Refactoring class to get rid of switch case

Question

Say I have a class like this for calculating the cost of travelling different distances with different modes of transportation:

public class TransportationCostCalculator {     public double DistanceToDestination { get; set; }      public decimal CostOfTravel(string transportMethod)     {         switch (transportMethod)         {             case "Bicycle":                 return (decimal)(DistanceToDestination * 1);             case "Bus":                 return (decimal)(DistanceToDestination * 2);             case "Car":                 return (decimal)(DistanceToDestination * 3);             default:                 throw new ArgumentOutOfRangeException();         }     } 

This is fine and all, but switch cases can be a nightmare to maintenance wise, and what if I want to use airplane or train later on? Then I have to change the above class. What alternative to a switch case could I use here and any hints to how?

I'm imagining using it in a console application like this which would be run from the command-line with arguments for what kind of transportation vehicle you want to use, and the distance you want to travel:

class Program {     static void Main(string[] args)     {         if(args.Length < 2)         {             Console.WriteLine("Not enough arguments to run this program");             Console.ReadLine();         }         else         {             var transportMethod = args[0];             var distance = args[1];             var calculator = new TransportCostCalculator { DistanceToDestination = double.Parse(distance) };             var result = calculator.CostOfTravel(transportMethod);             Console.WriteLine(result);             Console.ReadLine();         }     } } 

Any hints greatly appreciated!

Answer 1

You could do something like this:

public class TransportationCostCalculator {     Dictionary<string,double> _travelModifier;      TransportationCostCalculator()     {         _travelModifier = new Dictionary<string,double> ();          _travelModifier.Add("bicycle", 1);         _travelModifier.Add("bus", 2);         _travelModifier.Add("car", 3);     }       public decimal CostOfTravel(string transportationMethod) =>        (decimal) _travelModifier[transportationMethod] * DistanceToDestination; } 

You could then load the transportation type and it's modifier in a configuration file instead of using a switch statement. I put it in the constructor to show the example, but it could be loaded from anywhere. I would also probably make the Dictionary static and only load it once. There is no need to keep populating it each time you create a new TransportationCostCalculator especially if it isn't going to change during runtime.

As noted above, here is how you could load it by a configuration file:

void Main() {   // By Hard coding.    /*     TransportationCostCalculator.AddTravelModifier("bicycle", 1);     TransportationCostCalculator.AddTravelModifier("bus", 2);     TransportationCostCalculator.AddTravelModifier("car", 3);   */     //By File      //assuming file is: name,value     System.IO.File.ReadAllLines("C:\\temp\\modifiers.txt")     .ToList().ForEach(line =>         {            var parts = line.Split(',');         TransportationCostCalculator.AddTravelModifier             (parts[0], Double.Parse(parts[1]));         }     );  }  public class TransportationCostCalculator {     static Dictionary<string,double> _travelModifier =           new Dictionary<string,double> ();      public static void AddTravelModifier(string name, double modifier)     {         if (_travelModifier.ContainsKey(name))         {             throw new Exception($"{name} already exists in dictionary.");         }          _travelModifier.Add(name, modifier);     }      public double DistanceToDestination { get; set; }      TransportationCostCalculator()     {         _travelModifier = new Dictionary<string,double> ();     }       public decimal CostOfTravel(string transportationMethod) =>        (decimal)( _travelModifier[transportationMethod] * DistanceToDestination); } 

Edit: It was mentioned in the comments that this wouldn't allow the equation to be modified if it ever needed to change without updating the code, so I wrote up a post about how to do it here: http://structuredsight.com/2016/03/07/configuring-logic.