How can I build a string resource from other string resources in WPF?

Question

Here's my 'sample code', including what I'm trying to do. Obviously, it doesn't work at the moment, but is there any way I can make it work?

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:System="clr-namespace:System;assembly=mscorlib"
    >
    <System:String x:Key="ProductName">Foo</System:String>
    <System:String x:Key="WindowTitle">{ProductName} + Main Window</System:String>
</ResourceDictionary>

Answer 1

The only way to add a computed string to a ResourceDictionary in this way is to create a MarkupExtension. Your MarkupExtension would be used like this:

<ResourceDictionary ...>
  <sys:String x:Key="ProductName">Foo</sys:String>
  <local:MyStringFormatter
    x:Key="WindowTitle"
    StringFormat="{0} Main Window"
    Arg1="{StaticResource ProductName}" />
</ResourceDictionary>

This assumes you have created a MarkupExtension subclass in your "local" namespace named MyStringFormatterExtension that has properties named "StringFormat", "Arg1", "Arg2", etc, and has a ProvideValue() method that does the obvious.

Note that as Aran points out, a Binding using StringFormatter would be a more common way to achieve the same effect, and is generally the better design. The tradeoff is it would not allow the result to be used as part of a ResourceDictionary.