StartSession.cs

public interface IStartSession
{
    void InvokeStartSesssionActions(ISession session);

    void AddToStartSession(Action<ISession> action);
}

public class StartSession : IStartSession
{
    private static readonly IList<Action<ISession>> StartSessionActions = new List<Action<ISession>>();

    public void InvokeStartSesssionActions(ISession session)
    {
        foreach (var action in StartSessionActions)
        {
            action.Invoke(session);
        }
    }

    public void AddToStartSession(Action<ISession> action)
    {
        StartSessionActions.Add(action);
    }
}