Palindromes in C# 7. Test-driven using xUnit

PalindromeChecker.cs

// This code is API-compatible with .NET Standard 1.2.
// https://docs.microsoft.com/en-us/dotnet/standard/net-standard#which-net-standard-version-to-target

using System;
using System.Linq;

namespace Palindrome
{
    public class PalindromeChecker : IPalindromeChecker
    {
        public bool IsPalindrome(string input)
        {
            if (string.IsNullOrWhiteSpace(input))
            {
                return false;
            }

            var loweredLettersAndNumbers = input
                .Where(char.IsLetterOrDigit)
                .Select(char.ToLower);

            if (!loweredLettersAndNumbers.Any())
            {
                // we need letters or numbers to qualify as a palindrome
                return false;
            }

            // do the reversed characters match? if so, we have a palindrome.
            return loweredLettersAndNumbers
                .Reverse()
                .SequenceEqual(loweredLettersAndNumbers);
        }
    }
}

PalindromeTests.cs

using Xunit;

namespace XUnitTestProject1
{
    public class PalindromeTests
    {
        [Theory]
        [InlineData("madam", true)]
        [InlineData("nurses run", true)]
        [InlineData("Nurses run", true)]
        [InlineData("nur sesrun", true /*?*/)]  // needs requirements clarification - this is nonsensical, but is it a palindrome?
        [InlineData("Madam, I'm Adam.", true)]  // from https://www.dictionary.com/browse/palindrome
        [InlineData("12321", true)]
        [InlineData("12320", false)]
        [InlineData("a", true)]         // single-letter word
        [InlineData("b", true /*?*/)]   // needs requirements clarification. - While "a" is a single-letter word, "b" is NOT a word by itself so it doesn't really "read" backwords nor forwards according to the definition of palindrome. 
        [InlineData("ab", false)]
        [InlineData("", false)]
        [InlineData(" ", false)]
        [InlineData(" , ", false)]
        [InlineData("_", false)]        // _ is part of the \w regex character class (https://gist.github.com/antmdvs/b8c77a583284f72eb439c2da0a1c37bb/revisions#diff-7c21bc6041149a1858ff9271bd964b41L21)
        [InlineData(null, false)]       // since we don't have non-nullable reference types (yet) :)
        public void ShouldCorrectlyIdentifyPalindromes(string input, bool expected)
        {
            var sut = new PalindromeChecker();
            var result = sut.IsPalindrome(input);
            Assert.Equal(expected, result);
        }
    }
}