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Fundamentals
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What is an Attached DependencyProperty, and what is the difference with a regular DependencyProperty?
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An Attached DependencyProperty (also called attached property for simplicity) is a DependencyProperty which can be set and read to/from any instance of any DependencyObject (not limited to the declared owner types, as with a regular DependencyProperty).
With this approach, some classes can interact with objects that know nothing about these classes.
A perfect example of attached properties is the Grid class.
The Grid class defines two attached properties: Grid.Row and Grid.Column.
These two attached properties can be used to alter the layout of controls nested under the Grid control without having them know a thing about the Grid class or how the layout is performed in the Grid class.
Example:
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition>
<ColumnDefinition>
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition>
<RowDefinition>
</Grid.RowDefinitions>
<Button Grid.Column="0" Grid.Row="1" />
<Button Grid.Column="1" Grid.Row="0" />
</Grid>
Note: Be aware that attached properties do not require an actual instance of the class that defines them in order to be used. For example, with KeyboardNavigation, you define no instance, but can use its attached properties.
C# Version:
Grid grid1 = new Grid();
ColumnDefinition coldef1 = new ColumnDefinition();
coldef1.Width = GridLength.Auto;
grid1.ColumnDefinitions.Add(coldef1);
ColumnDefinition coldef2 = new ColumnDefinition();
coldef2.Width = GridLength.Auto;
grid1.ColumnDefinitions.Add(coldef2);
RowDefinition rowdef1 = new RowDefinition();
rowdef1.Height = GridLength.Auto;
grid1.RowDefinitions.Add(rowdef1);
RowDefinition rowdef2 = new RowDefinition();
rowdef2.Height = GridLength.Auto;
grid1.RowDefinitions.Add(rowdef2);
Button btn1 = new Button();
grid1.Children.Add(btn1);
Grid.SetColumn(btn1, 0);
Grid.SetRow(btn1, 1);
Button btn2 = new Button();
grid1.Children.Add(btn2);
Grid.SetColumn(btn2, 1);
Grid.SetRow(btn2, 0);
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