#166 – You Can Override Metadata for Any Dependency Property

December 25, 2010 1 Comment

Recall that you can set a dependency property value on a DependencyObject for any dependency property, not just the properties that the class implements or inherits.

Since any dependency property can be attached to any dependency object, you can override metadata for any dependency property in a class.

Here’s an example where we override the Grid.Row and Grid.Column properties in a class that derives from Button.  If this ButtonLoner object is defined in a Grid in XAML, it will automatically appear in Row 1, Col 1, rather than Row 0, Col 0.

    public class ButtonLoner : Button
    {
        static ButtonLoner()
        {
            Grid.RowProperty.OverrideMetadata(typeof(ButtonLoner), new FrameworkPropertyMetadata(1));
            Grid.ColumnProperty.OverrideMetadata(typeof(ButtonLoner), new FrameworkPropertyMetadata(1));
        }
    }

Here’s an example of the button being used:

    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition/>
            <RowDefinition/>
        </Grid.RowDefinitions>
        <Grid.ColumnDefinitions>
            <ColumnDefinition/>
            <ColumnDefinition/>
        </Grid.ColumnDefinitions>
        <app:ButtonLoner x:Name="btnLoner" Content="Loner" Width="100" Height="25"/>
    </Grid>

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#165 – Overriding Metadata Can Result in Merged Metadata

December 24, 2010 Leave a comment

If you override metadata for a dependency property that a class inherits from its parent, you can specify one or more things to override:

  • The default value
  • The PropertyChangedCallback
  • The CoerceValueCallback
  • Other flags specified for UIPropertyMetadata or FrameworkPropertyMetadata

If you don’t specify all of these things, however, the items that you do not specify will be inherited from the parent.

For example, we created a child class of Slider and overrode metadata to provide a new default value for the Maximum property.  But the coercion behavior of Maximum in our new class behaves as it does for Slider.Maximum.  (Maximum can’t be set below Minimum).

Because we didn’t specify a CoerceValueCallback, the existing coercion logic is used.

When you do specify new metadata:

  • New default value replaces the existing one
  • PropertyChangedCallback is merged with the existing one  (both are invoked)
  • CoerceValueCallback replaces the existing one

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#164 – Overriding Metadata for an Inherited Dependency Property

December 23, 2010 3 Comments

When deriving a new child class, you inherit the parent’s dependency properties.  You might also want to change the behavior of one or more of the dependency properties.  You can do this to some extent by overriding the metadata for a dependency property.

Overriding the metadata in the child class doesn’t change the behavior of the dependency property in the parent class.

In the example below, we create a new class and override metadata for several properties, changing the default values.

    public class ThermometerSlider : Slider
    {
        static ThermometerSlider()
        {
            // Defaults for standard Slider: Min = 0, Max = 10, Value = 0
            // Defaults for ThermometerSlider: Min = -40, Max = 120, Value = 70

            // Change default Minimum to -40.0
            MinimumProperty.OverrideMetadata(typeof(ThermometerSlider), new FrameworkPropertyMetadata(-40.0));

            // Change default Maximum to 120
            MaximumProperty.OverrideMetadata(typeof(ThermometerSlider), new FrameworkPropertyMetadata(120.0));

            // Change default Value to 70
            ValueProperty.OverrideMetadata(typeof(ThermometerSlider), new FrameworkPropertyMetadata(70.0));
        }
    }

The metadata type must match the type in the original property (e.g. FrameworkPropertyMetadata, UIPropertyMetadata, or PropertyMetadata).

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