c# Dictionary<String, List<ConcreteClass> Help

I have this structure below. Essentially UserID and Session is the primary key or the unique identifiers in the dataset. I need help setting up the linq to  Print out the DetailList groupby UserId and Session using Linq. I am struggling writing the LinQ

  Dictionary> dataList = Reporter.GetSiteData();

   public class DetailRoot
    {

        public string UserId{ get; set; }
        public string Session{ get; set; }
        public IReadOnlyList DetailList { get; set; }

    }

 public class DetailModel
    {
        public string ApplicationGuid { get; set; }
        public string EventType { get; set; }

  }