IsPalindrome.cs

using System;
using System.Collections.Generic;
using System.Linq;

public class Program
{
	public static void Main()
	{
		Check("abcba", true);
		Check("abcde", false);
		Check("Mr owl ate my metal worm", true);
		Check("Never Odd Or Even", true);
		Check("Never Even Or Odd", false);
		
		Check("Do geese see God?", true);
		Check("A man, a plan, a canal, Panama!", true);
		Check("Ītati", true);
		
		Check("12321", true);
		Check("12345", false);
	}
		
	private static void Check(string s, bool shouldBePalindrome)
	{
		Console.WriteLine(s.IsPalindrome() == shouldBePalindrome ? "pass" : "FAIL");
	}
}

public static class StringExtensions
{
	public static bool IsPalindrome(this string s)
	{
        	var normalised = s.Normalise();

		return normalised.SequenceEqual(normalised.Reverse());
	}

	public static IEnumerable<char> Normalise(this string value)
	{
		return value
			.ToLower()
			.Normalize(System.Text.NormalizationForm.FormKD)
			.Where(c => char.IsLetter(c) || char.IsDigit(c));
	}
}