public void Test_VerifyCheckSum() { var serialNumber = SerialNumber.Create("ABC"); Assert.IsTrue(SerialNumber.IsCheckSumValid(serialNumber)); var modified = "D" + new string(serialNumber.Skip(1).ToArray()); Assert.IsFalse(SerialNumber.IsCheckSumValid(modified)); Assert.IsFalse(SerialNumber.IsCheckSumValid(modified + "A")); TestHelper.AssertException <ArgumentNullException>(() => SerialNumber.IsCheckSumValid(null)); Assert.IsFalse(SerialNumber.IsCheckSumValid("")); Assert.IsFalse(SerialNumber.IsCheckSumValid("ABCDER")); }