XObject.Annotation<T> Method

public T Annotation<T>()
where T : class

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

T1