string choice = Console.ReadLine();
string ch = choice.ToLower();
while (ch != "a" && ch != "b" && ch != "c")
{
   Console.WriteLine("There is no such choice,input again!");
   choice = Console.ReadLine();
   ch = choice.ToLower();
}
switch(ch)
{
   case "a":
    StartGame();
    break;
   case "b":
    Help();
    break;
   case "c":
    break;
}