Why the StaticResource cannot be resolved in this case?

Question

I have got an exception "Cannot find resource named 'mrg'. Resource names are case sensitive." when I try to do the following:

MainWindow.xaml:

<Window.Resources>
  <Thickness Left="0"
             Right="1"
             Bottom="2"
             Top="3"
             x:Key="mrg" />
</Window.Resources>
<Grid>
  <ItemsControl ItemsSource="{Binding}">
    <ItemsControl.ItemTemplate>
      <DataTemplate>
        <local:UserControl1 />
      </DataTemplate>
    </ItemsControl.ItemTemplate>
  </ItemsControl>
</Grid>

MainWindow.xaml.cs:

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
        List<string> source = new List<string>()
        {
            "item1",
            "item2",
            "item3",
            "item4",
            "item5",
        };
        DataContext = source;
    }
}

and UserControl1.xaml:

<Grid>
    <TextBlock Text="{Binding}" Margin="{StaticResource mrg}" />
</Grid>

According to the msdn article:

Static resource lookup behavior

1. The lookup process checks for the requested key within the resource dictionary defined by the element that sets the property.

2. The lookup process then traverses the logical tree upward, to the parent element and its resource dictionary. This continues until the root element is reached.

3. Next, application resources are checked. Application resources are those resources within the resource dictionary that is defined by the Application object for your WPF application.

So the resource had to be found because of step 2. But, as I can see in the Locals window when exception is catched, the UserControl1.Parent == null.

I'm confused in this problem. The way I can solve it is to put the resource to the Application level.

My question is: why the StaticResource connot be found ?

Answer 1

The DataTemplate forms a logical tree of its own, which is disconnected from the logical tree of the ItemsControl. Hence the lookup by traversing the logical tree won't find the resource.

I wasn't able to find a reference in MSDN, just this article on CodeProject, where it reads:

The elements that are part of an expanded template, hereafter referred to as "template elements", form their own logical tree which is disconnected from the logical tree of the object for which they were created.

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Using DynamicResource instead of StaticResource will overcome the problem. However i can't tell exactly why. Maybe an explanation can be found in the Static resource lookup behavior and Dynamic resource lookup behavior sections in Static and Dynamic Resources, but i'm not sure.