Modify content of resource - wpf

Question

I have a UserControl that contains a TabControl that has an ItemTemplate that in turn has a Button. I want the user be able to change the content of that button, e.g. change it from TextBlock to Image.
A solution I thought of was to set the button's content from the Resources of the UserControl and overwrite the Resource by setting it on the ResourceDictionary of the entailing Window. Of course that does not work as StaticResource always resolves to the "closest" instance it can find.
I then thought of modifying the resource in the constructor of my UserControl, depending on some property. But it seems, one cannot change a resource. Below is a close sample showing the idea with a simple ListBox in a Window in which I try to change "What" to "How".
How would you approach this?
<Window.Resources>
<TextBlock x:Key="key" Text="What: " x:Shared="false" />
</Window.Resources>
<Grid Margin="10">
<ListBox Name="lbTodoList" HorizontalContentAlignment="Stretch">
<ListBox.ItemTemplate>
<DataTemplate>
<StackPanel Orientation="Horizontal">
<StaticResource ResourceKey="key" />
<TextBlock Text="{Binding}" />
</StackPanel>
</DataTemplate>
</ListBox.ItemTemplate>
</ListBox>
</Grid>
public partial class MainWindow : Window
{
public MainWindow()
{
InitializeComponent();
TextBlock tb = FindResource("key") as TextBlock;
tb.Text = "How: ";
List<string> items = new List<string>();
items.Add("Item 1");
items.Add("Item 2");
items.Add("Item 3");
lbTodoList.ItemsSource = items;
}
}

Answer 1

Instead of trying to override a resource, you should just treat it like any other XAML data-binding templating issue.
Instead of:
<StaticResource ResourceKye="key" />
Do something like this:
<TextBlock Text="{Binding LabelText}" />
In your UserControl set the default value of LabelText to "What:" and then allow the user to override the value. The data binding will take care of the rest.
If you want to have more dynamic content, then use a ContentControl instead and have properties for Content, ContentTemplate, and even ContentTemplateSelector depending on what you need to do.
<ContentControl
Content="{Binding MyContent}"
ContentTemplate="{Binding MyContentTemplate}"
ContentTemplateSelector="{Binding MyContentTemplateSelector}" />
This opens up a lot of flexibility.