yield return, yield break

public static IEnumerable<char> AlphabetSubset2(char start, char end)
 {
     char[] chars = new char[end - start];
     int i = 0;
     for (var c = start; c < end; c++)
     {
         chars[i++] = c;
     }

     return chars;
 }


public static IEnumerable<char> AlphabetSubset(char start, char end)
{
    for (var c = start; c < end; c++)
        yield return c;
}