Keeping an object graph consistent

interface IGraphNode
{
    IGraphNode From { get; set; }
    IGraphNode To { get; set; }
}

IGraphNode A = new GraphNode();
IGraphNode B = new GraphNode();
A.To = B;

IGraphNode From
{
    get { return from; }
    set
    {
        from = value;
        if (value.To != this) {
            value.To = this;
        }
    }
}

IGraphNode To
{
    get { return to; }
    set
    {
        to = value;
        if (value.From != this) {
            value.From = this;
        }
    }
}

A.To = B;
B.From = A;

IGraphNode A = new GraphNode();
IGraphNode B = new GraphNode();
IGraphNode C = new GraphNode();

A.To = B;
C.To = B;

// what should B.From return?

IGraphNode A = new GraphNode();
IGraphNode B = new GraphNode();

A.To = B;
A = null;  // should the reference from B keep the object
           // that A used to reference alive?

void ConnectNodes(IGraphNode a, IGraphNode b)
{
    a.to = b;
    b.from = a;
}

void ConnectNodes(IGraphNode b)
{
    this.to = b;
    b.from = this;
}