XObject.AddAnnotation-Methode

public void AddAnnotation(
	Object annotation
)

public class MyAnnotation {
    private string tag;
    public string Tag {get{return tag;} set{tag=value;}}
    public MyAnnotation(string tag) {
        this.tag = tag;
    }
}

public class Program {
    public static void Main(string[] args) {   
        MyAnnotation ma = new MyAnnotation("T1");
        XElement root = new XElement("Root", "content");
        root.AddAnnotation(ma);

        MyAnnotation ma2 = (MyAnnotation)root.Annotation<MyAnnotation>();
        Console.WriteLine(ma2.Tag);
    }
}

T1