Gist for demonstration of Jodrell's answer to http://stackoverflow.com/questions/17681197/an-elegant-way-to-do-this-may-be-generics

main.cs

class Transaction {}
class ServiceCharge : Transaction {}
class Sale : Transaction {}

interface IChargable<out T> where T : Transaction
{
    Transaction Charge();
}


class BusinessService : IChargable<ServiceCharge>
{
    public ServiceCharge Charge()
    {
        return new ServiceCharge();
    }
}

class Product : IChargable<Sale>
{
    public Sale Charge()
    {
        return new Sale();
    }
}

class Program
{
    static void Main()
    {
        var chargables = new IChargable<Transaction>[]
        {
            new BusinessService(),
            new Product()
        };
        
    }
}