private void RejectDuplicates(string name) { var existing = new ListOf <string>( new Yaapii.Atoms.Enumerable.Mapped <XElement, string>( elem => elem.Name.LocalName.ToLower(), this.node[0].Elements() ) ); if ( existing.Contains(name.ToLower()) ) { throw new InvalidOperationException($"Cannot add '{name}' because it already exists."); } }