Setting attached property in App.xaml

Question

I have the following app.xaml:

<BaseApp x:Class ="MyApp"
         xmlns ="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
         xmlns:x ="http://schemas.microsoft.com/winfx/2006/xaml"
         xmlns:local ="clr-namespace:MyApp"
        Props.MyCustom="test"
         Startup ="Application_Startup"                                 
         >

<Application.Resources >
    < ResourceDictionary>
        ...
    </ ResourceDictionary>        

</Application.Resources>   

I then need to be able to read the property from the base application:

 public partial class BaseApp : Application
{        

      static void MyFunc()
      {
           // Access property from here
           var myvar = Props.MyCustom
      }
}

I'm currently working on the belief that this needs to be in a separate class, as follows:

 public class Props : DependencyObject
{
    public string MyCustom
    {
        get { return ( string)GetValue(MyCustomProperty); }
        set { SetValue(MyCustomPropertyKey, value); }
    }

    public static readonly DependencyPropertyKey MyCustomPropertyKey =
        DependencyProperty.RegisterAttachedReadOnly("MyCustom" , typeof (string ), typeof (Props), new UIPropertyMetadata (0));           

    public static readonly DependencyProperty MyCustomProperty = MyCustomPropertyKey.DependencyProperty;

}

Am I approaching this in the right way, and if so, what do I need to do to be able to access this from app.xaml?

EDIT:

For future travellers, the way I finally managed this was to simply declare an abstract read-only property in the base class and override it in code behind. Not as neat as I would have liked, but it works.

Answer 1

Attached properties should have static getter and setter methods, in your case:

public static string GetMyCustom(DependencyObject obj)
{
    return (string)obj.GetValue(MyCustomProperty);
}

public static void SetMyCustom(DependencyObject obj, string value)
{
    obj.SetValue(MyCustomProperty, value);
}

---

Having said that you would still have to have an instance of a DependencyObject-derived class to set the property on, which System.Windows.Application is not. In other words, you can't set an attached property on your MyApp object.

---

Instead of using an attached property, you might just add a plain CLR property to your BaseApp class:

public class BaseApp : Application
{
    public string MyCustom { get; set; }
}

and use it like this:

<local:BaseApp x:Class="MyNamespace.MyApp" ...
               MyCustom="Hello">
    <Application.Resources>

    </Application.Resources>
</local:BaseApp>