This solution uses the notions that business interfaces know they must create a return and that their concrete implementations know what kind of concrete return to create.
Step 1 Split InvestmentReturn into two interfaces; the original minus the Business property and a new generic subclass:
public abstract class InvestmentReturn
{
public double ProfitElement { get; set; }
public abstract double GetInvestmentProfit();
public double CalculateBaseProfit()
{
// ...
}
}
public abstract class InvestmentReturn<T>: InvestmentReturn where T : IBusiness
{
public T Business { get; set; }
}
Step 2 Inherit from the generic one so you can use Business without casting:
public class RetailInvestmentReturn : InvestmentReturn<IRetailBusiness>
{
// this won't compile; see **Variation** below for resolution to this problem...
public RetailInvestmentReturn(IRetailBusiness retail)
{
Business = retail;
}
public override double GetInvestmentProfit()
{
ProfitElement = Business.GrossRevenue;
return CalculateBaseProfit();
}
}
Step 3 Add a method to IBusiness that returns an InvestmentReturn:
public interface IBusiness
{
InvestmentReturn GetReturn();
}
Step 4 Introduce a generic sublcass of EntityBaseClass to provide the default implementation of the above method. If you don't do this you'll have to implement it for all the businesses. If you do do this it means all of your classes where you don't want to repeat the GetReturn() implementation must inherit from the class below, which in turn means they must inherit from EntityBaseClass.
public abstract class BusinessBaseClass<T> : EntityBaseClass, IBusiness where T : InvestmentReturn, new()
{
public virtual InvestmentReturn GetReturn()
{
return new T();
}
}
Step 5 Implement that method for each of your subclasses if necessary. Below is an example for the BookShop:
public class BookShop : BusinessBaseClass<RetailInvestment>, IRetailBusiness
{
public double GrossRevenue { get; set; }
public BookShop(double grossRevenue)
{
GrossRevenue = grossRevenue;
}
// commented because not inheriting from EntityBaseClass directly
// public InvestmentReturn GetReturn()
// {
// return new RetailInvestmentReturn(this);
// }
}
Step 6 Modify your Main to add the instances of InvestmentReturn. You don't have to typecast or type-check because that's already been done earlier in a type safe way:
static void Main(string[] args)
{
var allMyProfitableBusiness = new List<IBusiness>();
// ...
var investmentReturns = allMyProfitableBusiness.Select(bus => bus.GetReturn()).ToList();
// ...
}
If you don't want your concrete businesses to know anything about creating an InvestmentReturn—only knowing that they must create one when asked—then you'll probably want to modify this pattern to incorporate a factory that creates returns given input (e.g. a map between IBusiness implementations and InvestmentReturn subtypes).
Variation
All of the above works fine and will compile if you remove the investment return constructors that set the Business property. Doing this means setting Business elsewhere. That might not be desirable.
An alternative to that would be to set the Business property inside GetReturn. I found a way to do that, but it really starts to make the classes look messy. It's here for your evaluation as to whether its worth it.
Remove the non-default constructor from RetailInvestmentReturn:
public class RetailInvestmentReturn : InvestmentReturn<IRetailBusiness>
{
public override double GetInvestmentProfit()
{
ProfitElement = Business.GrossRevenue;
return CalculateBaseProfit();
}
}
Change BusinessBaseClass. This is where it gets messy with a double-cast, but at least it's limited to one place.
public abstract class BusinessBaseClass<T, U> : EntityBaseClass, IBusiness
where T : InvestmentReturn<U>, new()
where U : IBusiness
{
public double GrossRevenue { get; set; }
public virtual InvestmentReturn GetReturn()
{
return new T { Business = (U)(object)this };
}
}
Finally change your businesses. Here's an example for BookShop:
public class BookShop : BusinessBaseClass<RetailInvestmentReturn, IRetailBusiness>, IRetailBusiness
{
// ...
}
