Assign an object to a BindableProperty in ResourceDictionary

Question

I try to initialize and assign an instance of a custom class to a BindableProperty in a ResourceDictionary (styles.xaml).

My class:

namespace Foo {
  public class Colors {
    public Color Positive { get; set; }
    public Color Negative { get; set; }
    public Color Neutral { get; set; }

    public Colors() { }
  }
}

My class with BindableProperty:

namespace Foo {
  public class Bar : ContentView {
    public Colors Colors {
      get => (Colors)GetValue(ColorsProperty);
      set => SetValue(ColorsProperty, value);
    }

   public static BindableProperty ColorsProperty = 
            BindableProperty.Create(nameof(Colors), typeof(Colors), 
            typeof(Bar), null, BindingMode.OneWay);

styles.xaml

<?xml version="1.0" encoding="utf-8" ?>
<ResourceDictionary xmlns="http://xamarin.com/schemas/2014/forms"
                    xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
                    xmlns:foo="clr-namespace:Foo;assembly=Foo">
  <foo:Colors
        x:Key="colors"
        Positive="Green"
        Negative="Red"
        Neutral="Blue"/>

  ...

  <Style TargetType="foo:Bar">
    <Setter Property="Colors" Value="{StaticResource colors}"/>
  </Style>
</ResourceDictionary>

A breakpoint in the constructor of class Foo.Colors is hit, so an instance of this class is created during loading styles.xaml. However loading styles.xaml crashes with an unhandled NullReferenceException inside Mono.Android. When i delete the assignment of the Color-Property everything works fine, so the assignment must be the problem.

Any suggestions?

Exception Details:

System.NullReferenceException: Object reference not set to an instance of an object.
  at Android.Runtime.JNINativeWrapper._unhandled_exception (System.Exception e) [0x0000e] in /Users/runner/work/1/s/xamarin-android/src/Mono.Android/Android.Runtime/JNINativeWrapper.g.cs:12 
  at Android.Runtime.JNINativeWrapper.Wrap_JniMarshal_PPL_V (_JniMarshal_PPL_V callback, System.IntPtr jnienv, System.IntPtr klazz, System.IntPtr p0) [0x0001d] in /Users/runner/work/1/s/xamarin-android/src/Mono.Android/Android.Runtime/JNINativeWrapper.g.cs:111 
  at (wrapper native-to-managed) Android.Runtime.JNINativeWrapper.Wrap_JniMarshal_PPL_V(intptr,intptr,intptr)

The Exception occures in App.xaml.g.cs, after global::Xamarin.Forms.Xaml.Extensions.LoadFromXaml(this, typeof(App));

Answer 1

I found a simple solution that works for me:

In styles.xaml I replaced StaticResource with DynamicResource:

<Style TargetType="foo:Bar">
  <Setter Property="Colors" Value="{DynamicResource colors}"/>
</Style>

Thanks for all suggestions!