private string shift(string str, int offset)
{
if (str == null)
{
throw new ArgumentNullException();
}
StringBuilder res = new StringBuilder();
foreach (char ch in str)
{
if (!Abc.Contains(ch) || char.IsControl(ch))
{
throw new ArgumentOutOfRangeException();
}
else
{
res.Append(shift(ch, offset));
}
}
return(res.ToString());
}