Whats wrong?

    using QuickGraph;
    using GraphSharp;

     public class State
        {
            public string Name { get; set; }
            public override string ToString()
            {
                return Name;
            }
        }

     public class Event
        {
            public string Name;
            public override string ToString()
            {
                return Name;
            }
        }

    BidirectionalGraph<State, TaggedEdge<State, Event>> x =
                    new BidirectionalGraph<State, TaggedEdge<State, Event>>();

    GraphLayout graphLayout = new GraphLayout();
    graphLayout.Graph = x;

Error:

Cannot implicitly convert type 'QuickGraph.BidirectionalGraph>' to 'QuickGraph.IBidirectionalGraph>'. An explicit conversion exists (are you missing a cast?)

If I put the cast, then application gets fault error on start without any information

Whats wrong?