Resource dictionary in User control

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.