title: Updating Custom Attached Property in Style Trigger with Setter
body: <p>I was trying out attached properties and style triggers hoping to learn more about it.
  I wrote a very simple WPF windows app with an attached property:</p>
  
  <pre><code>  public static readonly DependencyProperty SomethingProperty = 
        DependencyProperty.RegisterAttached(
            "Something", 
            typeof(int), 
            typeof(Window1),
            new UIPropertyMetadata(0));
  
    public int GetSomethingProperty(DependencyObject d)
    {
        return (int)d.GetValue(SomethingProperty);
    }
    public void SetSomethingProperty(DependencyObject d, int value)
    {
        d.SetValue(SomethingProperty, value);
    }
  </code></pre>
  
  <p>And I was trying to update the 'Something' attached property with a property trigger defined in the button style section:</p>
  
  <pre><code>  &lt;Window x:Class="TestStyleTrigger.Window1"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:TestStyleTrigger;assembly=TestStyleTrigger"
        Title="Window1" Height="210" Width="190"&gt;
        &lt;Window.Resources&gt;
            &lt;Style x:Key="buttonStyle" TargetType="{x:Type Button}"&gt;
                &lt;Style.Triggers&gt;
                    &lt;Trigger Property="IsPressed" Value="True"&gt;
                        &lt;Setter Property="local:Window1.Something" Value="1" /&gt;
                    &lt;/Trigger&gt;
                &lt;/Style.Triggers&gt;
            &lt;/Style&gt;
        &lt;/Window.Resources&gt;
  
        &lt;Button Style="{StaticResource buttonStyle}"&gt;&lt;/Button&gt;
    &lt;/Window&gt;
  </code></pre>
  
  <p>However, I kept getting following compilation error:</p>
  
  <p>error MC4003: Cannot resolve the Style Property 'Something'. Verify that the owning type is the Style's TargetType, or use Class.Property syntax to specify the Property. Line 10 Position 29.</p>
  
  <p>I can't understand why it gives me this error because I did use the 'Class.Property' syntax in the  tag of the  section. Can any one tell me how can I fix this compilation error?</p>