Keyboard-Calculator/Calculator/CalculationResponse.cs

using System;

namespace Calculator
{
    public class CalculationResponse
    {
        public static CalculationResponse Ok = new CalculationResponse(Response.Ok, null);
        public static CalculationResponse ClearedMemory = new CalculationResponse(Response.ClearedMemory, null);
        public static CalculationResponse WrongInput = new CalculationResponse(Response.WrongInput, null);

        public Response Response { get; }
        public string Result { get; }

        public CalculationResponse(Response response, string result)
        {
            if (response == Response.Result && result == "")
                throw new ArgumentNullException("CalculationResponse cannot have Response = RESULT and result = null");
            Response = response;
            Result = result;
        }
    }

    public enum Response
    {
        Ok,
        Result,
        ClearedMemory,
        WrongInput
    }
}