is a great approach. You can define as many parameters as you need this way. No need to implement a tokenizer in your converter.

V2

V3

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