String Truncate method with test coverage

StringExtensions-Truncate.cs

// CC-BY-SA 3.0, implementation by @LBushkin from http://stackoverflow.com/a/2776689/419956
public static class StringExtensions
{
    public static string Truncate(this string value, int maxLength)
    {
        if (maxLength < 0) throw new ArgumentOutOfRangeException("maxLength");
        if (string.IsNullOrEmpty(value)) return value;
        return value.Length <= maxLength ? value : value.Substring(0, maxLength);
    }
}

// NUnit TestFixture for coverage
[TestFixture]
public class StringExtensionsTests
{
    [TestCase((string)null)]
    [TestCase("")]
    [TestCase(" ")]
    [TestCase("\t")]
    [TestCase("\r")]
    [TestCase("\n")]
    public void Truncate_WhenStringIsNothing_ReturnsOriginal(string value)
    {
        Assert.That(value.Truncate(1), Is.EqualTo(value));
    }

    [Test]
    public void Truncate_WhenStringLengthIsEqualToMaxLength_ReturnsString()
    {
        Assert.That("four".Truncate(4), Is.EqualTo("four"));
    }

    [Test]
    public void Truncate_WhenStringLengthIsSmallerThanMaxLength_ReturnsString()
    {
        Assert.That("one".Truncate(4), Is.EqualTo("one"));
    }

    [Test]
    public void Truncate_WhenStringLengthIsLargerThanMaxLength_ReturnsString()
    {
        Assert.That("very long".Truncate(4), Is.EqualTo("very"));
    }

    [Test]
    public void Truncate_WhenMaxLengthIsZero_ReturnsEmptyString()
    {
        Assert.That("text".Truncate(0), Is.EqualTo(""));
    }

    [Test]
    public void Truncate_WhenMaxLengthIsNegative_ThrowsArgumentOutOfRangeException()
    {
        var ex = Assert.Throws<ArgumentOutOfRangeException>(() => "text".Truncate(-1));
        Assert.That(ex.ParamName, Is.EqualTo("maxLength"));
    }
}