public void AddExistingElementToIntCollection()
            {
                int count = 10;
                UniqueCollectionClass <int> IntCollection = new UniqueCollectionClass <int>();

                for (int i = 0; i < count; i++)
                {
                    IntCollection.Add(i);
                }

                Assert.Throws <ArgumentException>(() => IntCollection.Add(count - 1));
            }
            public void AddNonexistingElementToIntCollection()
            {
                int count = 10;
                UniqueCollectionClass <int> IntCollection = new UniqueCollectionClass <int>();

                for (int i = 0; i < count; i++)
                {
                    IntCollection.Add(i);
                }

                Assert.DoesNotThrow(() => IntCollection.Add(count + 1));
            }