How to I access an attached property in code behind?

Question

I have a rectangle in my XAML and want to change it's "Canvas.Left" property in code behind:

<UserControl x:Class="Second90.Page"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
    Width="400" Height="300" KeyDown="txt_KeyDown">
    <Canvas>
        <Rectangle 
            Name="theObject" 
            Canvas.Top="20" 
            Canvas.Left="20" 
            Width="10" 
            Height="10" 
            Fill="Gray"/>
    </Canvas>
</UserControl>

But this doesn't work:

    private void txt_KeyDown(object sender, KeyEventArgs e)
    {
        theObject.Canvas.Left = 50;
    }

Does anyone know what the syntax is to do this?

Answer 1

Try this

theObject.SetValue(Canvas.LeftProperty, 50d);

There is a group of methods on DependencyObject (base of most WPF classes) which allow the common access to all dependency properties. They are

• SetValue
• GetValue
• ClearValue

Edit Updated the set to use a double literal since the target type is a double.

Answer 2

 Canvas.SetLeft(theObject, 50)

Answer 3

As we are changing the property of the 'object', it would be better to use method suggedte by JaredPar:

theObject.SetValue(Canvas.LeftProperty, 50d);