Creating instances of resources?

Question

I'm brand spanking new to WPF and am trying to play around with projects to better understand what I'm reading.

My understanding of a resource is that it is the instance, you can't use it like a factory and create instances of it. For example, a XAML-defined rectangle. You can reference it, but you can't have numerous instances of it all over the surface.

In WPF, what would be the way to do that? If I define a Rectangle as a resource with specific properties and wanted to have multiple instances of that within a dynamically-generated grid, how should I be going about it? Or is there a different way I should be trying to do this?

Purely academic exercise with no real-world application.

Answer 1

To get multiple instances replicated across a grid or listbox, you need to set the data template to define the UI controls for each row of data, and then databind the grid or listbox to a collection of data that determines how many rows and the individual field values.

Key term for you to research first: data template.

Answer 2

Actually there's nothing about resources in particular that prevents you from using it multiple times. A perfect example of this is brush resources, style resources, etc. You define them in XAML and the XAML parser creates a single instance of the resources and stores them in the resource dictionary and these brushes, styles, etc can be used as property values many times even though only a single instance of the resource was created.

But having said that, as you noted, you can't really define a Rectangle resource and use it multiple times in the visual tree. This has nothing to do with the fact that it's a resource, but rather it has to do with the fact that a FrameworkElement cannot be a child of more than one parent element.

So what we have instead is called "templates". These tell WPF how to create an element tree but does not actually create the tree until you instantiate the template. Below is an example.

<UserControl>
    <ItemsControl ItemsSource="{Binding WholeBunchOfItems}">
        <ItemsControl.ItemTemplate>
            <DataTemplate>
                <Grid>
                    <Rectangle Fill="Yellow" />
                    <ContentPresenter Content="{Binding}" />
                </Grid>
            </DataTemplate>
        </ItemsControl.ItemTemplate>
    </ItemsControl>
</UserControl>

In this example I've bound an ItemsControl to a collection of some sort. For each item in the collection, the ItemsControl will use my DataTemplate to render the item. Within a DataTemplate you can use data binding to access the current item.

I would suggest reading up on MSDN about ControlTemplate, DataTemplate, and Style. These are all important concepts in WPF/Silverlight.