CSharpCode

Code:

    public class FizzBuzz
    {
        public string DoFizzBuzz(uint value)
        {
            string result = string.Empty;
            if (value == 0) return result;

            if (value%3 == 0) result = "fizz";
            if (value%5 == 0) result = result + "buzz";

            if (string.IsNullOrEmpty(result)) result = value.ToString();

            return result;
        }

        public string DoFizzBuzzExtended(uint value)
        {
            string result = string.Empty;
            string text = value.ToString();
            if (value == 0) return result;

            if (value % 3 == 0 || text.Contains("3"))
            {
                result = "fizz";
            }
            if (value % 5 == 0 || text.Contains("5"))
            {
                result = result + "buzz";
            }

            if (string.IsNullOrEmpty(result)) return text;

            return result;
        }
    }

Test:

    [TestFixture]
    public class FizzBuzzTest
    {
        [TestCase(0, "")]
        [TestCase(1, "1")]
        [TestCase(2, "2")]
        [TestCase(3, "fizz")]
        [TestCase(4, "4")]
        [TestCase(5, "buzz")]
        [TestCase(6, "fizz")]
        [TestCase(10, "buzz")]
        [TestCase(15, "fizzbuzz")]
        public void DoFizzBuzzTest(int value, string expected)
        {
            // arrange
            FizzBuzz target = new FizzBuzz();

            // act
            string actual = target.DoFizzBuzz((uint)value);

            // assert
            Assert.AreEqual(expected, actual);
        }

        [TestCase(0, "")]
        [TestCase(1, "1")]
        [TestCase(2, "2")]
        [TestCase(3, "fizz")]
        [TestCase(4, "4")]
        [TestCase(5, "buzz")]
        [TestCase(6, "fizz")]
        [TestCase(10, "buzz")]
        [TestCase(13, "fizz")]
        [TestCase(15, "fizzbuzz")]
        [TestCase(25, "buzz")]
        [TestCase(35, "fizzbuzz")]
        [TestCase(52, "buzz")]
        [TestCase(53, "fizzbuzz")]
        public void DoFizzBuzzExtendedTest(int value, string expected)
        {
            // arrange
            FizzBuzz target = new FizzBuzz();

            // act
            string actual = target.DoFizzBuzzExtended((uint)value);

            // assert
            Assert.AreEqual(expected, actual);
        }
    }