I have copied a useful class StringExtensions from Given full path, check if path is subdirectory of some other path, or otherwise but when I put it in my solution (VS 2013 Pro) I get the error message The Type or Namespace [CanBeNull] could not be found and The Type or Namespace [NotNull] could not be found
from the following code lines
public static string WithEnding([CanBeNull] this string str, string ending)
public static string Right([NotNull] this string value, int length)
My project include using System; How can I get rid of these errors?