Capitalise just the first letter of the whole sentence

Capitalise.cs

private string Capitalise(string str)
{
  if(String.IsNullOrEmpty(str))
    return string.Empty;
  
  var firstLetter = string.Empty;
  if(str.Length > 0)
    firstLetter = Char.ToUpper(str[0]);
  
  return (str.Length == 1) ? firstLetter : firstLetter + str.Substring(1).ToLower();
}