public void SubFieldCollection_AsReadOnly1()
        {
            SubFieldCollection collection = new SubFieldCollection();
            SubField           subFieldA  = new SubField('a', "Title1");

            collection.Add(subFieldA);
            SubField subFieldE = new SubField('e', "Subtitle");

            collection.Add(subFieldE);

            collection = collection.AsReadOnly();
            collection.Add(new SubField());
        }