Map rental cost strategy

Question

It is the case where data contains some field indicating what type of strategy should be picked.

I mean, I often ignore solving the problem like shown below and duplicate the problems that strategy pattern is out to solve in the first place. Is there a pattern for it or is it not viable at all in this case?

Recent example on how I have solved it:

public class MapRentalCostStrategy : IMapper<EquipmentType, IRentalCostStrategy>
{
    private readonly Dictionary<EquipmentType, IRentalCostStrategy> _knownRentalCostStrategy;

    public MapRentalCostStrategy(IMutablePriceConfiguration mutablePriceConfiguration)
    {
        _knownRentalCostStrategy = new Dictionary<EquipmentType, IRentalCostStrategy>
        {
            { EquipmentType.Heavy, new HeavyRentalCostStrategy(mutablePriceConfiguration)},
            { EquipmentType.Specialized, new SpecializedRentalCostStrategy(mutablePriceConfiguration)},
            { EquipmentType.Regular, new RegularRentalCostStrategy(mutablePriceConfiguration)}
        };
    }

    public Func<EquipmentType, IRentalCostStrategy> Create => equipment =>
    {
        if (!_knownRentalCostStrategy.ContainsKey(equipment))
            throw new ArgumentException();

        return _knownRentalCostStrategy[equipment];
    };
}

where EquipmentType is an enum.

Maybe I should inject a service that returns this dictionary? In any case, it seems I am delegating the responsibility (in this case its "object creation", it should not be mappers responsibility to deal with the instantiation of the dictionary).

Maybe I am on the wrong track? Is there a better way?

Answer 1

I think a singleton/enum like pattern would be beneficial. The curious part is the IMutablePriceConfiguration object seems to vary based on the situation.

Remove the IMutablePriceConfiguration as a dependency for the cost strategy objects.

Next:

public sealed class EquipmentType
{
    public static readonly Heavy = new EquipmentType(1, "Heavy", new HeavyRentalCostStrategy());
    public static readonly Specialized = new EquipmentType(2, "Specialized", new SpecializedRentalCostStrategy());
    public static readonly Regular = new EquipmentType(3, "Regular", new RegularRentalCostStrategy());

    public static readonly IEnumerable<EquipmentType> All = new EquipmentType[]
    {
        Heavy,
        Specialized,
        Regular
    };

    public static EquipmentType Find(int id)
    {
        return All.SingleOrDefault(e => e.Id == id);
    }

    public static EquipmentType Find(string name)
    {
        return All.SingleOrDefault(e => e.Name == name);
    }

    private IRentalCostStrategy CostStrategy { get; set; }
    public string Name { get; private set; }
    public int Id { get; private set; }

    private EquipmentType(int id, string name, IRentalCostStrategy costStrategy)
    {
        Id = id;
        Name = name;
        CostStrategy = costStrategy;
    }

    public double CalculateRentalCost(IMutablePriceConfiguration mutablePriceConfiguration)
    {
        return CostStrategy.CalculateRentalCost(mutablePriceConfiguration);
    }
}

• The IMutablePriceConfiguration becomes an argument to the CalculateRenderCost method on the EquipmentType class

• The EquipmentType class has a private constructor so you are guaranteed to only have those three instances available. No one can incorrectly (or maliciously) create equipment types and their cost strategies.

• You can access each equipment type in an enum-like syntax:

  EquipmentType.Regular
  

• You can find by id or name:

  EquipmentType.Find(1);
  EquipmentType.Find("Specialized");
  

• You can blindly loop over all available equipment types:

  foreach (var equipmentType in EquipmentType.All)
  {
      // ...
  }
  

• As long as you have an IMutablePriceConfiguration object, you can blindly call the CalculateRentalCost method and get a result back. No need for ifs or switchs peppered all over your code.