Untitled

{
    public class Controller
    {
        private List<string> errorList = new List<string> { "-", "+", "*", "/", "q", "c" };
        private double number;
        private IUI ui = new ConsoleIO();

        public double CheckValidDoubleInput(string input)
        {
            
            while (!double.TryParse(ui.ReadInput(), out number))
            {
                ui.WriteOutput("You need to write a valid double.");
            }
            return number;
        }

        public string CheckValidLetterInput(string input)
        {

            while (input.Length != 1 || errorList.Contains(input))
            {
                ui.WriteOutput("You need to write a single letter (c or q) or an arithmetic symbol.");
                input = ui.ReadInput().ToString();
            }

            return input;
        }
    }