namespace ConsoleLinq
{
    using System.Collections.ObjectModel;
    using System.Linq;
    class Program
    {
        static void Main(string[] args)
        {
            Collection<ElementBase> ELementList = new Collection<ElementBase>();
            ElementBase elementObjekt = new ElementBase { Name = "Node1" };
            elementObjekt.DiagramListe.Add(new Diagram { Name = "Diagram1", LayerName = "Layer2" });
            elementObjekt.DiagramListe.Add(new Diagram { Name = "Diagram2", LayerName = "Layer1" });
            ELementList.Add(elementObjekt);
            elementObjekt = new ElementBase { Name = "Node2" };
            elementObjekt.DiagramListe.Add(new Diagram { Name = "Diagram1", LayerName = "Layer1" });
            ELementList.Add(elementObjekt);
            elementObjekt = new ElementBase { Name = "Node3" };
            elementObjekt.DiagramListe.Add(new Diagram { Name = "Diagram2", LayerName = "Layer1" });
            ELementList.Add(elementObjekt);

            // is there a way to select any item in ELementList wich are "Diagram1" and sort the items by LayerName?
            var searchList = (from item in ELementList
                          where item.DiagramListe.GetDiagram("Diagram1") != null
                          select item).ToList();
            // output should
            // searchList[0].Name = "Node2";
            // searchList[1].Name = "Node1";

            foreach (var item in searchList) item.DoSomething();
        }
    }

    public class ElementBase
    {
        public string Name { get; set; }

        public DiagramListe DiagramListe = new DiagramListe();

        public void DoSomething() { }
    }

    public class DiagramListe : Collection<Diagram>
    {
        public Diagram GetDiagram(string diagramName)
            => (from obj in this
                where obj.Name == diagramName
                select obj).FirstOrDefault();
    }

    public class Diagram
    {
        public string Name { get; set; }
        public string LayerName { get; set; }
    }

}