CSharpCode

Code:

public class OddEven
{
    private const string s_Even = "Even";
    private const string s_Odd = "Odd";

    public string Convert(int number)
    {
        if (number % 2 == 0) return s_Even;
        if (IsPrimeNumber(number)) return number.ToString();

        return s_Odd;
    }

    public string ConvertRange(int start, int end)
    {
        string result = string.Empty;

        for (int i = start; i <= end; i++)
        {
            result += Convert(i) + " ";
        }

        return result.Trim();
    }

    private bool IsPrimeNumber(int number)
    {
        if (number == 1) return false;

        for (short i = 3; i <= Math.Sqrt(number); i += 2)
        {
            if (number % i == 0) return false;
        }

        return true;
    }
}

Test:

[TestFixture]
public class OddEvenTest
{
    [TestCase(1, "Odd")]
    [TestCase(2, "Even")]
    [TestCase(3, "3")]
    [TestCase(4, "Even")]
    [TestCase(5, "5")]
    [TestCase(6, "Even")]
    [TestCase(7, "7")]
    [TestCase(8, "Even")]
    [TestCase(9, "Odd")]
    [TestCase(10, "Even")]
    public void SingleOddEvenTest(int number, string expected)
    {
        // arrange
        var oddEven = new OddEven();

        // act
        string actual = oddEven.Convert(number);

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

    [Test]
    public void MultipleOddEvenTest()
    {
        // arrange
        var oddEven = new OddEven();

        // act
        string actual = oddEven.ConvertRange(1, 10);

        // assert
        string expected = "Odd Even 3 Even 5 Even 7 Even Odd Even";
        Assert.AreEqual(expected, actual);
    }
}