FizzBuzz.FizzBuzzer.AddNumberStringPair C# (CSharp) Method

        public void AddNumberStringPair(int number, string numberString)
        {
            if (_numberStringPairs.ContainsKey(number))
                throw new FizzBuzzerHasThisKeyException();
            else if (number == 0)
                throw new ZeroCanNotBeKeyException();

            _numberStringPairs.Add(number, numberString);
        }