Software 1 - Class Exercise 4

Goals

• Add interfaces
• Use linq queries

Instructions

Interfaces

Interfaces are critical to modern programming. There are multiple concepts and programming patterns that rely on interfaces. While we aren't going to explore any of those in this course, we are going to learn how to use interfaces.

1. We have an existing class that would be great to have an interface for: ProductLogic.
2. Create a new Interface and call it IProductLogic. You do it the same way you do a class, you just select Interface isntead of Class. If you create a class by accident, you can just change the keyword class to interface.
3. In this interface, we're going to be saying what functions need to be implemented, not how they are.
4. So look at the ProductLogic class and add the already existing methods to the interface. Here's the first one: public void AddProduct(Product product);
5. Once all the existing methods have been added to the interface, head over to the ProductLogic class again and let's have the class implement the interface. You do this using the same syntax you use to have a class inherit from another :.
6. Once we have ProductLogic implementing IProductLogic`, let's add a new method to our interface.
7. Add a method called GetOnlyInStockProducts. It doesn't need to take any arguments and it will return List<Product>.
8. Heading back to ProductLogic you should see a build error. This is because our ProductLogic class isn't fully implementing the interface. We'll be doing that in the next section.

Using Linq

1. In the ProductLogic class, in the constructor where you are creating a new list of products, instead of having that list be empty, go ahead and add 2 or 3 products to it. One of those products should have a quantity of 0.
2. Add the method from the interface that hasn't yet been implemented.
3. In that method body, we are going to filter our product list to only return products with quantity greater than 0.
4. To normally do this, we would probably use a for loop to iterate through the whole list checking each one to see if its quantity is greater than 0. If it is, we would add it to a new list then return that list. We're going to use linq for this though which will greatly simplify our method.
5. In the method body, type of this: return _products.Where(x => x.Quantity > 0).ToList();
6. Let's take this a step further. Let's say we would only want to return the names of the products. We would do this by using the Select method. This one would sit between Where and ToList. Go ahead and add .Select(x=>x.Name). You will have build errors from this, so go ahead and fix that. You'll need to make changes in ProductLogic and IProductLogic.
7. Finally, add a UI option to view in stock products only.