Read-only attached property in trigger (WPF)

I don't know if there is something special with your read-only attached property, but if you declare it in default manner it works:

public class AttachedPropertyHelper : DependencyObject{    public static int GetSomething(DependencyObject obj)    {        return (int)obj.GetValue(SomethingProperty);    }    public static void SetSomething(DependencyObject obj, int value)    {        obj.SetValue(SomethingProperty, value);    }    // Using a DependencyProperty as the backing store for Something. This enables animation, styling, binding, etc...    public static readonly DependencyProperty SomethingProperty =        DependencyProperty.RegisterAttached("Something", typeof(int), typeof(AttachedPropertyHelper), new UIPropertyMetadata(0));}

EDIT

If you want the same as a readonly attached property you change it to:

public class AttachedPropertyHelper : DependencyObject{    public static int GetSomething(DependencyObject obj)    {        return (int)obj.GetValue(SomethingProperty);    }    internal static void SetSomething(DependencyObject obj, int value)    {       obj.SetValue(SomethingPropertyKey, value);    }    private static readonly DependencyPropertyKey SomethingPropertyKey =        DependencyProperty.RegisterAttachedReadOnly("Something", typeof(int), typeof(AttachedPropertyHelper), new UIPropertyMetadata(0));    public static readonly DependencyProperty SomethingProperty = SomethingPropertyKey.DependencyProperty;}