尝试以下代码:

string planets = "1 Mercury Gray \n"
                    + "2 Venus Yellow \n"
                    + "3 Earth Blue \n"
                    + "4 Mars Red \n";

var lines = planets.Split("\n".ToCharArray(), StringSplitOptions.RemoveEmptyEntries)
    .OrderBy(s => s.Split(' ')[2])
    .ToArray();

foreach (var line in lines)
{
    Console.WriteLine(line);
}

谢谢@Kevin！