Activator 2: XAML and Generics Day

First of all, you didn’t miss part one. Just thought it was a funny reference.

I’m currently working on a new release of WPF Contrib. In my TaskDialog class, I used a few classes I derived from ItemsControl (which I’m considering deleting in this version, but that’s another debate). The reason I created them was so that you could add any item to the ItemsControl and it would be wrapped in some specific ContentControl, such as a Button. But if you added a Button, it would know that it doesn’t need wrapping (this is what the IsItemItsOwnContainerOverride() and GetContainerForItemOverride() methods do).

I had three classes which were exactly the same, aside from the type of the ContentControl. Generics would very much fit here, but unfortunately, XAML has a very limited support for it (it seems the new .NET 4.0 XAML stack will improve this, and much more). As an exercise I decided to try and extend XAML so it will support generics – on a basic level.

XAML can be extended using MarkupExtensions. These are classes that have a single method, ProvideValue(), that when XAML (or BAML) is loaded, create objects according to some custom logic. You’re surely familiar with the TypeExtension (x:Type), which returns a System.Type from the specified qualified string. What I’ve done is create two new extensions:

• An alternative TypeExtension that supports type arguments. The following is equivalent to typeof(List<List<KeyValuePair<string, int>>>):

{t:Type gen:List, {t:Type gen:List, {t:Type gen:KeyValuePair, sys:String, sys:Int32}}}

Note that you don’t have to (but can) specify gen:List`1. The number of type arguments will be inferred. The constructors support up to 4 type arguments. In the unlikely case that you need more than that, you can use the unabbreviated form and specify the arguments as sub-elements.

• An ActivatorExtension that accepts a type (either generic or not) and a list of property-value pairs. For example, this will create an instance of ItemsControl<CheckBox> (a generic variation of ItemsControl I have in the demo), and assign some of its properties, including a binding:

<t:ActivatorExtension x:Name="ic" Type="{t:Type t:ItemsControl, {t:Type CheckBox}}">
    <t:Setter Name="Name" Value="ic" />
    <t:Setter Name="Background" Value="{Binding ElementName=tb, Path=Text}" />
    <t:Setter Name="ItemsSource">
        <x:ArrayExtension Type="{x:Type sys:String}">
            <sys:String>one</sys:String>
            <sys:String>two</sys:String>
            <sys:String>three</sys:String>
            <sys:String>four</sys:String>
        </x:ArrayExtension>
    </t:Setter>
</t:ActivatorExtension>

There are, however, a few severe limitations:

• The Name property has no effect, either on the extension or as a property. Not only does the XAML compiler not generate a member for it in the partial class, but the FrameworkElement.FindName() method also doesn’t work. In templates, however, FindName() will return the extension itself, not the constructed object (if you add the x:Name attribute to it). This can be a big problem if you want to use an activated object as a template part.
• Special treatment has been given for bindings to work in property values. Other expressions, such as DynamicResource or TemplateBinding, will not.
• XamlWriter.Save() fails to save an object that contains an activator with a generic type (which makes sense since XAML does not support generics, and after it’s loaded, the MarkupExtension is gone.)
• The ActivatorExtension cannot be used as the root of a template (fails at compile time). This can be easily circumvented though, by surrounding it with a Decorator.

As I said in the beginning, this was just an experiment I wanted to share, and in no way should be viewed as a production-quality solution. I will, however, be adding the TypeExtension to the upcoming drop of WPF Contrib. It can be used to specify DataTemplate resource keys for generic types.

Attachment: activator-demo.zip