Rot 13 kata in C# with xUnit in one line.

Rot13.cs

using System;
using System.Linq;

namespace Cipher
{
    public class Rot13
    {
        public static string Encode(string text)
            => new string(
                text.ToCharArray()
                  .Select(c => char.IsLetter(c) ?
                                char.IsLower(c) ?
                                  (char)((c - 'a' + 13) % 26 + 'a')
                                  : (char)((c - 'A' + 13) % 26 + 'A')
                                : c
                  ).ToArray());
    }
}

Rot13Tests.cs

using System;
using Xunit;

namespace Cipher.Tests
{
    public class Rot13Tests
    {
        [Fact]
        public void GivenEmptyItMustReturnEmpty()
        {
            var actual = Rot13.Encode(string.Empty);
           Assert.True(string.IsNullOrEmpty(actual));
        }

        [Theory]
        [InlineData("ABCDEFGHIJKLMNOPQRSTUVWXYZ", "NOPQRSTUVWXYZABCDEFGHIJKLM")]
        [InlineData("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz", "NOPQRSTUVWXYZABCDEFGHIJKLMnopqrstuvwxyzabcdefghijklm")]
        [InlineData("Why did the chicken cross the road?", "Jul qvq gur puvpxra pebff gur ebnq?")]
        [InlineData("Gb trg gb gur bgure fvqr!", "To get to the other side!")]
        public void GivenValueMustEncodeToExpectedValue(string given, string expected)
            => Assert.Equal(expected, Rot13.Encode(given));
    }
}

@MikeMKH too late.

Love this! To make it even shorter - you can ditch text.ToCharArray().Select use Select right off of the string.

> "ABC".ToCharArray().Select(c => (char)((c - 'A' + 13) % 26 + 'A')).ToArray()
> char[3] { 'N', 'O', 'P' }

> "ABC".Select(c => (char)((c - 'A' + 13) % 26 + 'A')).ToArray()
> char[3] { 'N', 'O', 'P' }