Set value of private field

Try this (inspired by Find a private field with Reflection?):

var prop = s.GetType().GetField("id", System.Reflection.BindingFlags.NonPublic
    | System.Reflection.BindingFlags.Instance);
prop.SetValue(s, "new value");

My changes were to use the GetField method – you are accessing a field and not a property, and to or NonPublic with Instance.