How to cross-reference objects in classes

c#, oop

Solution

Great question. Pure many-to-many relationships are actually quite rare, and it usually helps to introduce an intermediate object to model the relationship itself. This will prove invaluable if (when!) use cases emerge which require the capture of properties regarding the relationship (e.g. whether the child/parent relationship is natural, surrogate, adoptive, etc).

So, in addition to the Person, Parent and Child entities which you've already identified, let's introduce an object called ParentChildRelationship. An instance of ParentChildRelationship will have a reference to exactly one Parent and One Child, and both the Parent and Child classes will hold a collection of these entities.

It's a good idea to then identify the use cases you have for working with these entities, and add appropriate helper methods to maintain the inter-object references. In the example below I've just chosen to add a public AddChild method to the parent.

public abstract class Person
{
}

public class Parent : Person
{
    private HashSet<ParentChildRelationship> _children = 
        new HashSet<ParentChildRelationship>();

    public virtual IEnumerable<ParentChildRelationship> Children
    {
        get { return this._children; }
    }

    public virtual void AddChild(Child child, RelationshipKind relationshipKind)
    {
        var relationship = new ParentChildRelationship()
        {
            Parent = this,
            Child = child,
            RelationshipKind = relationshipKind
        };

        this._children.Add(relationship);
        child.AddParent(relationship);
    }
}

public class Child : Person
{
    private HashSet<ParentChildRelationship> _parents = 
        new HashSet<ParentChildRelationship>();

    public virtual IEnumerable<ParentChildRelationship> Parents
    {
        get { return this._parents; }
    }

    internal virtual void AddParent(ParentChildRelationship relationship)
    {
        this._parents.Add(relationship);
    }
}

public class ParentChildRelationship
{
    public virtual Parent Parent { get; protected internal set; }

    public virtual Child Child { get; protected internal set; }

    public virtual RelationshipKind RelationshipKind { get; set; }
}

public enum RelationshipKind
{
    Unknown,
    Natural,
    Adoptive,
    Surrogate,
    StepParent
}

Problem

Original source