Xaml definition of custom object only works for first instance - xaml

Question

I'm trying to use xaml to instantiate a set of objects as follows:
<Application.Resources>
<local:RelationshipDefinition x:Key="R1" Name="College"
Type="One2one" ForwardName="Attended"
/>
<local:RelationshipDefinition x:Key="R2" Name="HasPart"
Type="One2manyBi" ForwardName="Part" BackwardName="PartOf"
/>
<local:RelationshipDefinition x:Key="R3" Name="HasChild"
Type="One2manyBi" ForwardName="Child" BackwardName="Parent"
/>
</Application.Resources>
The first instance "R1" works, and is instantiated. The other two aren't, even though no errors are thrown. Whichever of the relationships I move into the first place works correctly, so it's clear that it will only "accept" one instance. Is there a way for me to do this? I've experimented with trying to create a Collection class to hold them, but haven't succeeded. Trying to designate the collection as a ContentProperty like this:
[ContentProperty("Definitions")]
produces this error:
Error CS1729 'ContentPropertyAttribute' does not contain a constructor that takes 1 arguments
I'm new to the ways of xaml, and imagine there must be some fairly easy way to accomplish this.
After my initial post I found that if I created a separate ResourceDictionary to hold each RelationshipDefinition and included each separately like this:
<ResourceDictionary>
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="HasChildrenRelationship.xaml"/>
<ResourceDictionary Source="CollegeRelationship.xaml"/>
<ResourceDictionary Source="HasPartRelationship.xaml"/>
</ResourceDictionary.MergedDictionaries>
</ResourceDictionary>
The RelationshipDefinition object is a simple declarative object that is used by a semantic net based data access method. Each user 'declares' the network relationships they well be using in their particular case. Here's a fragment of the code from the class. It's essentially all Dependency Properties with a constructor that registers/finds the RelationshipDefinition in a global store.
public class RelationshipDefinition : DependencyObject
{
public string UID { get; set; } = Guid.NewGuid().ToString();
public string Name
{
get { return (string)GetValue(NameProperty); }
set
{
SetValue(NameProperty, value);
RegisterRelationshipDefinition(this);
}
}
public static readonly DependencyProperty NameProperty =
DependencyProperty.Register("Name", typeof(string), typeof(RelationshipDefinition), new PropertyMetadata(null));
public RelationshipType Type
{
get { return (RelationshipType)GetValue(TypeProperty); }
set { SetValue(TypeProperty, value); }
}
public static readonly DependencyProperty TypeProperty =
DependencyProperty.Register("Type", typeof(RelationshipType), typeof(RelationshipDefinition), new PropertyMetadata(null));
...etc...
It's used like this:
Person chris = new Person(){ FirstName = "Chris", LastName = "Smith", Age = 19 };
Person annemarie = new Person() {FirstName = "Anne-Marie", LastName = "Smith", Age = 22 };
Person john = new Person() { FirstName = "John", LastName = "Smith", Age = 47};
SNet.Link(from: chris, to: nyu, via: "College");
SNet.Link(from: john, to: chris, via: "HasChild");
SNet.Link(from: john, to: annemarie, via: "HasChild");
Person p1 = SNet.GetBackLink<Person>(from: annemarie, via: "HasChild");
List<Person> children = SNet.GetForwardLinks<Person>(from: john, via: "HasChild");
All the RelationshipDefinition objects were instantiated correctly. I'm sure there must be a way to include them together that I'm missing.