Replacing a series of if statements with an interface-based approach - oop

Question

so I am trying to write a discounts method that will apply discount(s) on a product.
The current vanilla code goes like so:
void ApplyDiscount(List<DiscountRule> discountRules, Product objProduct)
{
foreach (var discountRule in discountRules)
{
// this is a very simple way of deciding on the available discounts
if (discountRule.Type==DiscountType.Percent)
{
// Process for percentage discount
}
if (discountRule.Type==DiscountType.Free)
{
// Process for flat discount
}
// and so on , there are like 5 more types,
// not mentioned here for the case of brevity.
}
}
What this method does is take a list of discount rules and apply on the product.
The discount rules are fetched by executing a SP # the server and that returns the
available discounts for that product.
The code review for this resulted in the following comment:
Please use an interface based approach and try to get rid of the IFs!
I can get rid of the IFs but they will be replaced by SWITCH.
How do I go about using an interface?
May be this question is not constructive, but I would want to know if some OOPs gurus here can guide me in writing this better.
Regards.

Answer 1

An interface / virtual-dispatch approach might look something like this:
// First we loosely define how a "discount" can be used.
// This could also be an abstract class, if common base-class
// functionality is desired.
public interface IDiscount
{
// This is called to apply this discount to a particular product
void ApplyDiscount(Product product);
}
// Here's one implementation that applies a percentage discount
public class PercentDiscount : IDiscount
{
private decimal m_percent;
public PercentDiscount(decimal percent) {
m_percent = percent;
}
#region IDiscount implementation
public void ApplyDiscount(Product product) {
product.Price -= product.Price * m_discount;
}
#endregion
}
// Here's another implementation that makes a product free
public class FreeDiscount : IDiscount
{
public FreeDiscount() {
}
#region IDiscount implementation
public void ApplyDiscount(Product product) {
product.Price = 0;
}
#endregion
}
public class SomeClass {
// Now applying the discounts becomes much simpler! Note that this function
// takes a collection of IDiscounts, and applies them in a consistent way,
// by just calling IDiscount.ApplyDiscount()
void ApplyDiscounts(IEnumerable<IDiscount> discounts, Product product) {
foreach (var discount in discounts) {
discount.ApplyDiscount(product);
}
}
}
Note that I also changed ApplyDiscounts to take an IEnumerable<T> instead of List<T>. This allows any arbitrary collection type to be passed, and also doesn't allow the function to inadvertently modify the collection.