public void CheckIfContainsPersonWithSuchName()
        {
            DatabaseExtended db = new DatabaseExtended();

            db.Add(new Person(12, "Todor"));
            Assert.Throws <InvalidOperationException>(() => db.FindByUsername(null), "The name should not be null!");
        }
        public void CheckIfContainsPersonWithSuchName(string name)
        {
            DatabaseExtended db = new DatabaseExtended();

            db.Add(new Person(12, "Todor"));
            Assert.Throws <InvalidOperationException>(() => db.FindByUsername(name), "There is not such name");
        }