Resource dictionary in User control - wpf

Question

I have a user control and it uses resource dictionaries. In that user control, there is another user control which uses same resource dictionaries.what I want to know is whether wpf actually loads it twice and if yes, is there any perfomance impact. Is there any better way to do this.
Thanks in advance.

Answer 1

Interesting question. I was intrigged enough to investigate. It appears that WPF loads a new ResourceDirectionary (and all resources defined and the dictionary which are used) for each appearance of element.
Take a look at the following code:
ViewModel:
public class Person
{
public string name { get; set; }
public int age { get; set; }
public Person() { }
}
Resource (Dictionary1.xaml):
<ResourceDictionary
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:so="clr-namespace:SO"
>
<so:Person x:Key="m" name="Methuselah" age="969" />
</ResourceDictionary>
View:
<Window
x:Class="SO.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:so="clr-namespace:SO"
Height="200" Width="300"
Title="SO Sample"
>
<Window.Resources>
<ResourceDictionary Source="Dictionary1.xaml" />
</Window.Resources>
<StackPanel DataContext={StaticResource m}>
<UserControl>
<UserControl.Resources>
<ResourceDictionary Source="Dictionary1.xaml" />
</UserControl.Resources>
<TextBlock x:Name="inner" DataContext="{StaticResource m}" Text="{Binding Path=name}" />
</UserControl>
<TextBlock x:Name="outer" Text="{Binding Path=name}" />
<Button Click="Button_Click">Change</Button>
</StackPanel>
</Window>
Put a breakpoint at the Person() constructor and notice the object is instantiated twice. Or, make Person implementing INotifyPropertyChange, and add the following code for Button_Click:
private void Button_Click( object sender, RoutedEventArgs e ) {
Person innerPerson = this.inner.DataContext as Person;
Person outerPerson = this.outer.DataContext as Person;
innerPerson.name = "inner person";
outerPerson.name = "outer person";
}
If you want to have a single instance of each resource, have the reousrces in the element of app.xaml file.