public void AddRangeShouldAddOnlyValuesThatAreNotInTheHashSetAlready() { //Arrange var customList = new CustomList <int>() { 100, 43, 40, 10, 10, 60, 5 }; //Act hashset.AddRange(customList); int[] expectedResult = new[] { 10, 20, 30, 40, 50, 60, 100, 43, 5 }; //Assert for (int i = 0; i < hashset.Count; i++) { Assert.AreEqual(true, hashset.Remove(expectedResult[i])); Assert.AreEqual(false, hashset.Remove(expectedResult[i])); } }