Markup Extensions in WPF/Silverlight

Question

Has anyone ever created a custom markup extension in WPF or Silverlight? When would you ever want or need to do this? Any tips or sources on how to do it?

Answer 1

An example would be for Localization:

A simple and effective way to localize application resources is to write a custom MarkupExtension that provides a localized value. The extension takes a parameter that is a unique resource key... [and then] looks up the value from a generic resource provider.

Note: You can not write custom markup extensions in silverlight.

Answer 2

Yes it is handy and I have created one myself. I created a markup extension called EvalBinding that takes a set of bindings as children and a C# evaluation string. It evaluates the C# to process the values from the child bindings so that I do not need to create many simple TypeConverter classes.

For example I can do this...

<EvalBinding Eval="(this[0] > this[1] ? 'GT' : 'LTE')">
    <Binding ElementName="element1" Path="Size"/>
    <Binding ElementName="element2" Path="Size"/>
<EvalBinding>

Where this is a reference to the array of child binding results.

For resources on implementing a MarkupExtension...

MSDN

Josh Smith Blog Entry

Rob Relyea Blog Entry

Answer 3

Hooray!!

This is implemented in Silverlight 5!!

And furthermore, now it's a generic interface instead of a class!!

Check it out.

Read this for an example.