WPF AttachedProperties

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03 February

An AttachedProperty is similar to a DependencyProperty, except that the owner of the value is not the UIElement itself but an external UIElement. For instance, the Canvas.Left and Canvas.Top are AttachedProperties.

In Xaml, you usually assign an AttachedProperty the following way:

<Button Canvas.Left="12" Canvas.Top="44">ButtonText<Button>

Now if you want to create an own AttachedProperty, you simply use the propdp code snippet to create a DependencyProperty first:

public int Order
{
get { return (int)GetValue(OrderProperty); }
set { SetValue(OrderProperty, value); }
}

// Using a DependencyProperty as the backing store for Order. This enables animation, styling, binding, etc...
public static readonly DependencyProperty OrderProperty =
DependencyProperty.Register("Order", typeof(int), typeof(MyContainer), new UIPropertyMetadata(0));

To change the DependencyProperty to an AttachedProperty, simply replace DependencyProperty.Register to DependencyProperty.RegisterAttached.

But if you try to attach this Property to a control, it doesn't work!

You need to Add static Getters and Setters instead of the local "Offset properties in the following way:

public static int GetOrder(UIElement e)
{
if (e == null) throw new ArgumentNullException("e");
return (int)e.GetValue(OrderProperty);
}

public static void SetOrder(UIElement e, int value)
{
if (e == null) throw new ArgumentNullException("e");
e.SetValue(OrderProperty, value);
}

And finally, you have created an AttachedProperty you can use with other Controls.

Here is the complete code:

[

ContentProperty("Items")]
public class MyContainer : ItemsControl
{
    static MyContainer()
    {
        DefaultStyleKeyProperty.OverrideMetadata(typeof(MyContainer), new FrameworkPropertyMetadata(typeof(MyContainer)));
    }

    public static int GetOrder(UIElement e)
    {
        if (e == null) throw new ArgumentNullException("e");
        return (int)e.GetValue(OrderProperty);
    }

    public static void SetOrder(UIElement e, int value)
    {
        if (e == null) throw new ArgumentNullException("e");
        e.SetValue(OrderProperty, value);
    }

    // Using a DependencyProperty as the backing store for Order. This enables animation, styling, binding, etc...
    public static readonly DependencyProperty OrderProperty =
        DependencyProperty.RegisterAttached("Order", typeof(int), typeof(MyContainer), new FrameworkPropertyMetadata((int)0));
    }

Now you can use it the following way:

Window x:Class="AttachedProp.Window1"
    xmlns=http://schemas.microsoft.com/winfx/2006/xaml/presentation
    xmlns:x=http://schemas.microsoft.com/winfx/2006/xaml
    xmlns:m="clr-namespace:AttachedProp"
    Title="Window1" Height="300" Width="300">
    <Grid>
        <m:MyContainer>
            <Button Canvas.Bottom="3" m:MyContainer.Order="3">OK</Button>
        </m:MyContainer>
    </Grid>
</Window>

Now if you want to use the AttachedProperty within a ControlTemplate you cannot simply use TemplateBinding. The following snippet shows the correct syntax.

Notice the surrounding bracket on the Path property which is mandantary as well as the appearance of Path. Thus you cannot use the following shortcut:

TextBlock Text="{Binding local:MyContainer.Order RelativeSource={RelativeSource TemplatedParent}}" Background="Red" MinHeight="24" />

12:34 PM | Blog it | Development

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